Travel Forecasting Glossary 2017
Michael G. McNally (mmcnally@uci.edu)
University of California, Irvine
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A

AADT

Annual Average Daily Traffic is the average of daily (24-hour) traffic volumes at a specified point over a year. Average Daily Traffic (ADT) is the corresponding average for a period less than a year (appropriate where traffic volumes fluctuate significantly over time).

AASHTO

American Association of State Highway and Transportation Officials represents state transportation departments in fostering the development, operation, and maintenance of an integrated national transportation system (formerly AASHO) [ web ].

AB 32

The "California Global Warming Solutions Act of 2006" is a comprehensive program of regulatory and market mechanisms designed to achieve a 30 percent reduction of greenhouse gases by 2020 (see SB 375) [ web ].

Abstract Mode

A mode alternative defined only by objective characteristics. Also, a (typically direct demand) mode choice model that represents modal attributes relative to the best or average mode and has only generic variables and parameters, facilitating the introduction of new mode alternatives by obviating model re-estimation.

Access

The right to enter and leave a location, facility, or service from a public right-of-way. Accessibility is typically defined as a measure of the ability of individuals or groups to exercise access.

Access & Egress

Access defines movement toward and egress defines movement from, as in access/egress to and from a development site or access/egress modes associated with getting to a transit stop (an access point reached, for example, via car as an access mode) and from the transit stop (an egress point) to a destination (via an egress mode, such as walking). See line-haul.

Access Management

Policies to "limit and consolidate access along major roadways, while promoting a supporting street system and unified access and circulation systems for development" (TRB) to promote safe operations and aesthetic design [ web ].

Accessibility

[1] A measure of the ability of individuals to travel between various activity locations within a region (see mobility); [2] An aggregate measure of the relative accessibility of a location to all other locations, often weighted by the number of opportunities for a particular activity type and represented by a variety of indices.

Active Transportation

Programs and policies for encouraging walking, biking, and other modes requiring physical exertion directed toward achievement of sustainable travel, land use, lifestyle, and health goals and objectives.

Activity

An activity occurs at the end of every trip and reflects the trip purpose, that is, what the tripmaker did at the trip destination. Activities are categorized according to schemes such as subsistence/maintenance/discretionary or work/shop/social/etc. The term is also used to describe the corresponding aggregate pattern of activity in time and space.

Activity Agenda

A set of activities planned by an individual or a household during a defined period of time.

Activity Analysis

A generic name for the study of activity and travel behavior, often called the activity-based approach.

Activity-based Approach

A modeling perspective focused on activities and reflecting the generally held belief that travel is a demand derived from activity participation. Activity-based models address individual and household travel / activity patterns that summarize the scheduling of travel and activities over the course of one or more days.

Activity-based Model

A model focused on activities rather than the conventional trip-based approach. The complexity of true activity-based models is such that they are still limited to research and controlled studies, but tour-based models are entering practice (circa 2010).

Activity Center

An area characterized by concentrated activity and a high density of trip ends, including the conventional CBD and major retail, employment, or mixed-use developments.

Activity Location Functions

In Transportation Systems Analysis, activity location functions represent the relationship between the location of activity (land uses) and general levels of prices for goods and services as well as the output levels of performance and demand. Location functions are used to project the future activity system in a region.

Activity Pattern

An activity pattern (AP) is a full description of the travel and activities completed by an individual (IAP) or a household (HAP) during a defined period of time, including spatial, temporal, interpersonal, and transportation attributes. It is the output of individual and household decision-making processes acting on an activity program and reflecting potential stochastic events that alter plans and constraints. The AP is the revealed outcome of unrevealed behavioral processes. Activity pattern is also used to describe the aggregate pattern of travel or activity in a population.

Activity Program

In most cases, synonymous with agenda, a set of activities planned by an individual or a household during a defined period of time (often this period is taken as one day).

Activity System

In the Manheim/Florian framework for transportation systems analysis, the Activity System constitutes a region's land uses and activities, excluding only the Transportation System. The Activity System in defined by location functions and is a key element in defining demand functions.

Actuated Control

A traffic control system where signal indications are determined by detection of vehicle or pedestrian arrivals at a sensor. When only selected movements are controlled by detection, the control is semi-actuated (see fixed time and adaptive control).

ADA

See Americans with Disabilities Act of 1990 (ADA).

Adaptive Control

A traffic control system where signal control parameters are adjusted in realtime based on ambient traffic measurements. (see fixed time and actuated control).

Adjacency Matrix

An N nodes by N nodes network topography matrix where cell (i,j) is non-zero if link (i,j) exists. The cell value is set equal to one to indicate a link exists but it can be used to store capacity or another link characteristic.

AFV

Alternate Fuel Vehicle (see AFV).

Agenda 21

A 1992 UN action plan regarding sustainable development, incorporating economic, environmental, public health, and other dimensions.

Agent

A (software) process to transparently act for a user by completing transactions, seeking information of specific interest, or communicating with other users and agents.

Agent-based Model

A computational process modeling approach for simulating the actions of autonomous agents, an approach well-suited for integrating learning and adaptation behaviors in activity-based models.

Agglomeration

Economic activity clustered in or around a single location. An agglomeration economy is a benefit that accrues due to being part of an agglomeration.

Aggregate Model

A model with units of analysis defined as aggregates of the basic behavior units. In travel forecasting, the basic behavioral unit is the individual decision-maker or the household. In aggregate travel forecasting models, these basic units are aggregated to spatially-defined Traffic Analysis Zones (TAZs) (see also Disaggregate Models).

Aggregation

The process of obtaining aggregate forecasts (such as link flows or modal shares) from disaggregate models. Techniques include naive aggregation and sample enumeration.

AHP

Analytical Hierarchy Process (see AHP).

AHS

Automated Highway System (see AHS).

AI

Artificial Intelligence, a multidisciplinary field dedicated to the development of methods or models that emulate human reasoning.

AIMSUN

A integrated suite of traffic microsimulation and travel forecasting software. Developed by TSS [ web ]. Other traffic microsimulation packages include CORSIM, Cube Dynasim, Paramics, SimTraffic, TransModeler, and VISSIM.

Air Pollution

The presence of one or more contaminants in the atmosphere in sufficient quantity and/or for sufficient duration to potentially impact human and/or environmental health. EPA defines standards for seven such criteria pollutants, including ozone, carbon monoxide, nitrogen oxides, particulate matter, and sulfur oxides [ web ].

Air Quality

An assessment of the presence in the atmosphere of defined pollutants, including carbon monoxide, nitrous oxides, particulate matter, and sulphur oxides. Air quality modeling is a critical task in metropolitan transportation planning for regions that have not attained national ambient air quality standards (NAAQS).

Air Quality Management District

A regional agency that adopts regulations to meet state and federal air quality standards.

Air Quality Modeling

Application of mathematical models for the estimation of emission rates, emissions by pollutant, dispersion of pollutants, and/or related air quality characteristics, typically applied as post-processors to the standard four-step model.

Akcelik LPF

A modification of Davidson's link performance function that reflects the original queueing theory elements but is amenable to application with standard iterative assignment algorithms (see also BPR).

ALINEA

Asservissement Lineaire d'Entree Autoroutiere, Papageorgiou's (1991) efficient, local feedback control, ramp metering algorithm.

Algorithm

A systematic, computational methodology for finding solutions to mathematical problems (e.g., the Frank-Wolfe Algorithm; Dijkstra's Algorithm; Dial's Algorithm).

All-or-Nothing Assignment

(AON) An elementary, deterministic trip assignment where the total trips between an origin and a destination are loaded on a single (typically) minimum travel time path. Link congestion effects are not considered, as compared with capacity restrained or equilibrium assignment (see also stochastic assignment).

All Red

The portion of a cycle for which all signal indications display red, providing a clearance interval for the intersection.

ALOGIT

An advanced software package for the estimation of logit models. Developed by The Hague Consulting Group [ web ].

Alternate Fuel Vehicle

A motorized vehicle that does not use gasoline or diesel fuels, instead using natural gas, methanol, hydrogen, and related fuels, or electric-powered.

Alternative

An option in a choice set, such as a mode choice alternative. In the context of travel forecasting, a transportation system alternative representing a future combination of current and planned system elements under consideration in a transportation planning study.

Alternative Modes

Non-automotive modes of travel including public transportation options and non-motorized modes such as bicycles and walking. Also includes evolving modes such as Segways and electric scooters that can utilize walkways and bikeways.

Alternative Planning Strategy

Under SB375, California MPOs must submit a Sustainable Communities Strategy (SCS) as part of the Regional Transportation Plan (RTP). The SCS must meet emission reduction targets if feasible to do so; if not, the MPO develops a separate plan, an Alternative Planning Strategy, that identifies impediments and a process to achieve targeted reductions if the impediments were not present.

Alternative Specific Constant

Alternative-specific indicates a variable or constant that is assigned to a particular choice alternative in a utility model. An alternative-specific constant takes on a value of one for the assigned choice and its estimated parameter is usually considered the constant itself. Variables that are assigned to all alternatives in a choice set are deemed generic.

Alternative Specific Variable

In discrete choice models, an alternative-specific variable appears in the utility function of a specific choice alternative with an independently estimated coefficient (compare with generic variable).

Alternative Work Schedule

Programs designed to reduce peak period work trips by having working hours start and/or end earlier and/or later than conventional work hours, often paired with a reduction in the number of work days. For example, four 10-hour work days (versus five 8-hour days) produce two fewer weekly commute trips (trip reduction) and the remaining trips may be shifted outside of the peak (peak spreading).

Alternatives Analysis

A systematic analysis of the engineering and economic feasibility of transportation system alternatives under consideration for a corridor or region, a process required before federal support can be allocated.

Amber Time

The portion of a cycle for which signal indications display amber (aka yellow) alerting drivers that a signal indication is about to change (from green) to red (aka a change interval).

American Community Survey

A new survey effort from the US Census Bureau to replace the decennial census long form, the ACS is sent to a small sample of the population on a rotating basis [ web ].

American Recovery and Reinvestment Act

The February 2009 Act that authorized $787 billion of public funds, including $48 billion for transportation (including $27.5 billion for highways, $8.4 billion for transit, and $8.0 billion for high speed rail) [ web ].

Americans with Disabilities Act

The Americans with Disabilities Act of 1990 defined federal requirements for accommodating individuals with disabilities in public facilities, including public transportation systems (terminals, vehicles, etc.).

AM-peak

The time period corresponding to the highest morning traffic volumes. The AM-peak hour is the highest volume hour within the AM-peak period, which is typically defined as a 2 to 4 hour-long period (see midday, PM-peak, and off-peak periods.

AMPO

Association of Metropolitan Planning Organizations, a non-profit organization that serves as a transportation advocate for MPOs [ web ].

Amtrak

The US domestic intercity rail service operated by the National Railroad Passenger Corporation since 1970.

ANalysis Of VAriance

ANOVA is a standard statistical technique for analyzing the components of variation. Among other applications, it is used with Category Analysis to develop categories for variables in Trip Generation models.

Analytical Hierarchy Process

A decision-making approach featuring an hierarchical structure of choice criteria for the ranking of choice alternatives.

ANN

Artificial Neural Network (see Neural Network).

ANOVA

ANalysis Of VAriance (see ANOVA).

API

An Application Programming Interface is any software that enables a program application to use utilities provided by another application. For example, microsimulation models such as Paramics can be extended with APIs for new ramp metering, actuated signal control, or vehicle routing algorithms.

Application, Model

Forecasting or sensitivity analysis with a model that has been estimated, calibrated, and validated. In travel forecasting, models are applied to predict or to investigate relationships between travel demand, system performance, and associated impacts for one or more defined system alternatives.

Appropriation

A federal allocation of authorized funding that permits federal agencies to obligate funds for specified purposes (see also authorization).

APTA

American Public Transportation Association, an organization that promotes the accessibility and availability of public transportation [ web ].

APTS

Advanced Public Transportation Systems, a component of Intelligent Transportation Systems.

APWA

American Public Works Association, a professional association of public agencies and private companies that promotes public infrastructure and services [ web ].

AQ

Air Quality (see AQ).

AQMD

Air Quality Management District (see AQMD).

ArcGIS

One of the most complete and extensible GIS available today. ArcView and ArcInfo are functional implementations of ArcGIS software. Developed by ESRI [ web ].

Area Sources

Point, line, and area sources are idealized zero-, one-, and two-dimensional locations that are sources for various pollutants. Small stationary and non-transportation pollution sources that are not included as point sources are treated collectively as area sources in estimating emissions.

Area Type

Describes the type and density of development through which a roadway traverses. In travel forecasting, facility type and area type are used to create a look-up table that provides default link speeds and capacities.

Army Corps of Engineers

The US federal agency responsible for the planning, design, construction, and operations of both civilian and military facilities maintains jurisdiction over US domestic waters [ web ].

ARRA

American Recovery and Reinvestment Act of 2009 (see ARRA, TIGER).

Arterial

In the functional classification of roads, arterials are traffic-controlled facilities that place relatively greater emphasis on service to through traffic and less emphasis on serving local land uses than do collectors and local streets. Arterials are often sub-categorized (as major/minor or primary/secondary) although specific operational characteristics vary over regions. Arterials and limited-access facilities account for about 10 percent of the lane-miles in the US, but almost 75 percent of the VMT. In travel forecasting networks, major (or primary) arterials are virtually always coded, as are typically minor (or secondary) arterials. A good rule of thumb in network coding is to include functional classifications one level lower than the level at which policy decisions will be made.

Assignment

Generic term for trip assignment, transit or vehicle.

ATIS

Advanced Traveler Information Systems. Advanced technologies to provide real-time information to travelers. A component of Intelligent Transportation Systems (see ITS).

ATMS

Advanced Transportation Management Systems. Advanced technologies such as decision support systems (DSS), sensors and control devices, vehicle tracking, video and other remote monitoring, etc. to improve the efficiency of transportation operations. A component of Intelligent Transportation Systems (see ITS).

Attainment

Meeting or exceeding NAAQS standards for criteria pollutants, including ozone, carbon monoxide, particulate matter, and nitrogen oxides. A region may be an attainment area for a given pollutant and a non-attainment area for other criteria pollutants [ web ].

Attitude

Attitudes describe an individual's state of mind, developed with experience, that exerts a dynamic influence on individual behavior.

Attitudinal Data

Survey data comprising individual self-assessments of attitudes with respect to cognition (via level of importance) and affect (via comparative ranking such as satisifaction level), including subjective attributes of a choice alternative such as comfort or convenience.

Attraction

The location or zone drawing a generated trip, and also used for the attracted trip itself: a zone is an attraction for N trip attractions. Contrast with production and destination.

Attraction Balancing

Same as Attraction Factoring.

Attraction Factoring

When applying a singly-constrained gravity model for trip distribution, the estimated attractions (column sums) will not match inputs from trip generation. Attraction Factoring is a technique to adjust the SCGM attractiveness term so that estimated attractions in subsequent iterations converge to those from trip generation (see also column & row factoring). Note that this process is often referred to as "attraction balancing", which can be confused with the balancing of attractions to match productions after trip generation.
Wj(n+1) = [ Ajobs / ΣiTijest(n) ] Wj(n) , where Wj(1) = Ajobs

Attrition

The loss of participants during the course of a study, often compromising results or necessitating replenishing the sample.

Authorization

Federal legislation that establishes the operation of a federal program or agency for a particular type of funding obligation (ISTEA, TEA-21, etc.) (see also appropriation).

Automated Highway Systems

An initial goal of intelligent transportation systems (ITS) was dedicated highway lanes providing fully autonomous, computer-controlled vehicle flow utilizing vehicle-roadway and inter-vehicle communications that removes drivers from control decisions once their vehicles join a platoon on the facility. Field demonstrations demonstrated the technical feasibility of AHS to increase capacity by decreasing headways and flow perturbations, but the AHS goal was eliminated in favor of semi-autonomous components and vehicle-infrastructure integration (VII).

Automobile Availability

The degree to which an automobile is available to an individual (compare to auto ownership). In general, vehicle availability.

Automobile Ownership

The number of motorized vehicles owned by or available to an individual or a household. Vehicle availability is highly correlated with trip generation and mode choice decisions, thus, auto ownership models are developed as an important input to travel forecasting.

Auto Ownership Model

A pre-processing step in most travel forecasting models that produces estimates of auto ownership, a critical variable in most trip production models. Category models typically require distributions of households by level of auto ownership (e.g., number of households with 0, 1, 2 or more cars per household).

AutoNET

An interoperable, distributed, transportation management system concept featuring a mobile, ad-hoc, peer-to-peer network integrating vehicles, information technology, and communication systems with transportation infrastructure (developed at ITS, Irvine).

Autoscope

A video traffic detection system that automates the process of vehicle detection and traffic data collection in lieu of loops [ web ]. Similar systems include Vantage [ web ].

Auxiliary Flow

In the relaxation of a non-linear program (e.g., UE), the linearization using the gradient (e.g., AON) is deemed the auxiliary problem and the results are the auxiliary flows.

Auxiliary Lane

A lane added to a freeway section to accommodate section-specific volumes such as between an on-ramp and the next off-ramp, but also associated with lanes designed for section-specific uses such as weaving or hill-climbing.

Auxiliary Problem

In trip assignment, the auxiliary problem is the linear program (implemented as All-or-Nothing Assignment) that is used as a convex combination with the current solution to determine a new solution to the original non-linear UE problem (see Frank-Wolfe).

AVCSS

Automatic Vehicle Control and Safety Systems, a component of Intelligent Transportation Systems (initially known as AVSC).

Average Cost

[1] The expected value of cost (see marginal cost).
[2] In travel forecasting, cost is usually taken as either travel time or generalized cost for network links or O-D pairs. In the trip assignment step, equilibrating average costs of link performance functions yields a user equilibrium result.

AVI

Automated Vehicle Identification.

AVO

Average Vehicle Occupancy is, for a given area and time period, the total number of persons traveling in private vehicles divided by the total number of private vehicles utilized (see also AVR).

AVR

Average Vehicle Ridership is, for a given area and time period, the total number of persons traveling by any mode, divided by the total number of private vehicles utilized (see AVO).

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Backward Bending

A typical performance curve has travel time increasing at an increasing rate as volume approaches capacity. Beyond capacity, congestion effects begin to reduce output volume so the curve bends back toward lower volumes as travel time continues to increase.

Balanced Transportation

A holistic view of transportation systems in which multiple individual modes jointly provide users with travel options, and for which system planning reflects the relative individual and system level advantages of alternative modes.

Balancing

See Trip Balancing.

Bandwidth

For a defined sequence of coordinated traffic signals, the length of green time during which vehicles leaving a signal will be able to reach subsequent signals during their green phases.

Barnes Dance

A traffic signal phase where red is displayed to all vehicle movements, allowing pedestrians to cross in all directions, including diagonally, simultaneously. Also known as a Scramble Phase. Can lead to greater delay for vehicular and pedestrian traffic by eliminating overlap phasing but may be appropriate when there are large pedestrian volumes.

Barrel

A unit of volume used in oil production and transportation equal to 42 US gallons (or 159 liters).

Baseline

A reference point in travel forecasting, representing the current state of the transportation and activity systems plus projects that have already been programmed for implementation in the forecast period, on which comparisons with future alternatives are based.

Basic Employment

Employment directed toward addressing non-local (export) demands, such as manufacturing, mining. and agricultural employment (see service employment).

Beckmann Transformation

The objective function of the user equilibrium optimization program, equal to the sum of the integrals of the link performance functions, proposed by Beckmann, McGuire, and Winsten (1956).

Before-After Study

The measurement of system performance outputs prior to an infrastructure or service improvement followed by remeasurement after implementation, with any performance changed attributed to the improvement. For certain projects, such as those that entail longer implementation periods, control samples may be required to ensure that factors other than the improvement are not responsible for the performance change.

Behavior

The observable outcome of an individual engaged in active decision-making as a function of intentions, attitudes, and perceptions. Short-term behavior includes activity and travel choices; long-term behaviors include location, life style, and role choices.

Behavioral Model

A model that reflects the revealed outcome of individual decision processes as a function of values, attitudes, and preferences.

Benefit

A result of an action expressed in terms of the utility gained from the action. In transportation, benefits are often expressed as cost savings.

Benefit-Cost Analysis

An evaluation technique that compares the societal benefits and costs, measured in monetary terms, of proposed projects or policies. Alternative actions are incrementally compared to find the greatest net benefits.

Benefit-Cost Ratio

The ratio of a project's total benefits to total costs, measured in monetary net present value. In benefit-cost analysis, alternative projects are incrementally compared to find the greatest net benefits.

Best Practices

Projects or processes that exemplify the state-of-the-practice in transportation planning, design, management, and operations, and that serve as models of effectiveness for adoption consideration in related applications.

Biased

An estimator that does not tend toward the population parameter as sample size increases.

Bid Rent

Bid-rent theory, which underlies some land use models, proposes that land uses organize geographically based on their capacity to compete for land rents, a capacity controlled by economic aspects of the land use demanded and accessibility to markets. Consumers bid for land at the location that maximizes their utility at current market rents (the bid rent).

Big Data

A generic but appropriate term for extremely large data sets associated with innovative data management and analysis techniques used to reveal underlying patterns and relationships. In human travel and activity behavior research and analysis, Big Data can capitalize on independent but spatially and temporally consistent data to identify statistical links, for example, between anonymous, discrete cell phone use and locational-referenced socio-demographic data, to estimate OD flows, travel times, and other system attributes.

Big Dig, The

The moniker assigned to Boston's Central Artery Project that re-routed I-93 under central Boston to address congestion but consumed 15 years and $15 billion between 1991 and 2007, and incurred massive cost overruns and construction problems.

Big O Notation

A measure of efficiency for an algorithm's computational complexity, providing a worst-case "Order of" magnitude for the number of elementary computations.
In general, O(log(n)) < O(n) < O(nlog(n)) < O(n2) < O(n3) < O(2n)

Bi-level Program

An optimization problem decomposed into a primary optimization problem with one set of variables and constraints and a second problem, embedded in the first, with a second set of variables and constraints. Applications include dynamic traffic assignment.

Bisection Search

A numerical root-finding algorithm that proceeds by halving the search interval and evaluating the derivatives of the function at subinterval endpoints. It is utilized in the Frank-Wolfe algorithm to find the minimum of the Beckmann objective function along the line between the current (convex combination) and auxiliary (all-or-nothing) solutions.

Bliss

In 1899, Henry Bliss became the first pedestrian fatality from an automobile accident.

Block

A standard spatial aggregation unit used by the U.S. Census, typically corresponding to actual city blocks. Block data is aggregated into the more commonly used census tracts.

Blueprint

A consensus-building program initiated in California at the regional level, incorporating all stakeholders, and integrating land use, transportation, and environmental planning in a visioning process involving scenario building with measurable performance outcomes.

Bluetooth

A short-range radio technology to standardize communications and data synchronization between microprocessor applications, such as cell phones, PDAs, and laptops, operating in the unlicensed 2.4 GHz frequency band.

Boondoggle

A colloquialism for a typically public-funded project characterized by continued or increasing expenditures while expected objectives are decreasingly likely to be achieved, due to understated costs or overstated performance projections (see success story).

Bottleneck

A section of a transportation facility where capacity is significantly lower than on the upstream section, restricting traffic volumes and creating queues (AKA chokepoint).

Bounded Rationality

The theory due to Herbert Simon that rational (i.e., utility maximizing) behavior is limited (bounded) because decision-makers consider only a restricted choice set and a finite decision time and thus may satisfy rather than optimize their choices. Compare with utility maximizing and satisficing approaches.

BPR

The former US Bureau of Public Roads, now FHWA.

BPR Function

An empirically estimated and commonly utilized link performance function developed by BPR for calculating travel time as a function of link volume [ t = t0(1+α(vol/cap)β) ].

Braess' Paradox

A counter-intuitive, theoretical state where, under user equilibrium assignment, a network link addition can produce an increase in the total cost of travel.

Brownfields

Abandoned industrial or commercial properties within urban areas where redevelopment is complicated by environmental contamination.

BRT

Bus Rapid Transit (see BRT).

BTS

Bureau of Transportation Statistics (see BTS).

Build Out

The final completion of all elements of a planned project or system.

Bureau of Transportation Statistics

A department within USDOT responsible for the collection, analysis, and reporting of transportation data [ web ].

Burrell's Algorithm

A Monte Carlo simulation-based trip assignment algorithm that samples perceived link costs for different users from a uniform distribution (Burrell, 1968).

Bus Rapid Transit

A generic term for the operation of buses on dedicated roadways or in restricted access lanes, with limited stops and often with signal priority or preemption, to emulate rail transit performance characteristics while maintaining the flexibility and cost of bus transit.

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CAA

The Clean Air Act of 1970 established national air quality standards, defined criteria pollutants, and required regions that do not conform to these standards to prepare an SIP. The Clean Air Act Ammendments (1977 and 1990) established criteria for attaining federal standards.

CAAA

The Clean Air Act of 1970 established national air quality standards, defined criteria pollutants, and required regions that do not conform to these standards to prepare an SIP. The Clean Air Act Ammendments (1977 and 1990) established criteria for attaining federal standards.

CAC

Citizen Advisory Committee (see CAC).

CAD

Computor Aided Design is the application of computer-based tools to facilitate design of systems and components (also see CAE).

CAE

Computor Aided Engineering is the application of information technology to the planning, analysis, and design of engineering systems (also see CAD).

CAFE

Corporate Average Fuel Economy Standards are established by Congress and require vehicle manufacturers to produce new vehicle fleets that meet a composite fuel economy standard for a given year.

Cal-Fred

The On-line California Freight Data Repository compiling a number of freight-related data sources and documentation maintained by UC Irvine [ web ].

Calibration

Calibration, an exercise in goodness-of-fit, is the fitting of a model to observed data, primarily by adjusting model parameters. Calibration might: (a) be performed following model estimation, (b) involve a pre-specified model that uses transferred structure and parameters (e.g., QRS), or (c) be linked with the process of model validation. In travel forecasting, models are typically calibrated every 10 years, corresponding to major regional data collection efforts. Models are validated prior to any forecasting application.
Note: In travel forecasting, the terms estimation and calibration often have been used interchangeably. Determining the structure of the model is explicitly part of the estimation process thus lending it greater statistical formality, but both terms reflect the same overall objective. If the structure of the model is pre-determined, then perhaps calibration is the better term. If model specification is involved, then perhaps estimation is the better term.

California Statewide Freight Forecasting Model

A statewide freight forecasting model for the state of California developed by UC Irvine (2011-2013) for Caltrans using state and federal data (FAF) and linked to the California Statewide Travel Demand Model.

California Statewide Travel Demand Model

A statewide travel forecasting model for the state of California developed by UC Davis (2010-2012) for Caltrans and linked to the California Statewide Freight Forecasting Model.

California Transportation Commission

A state commission established in 1977 to advise and assist the California legislature and Caltrans in implementing transportation policy, establishing priorities, and allocating funds for transportation investments, via the State Transportation Improvement Program (STIP).

California Transportation Plan

The California Transportation Plan (CTP) is the state's long-range transportation plan (LRTP) that forms the blueprint to guide future transportation decisions and investments to meet state goals and objectives [ web ].

California Urban Futures Model

A GIS- and rule-based model of housing development developed by Landis (1994), but not a general purpose land use model.

CALINE3

An air quality dispersion model designed to determine air pollution concentrations near transportation facilities (incorporated into the more refined CAL3QHC model). Developed by Caltrans [ web ].

Caltrans

The California Department of Transportation, the state agency responsible for design, construction, maintenance, and operations of designated California's state and interstate highways, provides inter-city rail services, and works with local agencies in all areas of transportation [ web ].

CAL3QHC

A CALINE3 based CO model with a traffic model to estimate queues and delays at signalized intersections.

Capacity

The maximum sustainable flow (typically measured in vehicles per hour) past a defined point (or over a uniform roadway segment) during a defined time period, under prevailing traffic and roadway conditions (see also LOS).

Capacity Analysis

Performance evaluation of roadway sections and intersections to estimate the portion of capacity that is utilized and the associated level-of-service, under current or proposed scenarios. Performance, a function of demand and supply (incorporating characteristics such as facility type, geometric design, and traffic control), is defined by level-of-service, cost, and capacity.

Capacity Restrained

The general term "capacity restrained" (or capacity constrained) reflects the increase in congestion as assigned link volume approaches link capacity. This is reflected in most link performance functions (LPF). The specific term "Capacity Restrained Assignment" (CRA) reflects procedures that attempt to correct the inadequacy of All-or-Nothing assignment to reflect congestion resulting from link capacity constraints. CRA algorithms recompute link travel times using assigned volumes as input to link performance functions. These times are used to recomputed minimum paths for additional iterations of AoN assignment. Heuristics such as travel time smoothing or link volume averaging were used in attempts to achieve convergence to equilibrium conditions (see also UE).

Capital Costs

The costs of infrastructure and long-term assets such as facilities, rights-of-way, and vehicles. Compare with operating costs.

Capital Budgeting

The process of project appraisal to determine the economic value of proposed project alternatives with assessment techniques such as net present value, equivalent annual cost, or internal rate-of-return.

Car Following Model

A microscopic theory of traffic flow based on the behavior of a vehicle following a lead vehicle. The General Motors car following models relate the acceleration at time (t+Δt) of a vehicle following a leader as a function of spacing and speed relative to the lead vehicle at time t, the following vehicle's speed at time (t+Δt), and a sensitivity parameter.

Car Sharing

An alternative to conventional individual ownership and use of motorized vehicles where a private company, public agency, or cooperative manages a fleet of vehicles that can be used by any registered members for any amount of time. Usage entails a computer- or phone-based reservation system and a keyless, smart card access to a reserved vehicle located in a fixed parking space. See station car.

CARB

California Air Resource Board is a part of the California Environmental Protection Agency and is responsible for statewide air quality [ web ].

Carbon Dioxide (CO2)

A colorless, odorless, non-toxic gas resulting from the combustion of carbon-based fuels. A normal component of air, CO2 is also a greenhouse gas and a suspected causal agent in global warming.

Carbon Monoxide (CO)

A colorless, odorless, toxic gas produced by the incomplete combustion of fuels. A criteria pollutant, over 80 percent of CO is produced by gasoline-fueled vehicles.

Carpool Lane

A roadway lane dedicated for use by high-occupancy vehicles, typically as a physically-separated lane on a freeway or other limited-access facility.

CART

Classification And Regression Trees are nonparametric, hierarchical modeling techniques to determine if-then conditions for the accurate classification of continuous or categorical dependent variables with continuous or categorical explanatory variables.

CARTESIUS

A multi-agent, real-time decision support expert system for congestion management that integrates real-time traffic control and simulation [ web ].

Catastrophe Theory

A mathematical theory that seeks to describe phenomena characterized by abrupt changes that follow from continuous processes. The limited number of ways that such changes can take place are defined as the elementary catastrophes.

Categorical Exclusion

Applies to a transportation project that has minimal environmental impact and thus does not require an EIS.

Category Analysis

A general statistical technique for exploring relationships among variables. Discrete categories are specified for one or more explanatory variables and then the mean of a dependent variable is computed over the observations in each defined cell. Used extensively in trip generation for household trip production models, this method is also known as Cross Classification Analysis (compare with regression).

CATI

Computer-Assisted Telephone Interview techniques integrate computer-based surveys with human interviewers to facilitate survey execution, coding, and error checking. Integration of GIS, GPS, and computer-based self-administered surveys (such as CHASE and REACT!) are evolving to minimize interviewer requirements.

CATS

The Chicago Area Transportation Study, one of the first comprehensive transportation studies, essentially introduced the basic four step modeling paradigm in 1959 (CATS, the MPO for the Chicago region, merged into the Chicago Metropolitan Agency for Planning in 2005).

Causal Model

A causal model provides a cause and effect versus a correlative relationship. A causal variable is an explanatory variable whose presence and value results in a corresponding presence and a value for a dependent variable.

CBD

Central Business District: The traditional downtown retail, commercial, service, and institutional employment center of a metropolitan area.

CCIT

California Center for Innovative Transportation, the University of California's Caltrans-sponsored center established to accelerate the implementation of research results and technologies [web ].

CCTV

Closed Circuit TeleVision utilizes cameras and fiber to transmit realtime video to traffic management centers for traffic surveillance.

Cellular Automata

An n-dimensional array of identical finite state cells whose state evolution is rule-based, defined by the current state of the cell and of its neighbors. CA form the basis of a class of simulation models from which complex macroscopic behavior can emerge using simple microscopic rules (examples include pedestrian and vehicle traffic behavior).

Census

The process of collecting, compiling, and disseminating demographic, economic, and social data describing a defined population at a specific time.

Census, US

[1] The decennial enumeration of the U.S. population including the Journey to Work (JTW), the American Community Survey (ACS), and the Public Use MicroSample (PUMS);
[2] The U.S. Census Bureau [ web ];
[3] The Census Transportation Planning Package (CTPP).

Census Block

A division of a census tract representing the smallest geographic unit for which the Census tabulates population-level data. The size distribution ranges from individual city blocks in dense urban areas to very large areas in rural or other low density areas.

Census Geography

The US Census Bureau and the Office of Management and Budget define Metropolitan Statistical Areas as core areas of substantial centralized population and the adjacent economically and socially integrated areas. Terminology has evolved from Standard Metropolitan Area (SMA), to Standard Metropolitan Statistical Area (SMSA), to Metropolitan Statistical Area (MSA), to the current (2000) Metropolitan Area (MA, which are either spatially distinct MSAs or Consolidated MSAs (CMSA) that comprise two or more primary MSAs (PMSA)) [ web ].

Census Tract

The US census aggregates household demographic and socio-economic data by spatially defined units called census tracts. Similar in design and spatial scale as TAZs, census tracts are mapable to TAZs but are not identical.

Center City

The largest city in a metropolitan area (see metropolitan classification).

Centerline-miles

One centerline-mile corresponds to a mile-long segment of a roadway, independent of the number of lanes in either direction. One mile of 8-lane freeway and one mile of 2-lane local street both are one centerline-mile of roadway. Total centerline-miles is a measure of the spatial density of roadway infrastructure. Compare to lane-miles.

Centers

Historically, metropolitan areas were defined by a single center, the Central Business District (CBD) or traditional downtown. With suburbanization, non-CBD activity centers developed. These centers were compact areas characterized by higher density, often mixed use development. While traditional centers were typically served by multi-modal transportation systems, new centers were more likely to be automobile dominated.

Central Limit Theorem

If a random sample of size N for variable X is drawn from a population with mean μ and finite variance σ2, then the sampling distribution of the mean of X will have mean μ and variance σ2/N and will tend toward normal as N becomes large.

Central Place Theory

A idealized location theory developed by Christaller (1933) that introduces a hierarchy of goods and services based on accessibility to market populations, resulting in a hexagonal distribution of cities, towns, and their hinterlands. CPT is limited by simplifying assumptions that are comparable to von Thünen's.

Centroid

A defined point within a TAZ from which all trips are assumed to start or end. It should be located to reflect the center of activity in a TAZ and not necessarily the geographic center. Centroids are connected to the network via centroid connectors, abstract links that represent general access onto the formal network. Some travel models load trips directly onto network links (such as microsimulation models).

Centroid Connector

Abstract links that connect centroids to network nodes and represent general access from a TAZ to the formal transportation network.

CEQA

The California Environmental Quality Act (1970) that requires state and local agencies to identify and mitigate significant environmental impacts, a process summarized in an EIR.

Chain

A trip chain: a sequence of tripstrips and activities, typically starting and ending at home (AKA tour). Trip chaining is the process of linking travel between non-home activities to reduce overall travel disutility.

Changeable Message Sign

An electronic sign used on roadways to provide travelers with information on traffic conditions, special events, etc. (also known as Variable Message Sign).

Chaos

A property of some dynamic systems that are highly sensitive to initial conditions and produce seemingly random effects despite being deterministic. See logistic model [2].

Charrette

A collaborative work session in which stakeholders and planning and design professionals draft potential solutions to a design problem, increasingly used in land use and transportation planning.

CHASE

Computerized Household Activity SchEduling survey, the first essentially self-administered, PC-based survey instrument to collect, process, and archive household travel/activity diaries, developed by Doherty (1998) based on HATS (see also REACT!).

Chicane

A series of curb extensions on alternating sides of a road to narrow the perceived lane width and calm traffic. Chokers are curb extensions directly opposite each other, often at intersections (see traffic calming).

Chlorofluorocarbon (CFC)

CFCs are a family of gases containing chlorine, fluorine, and carbon, that were used as coolants and contributed to ozone depletion.

Choker

Curb extensions directly opposite each other, often at intersections to narrow the perceived lane width and calm traffic. A chicane is a series of curb extensions on alternating sides of a road.

Choice-based Sample

Choice-based sampling sets sampling rates differentially for choice alternatives, which allows choices such as transit, which are selected less frequently, to be sampled at a higher rate to provide sufficient choice observations. Model estimation is made consistent by applying appropraite weights reflecting population shares.

Choice Set

The set of choice alternatives available to a decision maker. For example, the choice set of modes available to a particular decision maker might be car, bus, and walk.

Chokepoint

A section of a transportation facility where capacity is significantly lower than on the upstream section, restricting traffic volumes and creating queues (AKA bottleneck).

Circulation Element

An element of a general plan that includes the general location and characteristics of existing and proposed transportation faclities.

Citizen Advisory Committee

Public groups organized and utilized by MPOs and transportation authorities for continuous citizen input to the transportation planning process.

City Logistics

The process of optimizing metropolitan logistic activities, in consideration of social, environmental, economic, financial, and energy impacts of urban goods movement.

Civil Engineering

ASCE defines civil engineering as "the design and maintenance of public works such as roads, bridges, water and energy systems as well as public facilities like ports, railways and airports."

Civil Rights Act of 1964

In transportation, Title VI of this federal legislation ensures environmental justice in the location of transportation infrastructure and for improvements in system performance.

Clark Approximation

A means of estimating multinomial probit (MNP) models that utilizes an approximation, applied recursively, to obtain the approximate distribution of the maximum of a number of normal variables, allowing for the estimation of MNP choice probabilities. The method is efficient but is often limited in practical applications.

Clean Cities Program

A government-industry partnership, sponsored by the US Department of Energy's Vehicle Technologies Program, to reduce oil consumption in the transportation sector by advancing the use of alternative fuels and vehicles technologies [ web ].

Climate Change

A change in global or regional long-term climate patterns, due to increased atmospheric carbon dioxide and other greenhouse gases, with the combustion of fossil fuels believed to be a major causal factor [ see global warming ].

Closed Loop System

A control system that feedbacks measured output to adjust the control strategy. In an actuated control system, a control strategy is implemented via field controllers while system detectors feedback performance data to the control system. The advantage of closed loop control is that disturbances (e.g., flow variations) can be accommodated by adjusting the control strategy (compare with Open Loop System).

Cloud Computing

In the beginning there were a small number of massive but isolated mainframe computers, where all hardware, software, and data (both system and user files) were stored and accessible through remote terminals. Then came the Internet, which provided high speed connections between mainframes. Simulaneously, the introduction of Personal Computers (PCs) placed all software and data on discrete hardware platforms, with these millions of hardware/software devices soon also interconnected via the Internet. The cloud reassumes control of software and data on a vaporous, distributed hardware "cloud" which includes both system and user files, accessible by numerous individual devices such as tablets, game consoles, and smart phones. Individuals will no longer own software (nor necessarily own their user files) and will pay on essentially a usage-based contract similar to smart phones, music streaming, and, as in the beginning, vintage time-sharing mainframe computers.

CMA

[1] Critical Movement Analysis, a simplified approach to assessing intersection capacity utilization based on traffic movement volumes but independent of signal timing. See ICU.
[2] Congestion Management Agency, in metropolitan counties in California, an agency responsible for the development and implementation of a Congestion Management Program ( CMP ) required under Prop 111 since 1990.

CMAQ

Congestion Mitigation and Air Quality Program. A Federal program for projects to reduce congestion and improve air quality in regions that have not yet attained federal air quality standards.

CMEM

Comprehensive Modal Emission Model is a microscopic emissions model that estimates emissions for light-duty vehicles as a function of the vehicle's operating mode and is applicable to project-level or corridor-specific transportation control measures. Developed by UC Riverside for NCHRP [ web ].

CMIA

Corridor Mobility Improvement Account (see CMIA).

CMP

Congestion Management Plan (see CMP).

CMS

Changeable Message Sign (see CMS).

Cobb-Douglas

A production function relating level of production Q to the input levels of the factors of production (capital K and labor L). A sample Cobb-Douglas function is given by:
Q = c Ka Lb where a, b, and c estimated coefficients.

Coding

[1] See network coding;
[2] See survey coding.

Coefficient

A estimated parameter that modifies a variable in a model.

COG

Council of Governments (see COG).

Cognitive Mapping

Information processing of spatial data, where an individual perceives, contextualizes, and interprets relative locations and attributes of the spatial environment.

Cohort

Generically, a population subgroup homogeneous over specified characteristics. In demographic models, a cohort is usually defined by age and gender. In cohort component models, cohort-specific birth, death, and migration are used to forecast population.

Cohort Component

A population projection technique where population is split by gender and age cohorts and differential birth, death, and migrations rates are applied. Also called Cohort Survivor.

Cohort Survival

See cohort component models.

Cold Start

[1] A cold start occurs when a vehicle's engine is started more than an hour after the previous period of operation (see hot start). Cold start emission rates apply for the first 505 seconds after a cold start.
[2] The initial iteration of an algorithm such as trip assignment is considered a "cold start" (see warm start).

Collectors

In the functional classification of roads, collectors serve a role mid-way between arterials and local streets, placing similar weights on service to through traffic and service to local land uses. As such, collectors serve to channel arterial traffic to and from local streets. Collectors account for about a quarter of the lane-miles in the US, but less than 20 percent of the VMT. In travel forecasting networks, the inclusion of collectors varies based on the scale and scope of the model and the policies to be tested.

Column & Row Factoring

When applying a singly-constrained gravity model for trip distribution, the estimated attractions (column sums) will not match those from trip generation. Column and Row Factoring uses iterative proportional fitting to factor the estimated trip matrix so that the resulting productions and attractions match corresponding values from trip generation (see also attraction factoring).

Column Generation

[1] In large-scale network programming problems, most variables are non-basic and equal to zero, yielding very sparse matrices. Column generation is a bi-level solution approach of adding to the master problem only those variables that are identified in the sub-problem as improving the overall solution.
[2] The automatic process of identifying paths within an algorithm is referred to as column generation because each new path generated adds a column to the link-path incidence matrix (see also path enumeration).

Commons, The Tragedy of the

A commons is a shared resource (e.g., public land, fresh air). Maximization of individual utility can lead to overuse and destruction (a tragedy) of the commons, particularly when commons usage is not priced.

Commute

The repetitive trip between a traveler's home and work location, typically by the same mode and at the same time of day, and usually on the same route.

COMMUTER

The EPA's spreadsheet-based assessment model that estimates changes in travel and emissions associated with transportation control measures for commuting. The travel impacts component uses pivot point analysis; the emissions component is based on the MOBILE model [ web ].

Compensatory Model

A model structure where one model variable can compensate for a change in value of another variable. In choice situations, an individual can tradeoff a variable (such as travel time) for another variable (such as cost), in selecting an alternative from a choice set.

Complete Streets

Policies and plans defining roadways to safely accommodate all users, including motorized and non-motorized vehicles as well as pedestrians [ web ].

Composite Alternative

In hierarchical modeling, a composite alternative represents the specific alternatives for a defined level of choice. For example, several specific public transit mode choices may be represented by a composite "public transit" alternative, measured by the corresponding inclusive value.

Composite Cost

In hierarchical modeling, composite cost is a weighted combination of the costs of choice alternatives for a defined level, the correct specification being a logsum formulation. In conventional travel forecasting, feedback to trip distribution might utilize composite costs that reflect all relevant modes of travel.

Comprehensive Plan

The long-range plan for a community's future development, a comprehensive plan (also known as a general plan or a master plan) defines goals and objectives, policies and standards, and constraints for the growth and development of the community. The plan provides a land use and zoning plan indicating planned land uses (e.g., residential, commercial, institutional) for districts and parcels, and addresses all planning elements including transportation infrastructure and services, the natural and built environment, and demographic trends.

Computational Complexity

A measure of algorithm scalability defined by problem class, providing an upper-bound impact on memory and run-time as problem size increases. O(•), or "Order of" notation, describes the upper bound for the magnitude of a problem's computational complexity.

Computational Complexity Class

In computational complexity theory:
[1] P, or Polynomial class, are decision problems that can be solved in polynomial time on a deterministic Turing Machine (each state has a single rule to follow)
[2] NP, or Non-deterministic Polynomial, are decision problems that can be solved in polynomial time on a non-deterministic Turing Machine (each state has multiple rules)

Concentration

A term used synonymously with density in traffic flow as a measurement of vehicles per mile.

Concentric Ring Model

A descriptive model of urban development developed by Burgess (1924) depicting urban form as a series of concentric rings extending from a central business district and defined by different land uses and socio-economic characteristics. Compare with the Sector Model and the Multiple Nuclei Model.

Concentric Zone Model

See Concentric Ring Model.

Confidence Interval

A statistical measure to quantify uncertainty by bracketing a sample estimate by a range of values within which the estimate is expected to occur for a specified level of significance. Such assessments typically are not incorporated into any estimates made in transportation planning and modeling.

Conformity

A Clean Air Act requirement that regional transportation plans must be in agreement with commitments designed to meet federal air quality standards to receive federal approval and funding. Planned transportation projects must not worsen an area's air quality goals as established in the State Implementation Plan (SIP).

Congestion

Interference between vehicles as flow densities increase, causing reduced speed and increased travel time. At low volumes, limited interaction allows vehicles to proceed uninterrupted and flow is uncongested. As volume approaches capacity, vehicle interaction increases and queues begin to form (see also recurrent and non-recurrent congestion).

Congestion Cost

The economic value of travel time delay based on value of time and excess fuel consumption, usually averaged and presented for major metropolitan areas (see TTI's Urban Mobility Report).

Congestion Management Plan

Required by California's Proposition 111 (1990) in metropolitan counties to link land use, transportation, and air quality for growth management that effectively utilizes transportation funds, alleviates traffic congestion, and improve air quality and other congestion impacts (see CMA [2]).

Congestion Pricing

Transportation facility use fees that vary with the level of congestion or, by proxy, by time of day, and designed to optimize the performance efficiency of the facility (also known as the value pricing).

Conical LPF

A class of link performance functions (LPFs) developed by Speiss that are similar to, but address some theoretical limitations of, the BPR LPF. Conical LPFs can be geometrically interpreted as hyperbolic conical sections.

Connected Transportation

The evolution of transportation systems from infrastructure connectivity toward the integrated connectivity of travelers, vehicles, and infrastructure, communicating via integrated active and passive data streams. See Vehicle Infrastructure Integration ( VII ) and Intelligent Infrastructure.

Connectivity

A general network term for measures that define the degree that nodes are connected via paths. Perhaps most applicable to transit systems where paths can be defined as transit lines and connectivity can be defined in terms of the number of transfers required to reach a particular destination (not reachable or reachable with 0, 1, 2, etc. transfers), computable by powering an adjacency matrix of transit lines.

Conservation of Flow

A network property equivalent to Kirchhoff's 1st Law constraining the flow into a node to be equal to the flow out, corresponding to standard nodes (such as intersections).

Consistent Estimator

An estimator is consistent if it converges to the population parameter as the sample size increases.

Constraint

A restriction limiting a system's behavior and thus imposed on appropriate variables in the associated model.

Constraint, Authority

Hagerstrand defined authority constraints as limitations on travel and activity performance imposed by institutional factors, such as lack of a driver's license due to age.

Constraint, Capability

Hagerstrand defined capability constraints as limitations on travel and activity performance imposed by physical factors, such as inability to be at two places at the same time.

Constraint, Coupling

Hagerstrand defined coupling constraints as limitations on travel and activity performance imposed by requirements to travel or perform activities with another person or the requirement for a drop-off activity to preceed the subsequent pick-up activity.

Constraints, Hagerstrand

Hagerstrand's model of time geography included authority, capability, and coupling constraints.

Consumer Price Index

A statistical index that measures the change in the cost of living, reflecting prices of consumer products, transportation, housing, and other goods and services. It is a an indicator of the general level of prices, and thus of inflation, and is used to adjust income and costs in forecasting model applications.

Consumer Surplus

The difference between what decision-makers would be willing to pay and what they actually pay at a given equilibrium volume. Geometrically, it is the area under the demand curve and above the equilibrium cost.

Container

The standard shipping unit that facilitates freight tranportation over multiple modes and through multiple transfer facilities (e.g., between containerships, railcars, and trucks) throughout the supply chain.

Context-sensitive Design

A collaborative, interdisciplinary planning and design approach in which stakeholders are integral parts of the design process and objectives such as safety, mobility, environmental sustainability, and preservation of community values are simultaneously addressed.

Contingency Table

A discrete model of the relationship between two or more categorical variables, tabulating the joint distribution and with row and column sums providing the marginal distributions. Model statostical significance can be assessed with a chi-squared test. Non-categorical variables can be used by defining categories a priori. Category Analysis is similar to a contingency table except, in place of a joint frequency in each cell, the corresponding average value of a third variable is presented (e.g., average trip rates corresponding to the value of the explanatory variables).

CONTRAM

Dynamic traffic modelling software for modeling networks with time varying traffic flows. Developed by TRL [ web ].

Control Sample

In a before-after study, a sample with characteristics similar to the implementation site but which do not include the infrastructure or service improvements, but would be subject to similar external factors in these assessment of ex post performance.

Control System

The operations management components of a transportation system, including traffic signals, signs, controllers, detectors, and other hardware/software.

Convergence

In iterative models, achieving a defined tolerance of approaching a known value or reducing a defined gap.

Convex Combination

In iterative trip assignment, a weighted combination of the current solution Va(n-1) and a linear approximation Fa(n), or: Va(n)=(1-αn)Va(n-1) + αn Fa(n) (see Frank-Wolfe or MSA[2]).

Coordination

In traffic engineering, the linking of timing patterns at a sequence of intersections to facilitate the progression of through traffic.

Cordon Line

An abstract line defining the external boundary of a study area. Internal cordon lines are sometimes used as screenlines to compare estimated and observed flows into and out of a sub-area (such as a CBD).

Correlation

A standardized measure of the strength of a linear relationship between two variables
(-1 ≤ correlation ≤ +1). Correlation does not imply any causal effect.

Corridor

A linear study area along one or more transportation facilities for which estimates of travel demand and system performance are desired.

Corridor Mobility Improvement Account

Created by California's Proposition 1B in 2006, the CMIA provides funds for the California Transportation Commission to allocate for performance improvements to the state highway system to relieve congestion, enhance mobility, and improve safety.

Corridor System Management Plan

A corridor planning and management process required by California's Proposition 1B for the management of all modal infrastructure and services in a corridor to optimize the efficiency and effectiveness of transporting people and goods. CSMPs integrate transportation systems planning with systems management [ web ].

CORSIM

A comprehensive traffic microsimulation software package, part of TSIS (Traffic Software Integrated System). Developed by FHWA [ web ]. Other traffic microsimulation packages include AIMSUN, Cube Dynasim, Paramics, SimTraffic, TransModeler, and VISSIM.

Cost

A price paid or otherwise associated with a transaction. Costs represent trade-offs between alternate uses of resources, and can be measured by money and time expended, or opportunities lost, to obtain a benefit. Transportation costs directly incurred include travel time costs; out-of-pocket costs (fares, tolls, and parking charges); and vehicle expenses (capital and operating costs). Transportation costs indirectly incurred include infrastructure capital, maintenance, and operations costs; accident costs; and environmental costs. Transportation benefits are equivalent to a reduction in costs (such as reduced travel time).

Cost-Benefit Analysis

See Benefit-Cost Analysis.

Cost Effectiveness

A measure of input per unit of output, such as input cost per unit of an output performance measure (e.g., cost per passenger-mile traveled). Often used in comparing alternatives when output performance measures cannot be expressed in monetary terms, selecting the alternative that has the greatest effectiveness for a given input cost.

Council of Governments

A regional agency serving multiple jurisdictions (including counties and cities) to address specified regional issues such as transportation planning.

Count

The number of activations of an inductive loop detector in a specified counting interval (usually 30 seconds) that is converted to a volume estimate (compare to presence and occupancy).

Counting Interval

A defined period of time during which measurements are made (e.g., 30 seconds for an inductive loop detector or 24 hours for a trip survey).

CPI

See Consumer Price Index.

Criteria

A quantitative standard by which achievement of objectives can be measured.

Criteria Pollutant

The 1970 Clean Air Act identified six criteria pollutants, including ozone (O3), carbon monoxide (CO), nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter (PM10), and lead. The EPA establishes the associated standards (see NAAQS).

Cross Classification

A tabular classification of two or more categorized variables. See Category Analysis.

Cross Elasticity

An elasticity defined as the change in market share of an alternative due to a change in an attribute of a competing alternative (e.g., the change in transit demand due to auto fuel price increase). Also called an indirect elasticity. Compare with direct elasticity.

Cross Sectional

A survey of and data from a sample of observations collected at a single point in time. Compare with longitudinal surveys and data.

Cross-sectional Design

The cross-sectional design of full roadway right-of-way, including the roadway itself, median and shoulders, sidewalks and bike lanes, drainage and side slopes, and other components, and reflecting topography, safety, facility type, speed, and other design concerns (see geometric design as well as vertical and horizontal alignment).

CSD

Context Sensitive Design (see CSD). CSS is a Context Sensitive Solution.

CSFFM

California Statewide Freight Forecasting Model (see CSFFM ).

CSMP

A Corridor System Management Plan (see CSMP).

CSTDM

California Statewide Travel Demand Model (see CSTDM ).

CTC

See California Transportation Commission (CTC).

CTNet

Caltrans' Traffic Signal Management and Surveillance System developed for statewide deployment to monitor and control traffic signals.

CTP

See California Transportation Plan (CTP).

CTPP

The Census Transportation Planning Package is created from portions of the decennial US census to provide information on basic travel characteristics and associated socio-economic characteristics (for the ten percent sample) [ web ]

Cube

A software packages for travel forecasting, incorporating an integrated GIS. Developed by Citilabs [ web ]. Other travel forecasting packages include EMME/2, QRSII, MinUTP, T-Model, Tranplan, and TransCAD.

CUF

The California Urban Futures Model (see CUF).

Cutline

A control line for which estimated and observed performance measures are aggregated and compared for model validation or for performance assessment, a cutline is similar to a screenline but only partially splits a defined sub-area (or corridor), thus capturing local flows that cross through, but not flows bypassing, the sub-area. With definition of spatial scale somewhat arbitrary, these terms are often used interchangeably (see also cordon line).

CVO

Commercial Vehicle Operations, an original component of Intelligent Transportation Systems.

Cycle

A complete sequence of signal control indications.

Cycle Length

The total time for a traffic signal to complete a full sequence of signal indications (i.e., a cycle) and return to the initial indication.

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Data Mining

Previously a somewaht pejorative term for brute force analysis leading the generation of research hypotheses, data mining has become a generic term for a range of techniques used to seek new relationships, patterns, and hypotheses within large databases.

Davidson LPF

Davidson's Link Performance Function is based on queueing theory and assumes that arrivals (demand) can not exceed the service rate (capacity). Although theoretically sound, it is not amenable to standard iterative assignment algorithms that are designed to not restrict link flows to link capacity (thus, the BPR LPF is often used). Akcelik provides a modified version that resolves this issue.

Deadheading

Segment of a transit route driven by a transit vehicle not in revenue service. Similarly, an empty backhaul in freight transportation (usually trucking).

Dead Reckoning

A method of estimating position based on starting position, speed, and elapsed time.

Decide-Announce-Defend

The historical approach approach to transportation decision-making in which planners made decisions, then called public meetings to announce and defend those decisions to the public, without allowing time for integrated public involvement in the transportation planning process.

Decision Support System

Information technology and software that utilize decision models, expert knowledge, rule-based systems, and other methodologies for unstructured problem solving and decision-making in an interactive environment.

Decision Tree

An hierarchical mapping of sequential decision-making where a choice of any branch of the tree leads to further decision points (and usually eliminates choice on prior branches).

Dedicated
Short Range Communications

A short- to medium range wireless communications protocol designed for ITS applications such as vehicle to infrastructure integration (VII), including V2V (vehicle-to-vehicle) and V2R (vehicle-to-roadside) technologies, operating in the 5.9 GHz frequency band (also used by Wi-Fi).

Degrees of Freedom

Each of N sampled data points is free to take on any value. When a parameter is estimated for this data, this freedom to vary is restricted. The precision of an estimate is a function of degrees of freedom

DEIR/DEIS

See EIR/EIS.

Delay

The difference between the actual travel time on a link and the free-flow travel time. Often represented as total or average delay (taken over all vehicles in a defined period) and serving as a measure of congestion. Delay may be attributed to queueing based on congested flows and/or signal timing.

Delphi Technique

A planning, prediction, or decision-making tool in which a panel of experts, via interations of individual assessment and feedback to the panel, reach a consensus, often when input data and decision criteria are subjective rather than objective.

Demand

The quantity of transportation desired at a given price, typically defined for specific users in a specific time and place.

Demand Analysis

Travel demand analysis encompasses (a) forecasting future volumes and level of service for planned transportation facilities and services, (b) evaluating the associated benefits and costs, and (c) developing plans for system operations.

Demand Function

In Transportation Systems Analysis, a demand function reflects characteristics of the Activity System that together with performance functions (which reflect characteristics of the Transportation System) determine network flows (volumes and travel times).
A demand function relates volume to a function of characteristics of a given region and the associated level-of-service, or: volume = D( Asys, LOS )

Demand Responsive Transit

A form of paratransit that features reservation-based and/or on-demand, door-to-door transportation within a defined service area, operated by transit agencies, community groups, or private operators, and providing high quality transit service to primarily elderly and disabled riders.

Demographic Data

Characteristics of the population, including population and household counts as well as descriptors such as age and gender, usually defined at the zonal level. While often used interchangeably with the term socio-economic data, demographic data are best viewed as fixed population characteristics that define a state (such as age and gender) whereas socio-economic data correspond to time-varying attributes that define status within a state (such as auto ownership and income). Demographic and socio-economic data, together with land use data, are key inputs to trip generation.

Demography

The field of research and analysis on the size, structure, and distribution of population, including the dynamics of fertility, mortality, and migration, as well as related issues such as housing and socio-economic characteristics.

Density

[1] The number of flow units (vehicles) present on a defined section of roadway at a given time (typically measured in vehicles per mile). With volume and speed, density defines the fundamental diagram of traffic flow (which provides a direct link to performance functions).
[2] The number of units of some activity measure (population, employment, etc.) per unit area (e.g., population per square mile).
[3] In the 3D process, density, design, and diversity are measures to comparatively describe the built environment (in 3D, an area's density is defined as the sum of population and employment divided by total land area).

Density Gradient

The change in density of population (or other demographic or socio-economic variable) over distance from a defined spatial center (such as a CBD). In the standard monocentric land use model, the density gradient is theorized as a negative exponential distribution.

Department of Transportation

Federal or state Department of Transportation (see DOT).

Departure Time

The clock time that a trip begins. Departure time has historically been addressed in aggregate terms with trips assigned to the period in which it was assumed to start and end based on time in motion studies associated with travel surveys. Departure time is more explicitly addressed in activity-based microsimulation models.

Dependent Variable

A variable whose value depends on that of other (explanatory, or independent) variables.

Derived Demand

The demand for travel is derived from the demand to perform an activity that is located so as to require travel to access the activity.

Design

[1] Design is a systematic process to develop solutions to address a specified problem or need. Design reflects an open-ended problem-solving approach that recognizes alternate solutions and relevant constraints.
[2] Design also refers to the resulting solution itself, in general or specific terms.
[3] Together with density and diversity, design forms the 3 Ds, measures to comparatively describe the built environment. In the 3D process, an area's design is a weighted combination of sidewalk completeness, route directness, and street network density.

Design Build

A project delivery process where a single party is contracted for both project design and construction. In the conventional process, a construction contract would follow completion of a separate contract for project design.

Design Hourly Volume

The thirtieth highest hourly traffic volume (in vehicles per hour) for the design year.

Desire Line

A graphical representation of the overall demand for travel between two points, typically drawn as a straight line from an origin to a destination with line width scaled to overall demand and often color-coded by trip purpose, mode, or other relevant attribute.

Destination

The location or zone where a trip ends, but also used for the trip itself: a zone is a destination for N trip destinations. The origin is where a trip begins. Compare with production and attraction.

Destination Accessibility

Destination Accessibility is a measure of a zone's or an area's regional accessibility. Together with density, design, and diversity, destination accessibility forms the 4 Ds, measures to comparatively describe the built environment. An extension of the 3D process, destination is an index defined for a given area i as the denominator of a gravity model for a region, or Σj Aj f(cij), where Aj is the number of attractions in zone j and f(cij) is some function of the generalized cost from area i to destination j.

Destination Choice

The process by which a trip maker located at a specific origin seeks a location at which to perform an activity for a specific purpose. Most conventional models address destination choice via a deterministic trip distribution function, such as a gravity model, rather than an explicit choice model such as Logit.

Detector

A device for identifying the presence or passing of a vehicle at a particular location, often used interchangeably with sensor (see inductive loop detector). A loop is a passive detector, whereas a pedestrian push button for a walk signal is an active detector.

Detector, Count

A detector for counting the number of vehicle activations at a particular location over a defined period (e.g., 30 seconds for most freeway lane detectors -- such freeway detectors often also compute and convey occupancy data).

Detector, Extension

A detector located upstream from an intersection to identify vehicles on an approach to extend the current green time by a fixed or decreasing amount until a phase maximum is reached.

Detector, Presence

A detector for identifying only the presence of a vehicle at a particular location, sending a binary signal to the controller to address the corresponding movement according to the current control logic. Most common deployment is at intersection stoplines.

Deterrence

Same as impedance.

Deterministic

Not a probabilistic function or process (see stochastic).

Deterministic Assignment

A network loading process that assumes travelers have perfect knowledge of all path travel times and thus choose paths in a deterministic fashion (e.g., Wardrop's Principles; see also iterative assignment). Compare to Stochastic Assignment.

DHV

Design Hour Volume (see DHV).

Dial's Algorithm

A stochastic trip assignment algorithm that obviates path enumeration. Equivalent to a logit path assignment, Dial's follows a 2-pass algorithm to assign flows to "reasonable" paths (a path is reasonable if it moves further from the origin and closer to the destination). In the forward pass, link weights are computed based on link likelihoods, proceeding from the origin; in the backward pass, link volumes are computed based on weights, proceeding from the destination. A 1-pass algorithm uses a relaxed definition of "reasonableness".

Diamond Lane

A commonly used expression for a carpool lane, based on the controversial Santa Monica, CA experiment where an existing freeway lane was converted to exclusive use by HOVs and demarcated with painted diamonds on the pavement. Such delineation may also be used for lanes dedicted to other modes, such as bike lanes.

Differential GPS

A method to improve the accuracy of GPS measurements by referencing a known fixed location to compare errors from GPS satellites.

Dijkstra's Algorithm

The most efficient algorithm to find the minimum path tree from a given origin on a transportation network based on the iterative scanning of links outbound from a node on the tree. Alternate algorithms (e.g., Moore's) differ primarily in the sequence that nodes are evaluated for inclusion on the minimum path. At each iteration, Dijkstra's selects the node closest to the origin (and is thus a label-setting path algorithm).

Direct Demand Model

An aggregate demand model that represents multiple travel choices in a single equation. The inherent complexity is controlled by restricting the application to well-defined travel corridors.

Direct Elasticity

An elasticity defined as the change in market share of an alternative due to a change in an attribute of that alternative (e.g., the change in transit demand due to a transit fare increase). Compare with indirect elasticity.

Directional Split

The proportional distribution of total traffic volume on a two-way facility.

Disaggregate Model

A model with units of analysis defined as the actual behavior units. In travel forecasting, the behavioral unit is the individual decision-maker or the household (see Aggregate Model).

Discount Rate

Commonly used synonymously with interest rate, a measure of the time value of money used in "discounting" future costs and benefits to equivalent present day values (technically, for interest rate i, the discount rate d = [i/(1+i)]).

Discrete Choice

A modeling approach, also known as qualitative choice models, representing a decision-maker's choice among a finite set of distinct choice alternatives (such as the choice of alternate modes among car, bus, or walk).

Dispersion Model

A mathematical model that estimates the atmospheric distribution of pollutants as a function of emissions rates, source location, and other factors.

Distance

The basic measure of spatial separation and thus a measure of total travel. Defined alternatively as straight-line distance ("as the crow flies") or as actual travel distance from a trip origin to its destination. In policy studies, distance is often replaced by policy-sensitive equivalents such as travel cost, travel time, or generalized cost

Distance
(from Heavy Rail)

An area's distance from a heavy rail transit station is a measure of an area's ability to draw trips from street networks. Together with density, design, diversity, and destination accessibility, distance forms the 5 Ds, measures to comparatively describe the built environment. An extension of the 4D process, the distance measure, defined as an exponential function of population and employment within a half mile of a rail station, rail service frequency, and feeder bus service frequency), is only applicable in zones containing a heavy rail station.

Distracted Driving

A state where non-driving activities divert a driver's attention from the primary task of controlling the vehicle in a safe manner, including phone/text use, using auxiliary in-vehicle systems such as radios or navigations systems, and attending to other personal demands or demands of other vehicle occupants. Distracted driving may contribute to as many as ten percent of all traffic fatalities each year.

Distribution

Generic term for trip distribution.

District

Aggregations of TAZs to facilitate model development or display of model results. Trip attraction models often are estimated for districts since residential-based travel surveys do not usually provide enough observed attractions in each TAZ to estimate the zonal attraction model directly.

Disutility

Utility that is negatively valued is often referred to as disutility (e.g., travel time or cost).

Diurnal Emissions

Vehicular emissions that occur on a daily cycle and that are not necessarily related to vehicle use. For example, emissions from a vehicle's fuel tank that occur even when a vehicle is not running.

Diversion Curve

Utilized in early mode and route choice studies, diversion curves expressed the proportion of trips that would use a particular mode (e.g., car versus bus) or route alternative (e.g., freeway versus arterial) given relative travel costs of the alternatives.

Diversity

Diversity is a measure of an area's land use mix, or more specifically, its jobs-population balance. Together with density and design, diversity forms the 3 Ds, measures to comparatively describe the built environment. In the 3D process, an area's diversity is defined as {1 - [ abs( (E/P)p - e ) / ( (E/P)p + e) ] }, where E and P are regional employment and population and e and p are the corresponding local values.

Do Nothing Alternative

A somewhat misleading name for the No Build Alternative. "Do Nothing" means no actions other than those which have already been planned and programmed.

Dock, Wharf, Pier

A pier is a port facility for loading/unloading ships that extends perpendicular from the shore; a wharf differs only in that it is built parallel to the shore. A dock is the space in which a ship moors alongside a pier or wharf (a dry dock is this space with the water removed for ship building or repairs). Otis Redding was sitting on a pier or a wharf.

DOT

A federal or state Department Of Transportation. The federal USDOT includes FHWA, FTA, and other transport agencies [ web ]. Each state has an equivalent agency, in most cases also named a DOT (e.g., Caltrans)

Doubly Constrained Gravity Model

A gravity model used for trip distribution derived via the Most Probable Distribution (Maximum Entropy) approach where both trip productions and attractions are constrained to match those input from trip generation (see Singly Constrained Gravity Model).

Downstream

Traffic or infrastructure that is located ahead of the current position in the direction of movement (as opposed to upstream traffic or infrastructure that may be seen looking back over your shoulder).

DRAM/EMPAL

Direct Residential Allocation Model and EMPloyment ALlocation model are components of the Integrated Transportation Land Use Package, ITLUP.

DSRC

Dedicated Short Range Communications (see DSRC).

DSS

Decision Support Systems (see DSS).

DTA

Dynamic Traffic Assignment (see DTA).

DTIM

The Direct Travel Impact Model is a grid-based vehicle emissions forecasting model developed by Caltrans, combining EMFAC emission rates with network assignment output, increasing emission resolution by using trips defined by TAZ rather than VMT.

Dual Problem

In linear programming, the dual is complementary to the original (primal) problem. The minimization of a primal LP with n decision variables and m "≥" constraints is equivalent to the maximization of a dual LP with m dual variables and n "≤" constraints.

Dual Variable

In the dual mathematical program, each primal constraint is represented by a dual variable, a shadow price that represents the marginal change in the optimal value of the objective function for a unit change in a constraint (equivalent to the value of the Lagrange multiplier at the optimum).

DUE

Deterministic User Equilibrium (see UE).

Dumb

Processes and technologies that existed before the advent of "smart" or "intelligent" transportation systems introduced in the early 1990s (see humor).

Dummy

An abstraction of a real-world attribute to facilitate model analysis:
[1] dummy variable: a means of accommodating non-metric variables in a regression analysis via a binary variable indicating the presence or absence of a particular value of the original variable;
[2] dummy (supply or demand) node: in the transportation algorithm, total supply must equal total demand; a dummy demand node is added is supply exceeds demand, or a dummy supply node is added if demand exceeds supply.

Dwelling Unit

Type of housing unit (e.g., single family detached, apartment, mobile home). Has often been used in trip generation models due to high correlations with household trip rates and ability to directly observe. Same as Housing Unit.

Dynameq

Event-based, equilibrium dynamic traffic assignment software developed by Inro [ web ].

Dynamic Segmentation

In GIS, a process that allows attributes to be assigned to portions of a line (e.g., a network link) without physically modifying line geometry.

Dynamic Traffic Assignment

Network assignment with dynamic (versus fixed) demand. DTA extends conventional static trip assignment by integrating the travel demand and traffic flow components.

DynaMIT

DynaMIT is real-time software for ATMS and ATIS support within a Traffic Management Center (TMC). Developed by MIT [web ].

Dynasim, Cube

Cube-Dynasim is real-time software system for simulating, visualizing, and evaluating the effects of transportation and activity system changes. Developed by Citilabs and linked with Cube travel forecasting software [web ]. Other traffic microsimulation packages include AIMSUN, CORSIM, Paramics, SimTraffic, TransModeler, and VISSIM.

DYNASMART

[1] DYNASMART (1994) is a mesoscopic simulation model for dynamic network traffic analysis, featuring macroscopic updating with individual (microscopic) vehicle tracking.
[2] DYNASMART-P is an intelligent transportation network design, planning, evaluation, and simulation tool, initially developed at the University of Texas.
[3] DYNASMART-X is a real-time traffic estimation and prediction tool for dynamic traffic assignment (DTA), developed at the University of Maryland [ web ].

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Earmark

An appropriation of public funds to a specific program or project, placed in legislation by an elected official (AKA "pork").

Ecological Fallacy

The problem of inferring individual characteristics from population data, such as inferring individual household characteristics based on TAZ data.

Econometrics

The application of statistical methods to the estimation of economic models.

Economic Base

A fundamental technique for describing and forecasting economic activity, economic base sectors an economy (often measured by employment) into basic (export-oriented) and one or more non-basic (local-oriented, or service) employment sectors. The basic sector fuels the local economy, driving service employment and population growth. Local multipliers define functional relationships between basic employment, service employment, and population. The location quotient and minimum requirements sectoring methods can be used to estimate basic employment by economic sector.

Economies of Scale

In comparing changes in average cost with increasing output, economies of scale exist if average cost decreases with increased output (also known as increasing returns to scale).

Edge Cities

Cities of a recent vintage developed at the periphery of established metropolitan areas (a named popularized in Garreau's (1991) Edge City).

Edie Model

Edie (1961) proposed the first two-regime traffic stream model combining Underwood's model, which performed well for free-flow conditions, and Greenberg's model, which performed well for congested flow. Two-regime models have a discontinuity between regimes, often near capacity.

Effectiveness

A general system performance measure defined as the degree to which stated goals and objectives are achieved (see also efficiency).

Efficiency

A general system performance measure defined as the ratio of system output to system input, where system input is often cost (see also effectiveness).

Efficient Estimator

An estimator is efficient if it has a small sampling error.

EIR/EIS

Environmental Impact Report/Environmental Impact Statement (see EIR/EIS).

Elastic Demand

See variable demand.

Elasticity

The elasticity of y with respect to x is the percent change in y with respect to the percent change in x, or ey|x = (dy/y)/(dx/x). An elasticity may be estimated at a point directly from a known function (point elasticity) or may be estimated from two observations of x and y (arc elasticity). Elasticity-based demand models provide estimates of the sensitivity of demand to small changes in the model's explanatory variables (see fare elasticity).

Electronic Toll Collection

Electronic systems that collect vehicle tolls without requiring vehicles equiped with a transponder to stop.

Elimination by Aspects

A non-compensatory choice model developed by Tversky (1972) based on the elimination of alternatives from an individual's choice set for not meeting a threshold on the highest ranked attribute (most important aspect), then proceeding through lower ranked (less important) attributes until a single choice alternative remains.

EMFAC

California's software package for calculating regional motor vehicle emissions. EMFAC2014 is the current version (see also MOBILE, MOVES) [ web ].

Eminent Domain

A government's right to take private property for a public use, compensating the property owner at market rates.

Emissions

In transportation, the gases and particles discharged into the atmosphere as byproducts of the combustion process of converting fuel to motive power. These mobile source pollutants are primary contributors to air quality problems in metropolitan areas.

Emission Inventory

An estimate of total atmospheric pollutants from mobile, stationary, area-wide, and natural sources for a specified period of time.

EMME/2

A software package for travel forecasting and transportation network analysis. Developed by INRO [ web ]. Other travel forecasting packages include Cube, QRSII, MinUTP, T-Model, Tranplan, and TransCAD.

EMPAL

EMPloyment ALlocation model, a component of the Integrated Transportation Land Use Package, ITLUP.

EMPIRIC

An first generation, recursive, simultaneous equation, sub-regional land use allocation model developed by Hill (1965) for the Boston Regional Planning Project.

Empirical

Based on actual experience rather than theoretical expectation.

Employment

The number of persons hired to work in a zone, region, or other defined spatial unit, usually categorized by occupation or industry. An endogenous, economic variable in land use models and an exogenous explanatory variable in travel forecasting models, especially trip generation where employment variables (total, retail, or other categories) are critical in most trip attraction models. Note that employment is defined by work location while labor force is defined by residential location.

Endogenous

A variable whose value is determined as an output from a mathematical model (compare with exogenous ).

Energy

Transportation is movement, movement requires energy, and the transportation sector of the economy constitutes a significant portion of total domestic energy consumption, including about two thirds of domestic oil consumption. Critical energy concerns include domestic dependence on foreign oil supplies and the environmental impact of oil use in transportation.

Entropy Maximization

The statistical basis for a family of aggregate trip distribution models developed by Wilson (1967), including growth factor, gravity, and intervening opportunity models. The derivation is based on the most probable distribution of a trip matrix found by maximizing an entropy function subject to constraints on productions, attractions, and travel cost.

Environment

[1] The natural environment encompasses all biological and physical components of the earth or a defined region.
[2] The built environment is the man-made infrastructure serving human activity.

Environmental Assessment

A NEPA-required process and report that identifies the environmental impacts of project alternatives, leading to either a Finding of No Significant Impact (FONSI) or an Environmental Impact Study (EIS).

Environmental Impact Report

An Environmental Impact Report documents the comprehensive analysis of the environmental impacts of proposed transportation and land development projects, as required under CEQA. The parallel federal requirement is an Environmental Impact Statement, a report required under NEPA. Draft EIR/EIS are circulated for agency and public comment. The final EIR/EIS must address significant impacts and also provide means to mitigate adverse impacts.

Environmental Impact Study

An Environmental Impact Study is a comprehensive analysis of the environmental impacts of proposed transportation and land development projects. An Environmental Impact Statement (EIS) is the federal report under NEPA requirements; an Environmental Impact Report (EIR) is the California report under CEQA requirements. Draft EIR/EIS are circulated for agency and public comment. The final EIR/EIS must address significant impacts and also provide means to mitigate adverse impacts.

Environmental Justice

In transportation, the process of having planning, programs, and policies that reflect meaningful participation of all impacted parties and the provision of facilities and services that reflect an equitable distribution of environmental costs and benefits.

EPA

The United States Environmental Protection Agency, responsible for administering Federal environmental laws, including the Clean Air Act.

Equilibrium

A system state where overall demand and system performance are balanced. Any increase in demand corresponds to an increase in cost that reduces that demand. Network flow is in equilibrium when no traveler can unilaterally change route and be better off, thus there is no incentive to change.

Equilibrium Assignment

A trip assignment algorithm that solves for an equilibrium solution for network flows. Deterministic user equilibrium assumes that paths are chosen based on actual link costs; stochastic user equilibrium assumes that paths are chosen based on perceived link costs.

Equity

A normative measure of fairness among transportation system users and non-users relative to costs and benefits.

Equivalent Annual Cost

In project appraisal, the discounting of all current and future benefits and costs, over a defined project lifetime, to an equivalent annual value.

ESAL

Equivalent Single Axle Load (see ESAL).

Estimation

Estimation, part of the model specification process, is the formal computation of the parameters for an hypothesized model, to maximize the likelihood of fitting observed data. In comprehensive model systems such as the Four Step Model, individual models are independently estimated. The model system as a whole is validated prior to any policy application.
Note: In travel forecasting, the terms estimation and calibration often have been used interchangeably. Determining the structure of the model is explicitly part of the estimation process thus lending it greater statistical formality, but both terms reflect the same overall objective. If the structure of the model is pre-determined, then perhaps calibration is the better term. If model specification is involved, then perhaps estimation is the better term.

Estimator

An estimated or calibrated value for an unknown population parameter.

ETC

Electronic Toll Collection, the collection of roadway tolls based on automated vehicle identification (AVI).

Ethanol

A carbon-based, alternative fuel that is an alcohol fermented from corn or other bio-sources.

Euclid vs. Ambler

A landmark in zoning law was the United States Supreme Court's 1926 decision (Village of Euclid, Ohio vs. Ambler Realty Co) that supported the legal authority of local governments to control public and private land, including such building requirements as use category, size, height, parking, and setbacks via zoning ordinances.

Evaluation

As part of the Transportation Planning Process, evaluation is the process of systematically assessing the costs and benefits of competing alternatives. In addition to such a priori applications, evaluation is also performed in ex post performance assessment of existing transportation systems.

Evan's Algorithm

Proposed by Evans (1976) to find the equilibrium solution of a combined model of trip distribution and trip assignment, formulated as a constrained optimization problem.

Event

Any accident, incident, or planned occurance that affects transportation system performance. A primary role of Transportation Management Centers is to expediciously resolve events.

Evolutionary Algorithm

A broad family of algorithms for search and optimization first introduced by Fogel (1966) that include genetic algorithms but in general are not dependent on a defined fitness function but are more dependent on diversity and mutation and thus less likely to be caught in local optima.

Exogenous

A variable whose value is determined independently and is used as an input to a mathematical model (compare with endogenous ).

Expansion Factor

In travel surveys, the parameter that converts sample data to population-level equivalents, based on the designed sampling frame.

Expert System

A software system that incorporates quantitative and qualitative professional expertise and judgment in a knowledge base integrated with an inference engine to emulate the decision process of experts in a particular area ( also know as Knowledge-based System ).

Explanatory Variable

An exogenous variable hypothesized to influence the value of a dependent variable.

Exponential Growth

The assumption of unconstrained natural increase of a population yields the exponential growth model (Malthus, 1798) where the growth rate is directly proportional to the current population (the discrete equlivalent is the geometric growth model). Compare with the logistic growth model.

External Station

A "door" on the cordon line of a study area corresponding to major entry and exit points for external trips through, into, and out of the study area.

External Travel Survey

While a Household Travel Survey collects data on travel behavior for residents of a study area, an external survey collects travel data on non-residents who travel to, from, and through the study area. These trips are identified as internal/external (IE or EI) or external/external (EE, or thru trips). Usually conducted every 10 years at key external stations.

External Trip

A trip with either its origin or its destination located outside of the study area. The external trip end is assigned to an external station. Often referred to as "IE" for internal-to-external or "EI" for external-to-internal trips. A through trip has both trip ends outside the study area (often referred to as "EE" or for external-external). An internal trip has both trip ends inside the study area (often referred to as "II" for internal-internal).

Externality

An economic cost or benefit that is experienced by individuals not directly involved in an activity and thus not reflected in the price of the activity (e.g., air pollution).

Extrapolation

The process of forecasting values of a dependent variable beyond the range of observed explanatory variables, usually by estimating a model (for example, a trend projection or other time series model) and applying it beyond the observed range of the explanatory variables, thus increasing uncertainty in the estimated values.

Exurban

Outlying rural communities beyond traditional suburbs and characterized by significant commuting flows with the core urban area (see metropolitan classification).

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F Factor

See Friction Factor.

F Test

A test of model significance in least squares estimation, an F statistic is computed in an Analysis of Variance as the ratio of the mean sum of squares accounted for by the model to the residual mean sum of squares. If the calculated value is greater than the critical value, then the null hypothesis that all model coefficients are equal to zero is rejected (see t Test).

Facility

A component of transportation infrastructure, often taken as an operational facility such as a roadway or guideway, but may also including terminals. In general, any building or other dedicated infrastructure.

Facility Location

A branch of operations research that develops and applies methods and models for the spatial location of facilities with respect to transportation and activity systems.

Facility Type

Describes the operational design of a roadway based on access control, speed limits, and topography. Functional classifications include freeways, arterials, and local streets. In travel forecasting, facility type and area type are used to create a look-up table that provides default link speeds and capacities.

FAF

Freight Analysis Framework (see FAF) [ web ].

FAF Data

A national freight data survey and data base documenting flows by commodity (see also Cal-Fred ) [ web ].

FAHA

Federal-Aid Highway Act (see FAHA).

FAIR Lanes

Fast And Intertwined Regular lanes is a pricing concept that (physically) divides free, general-purpose traffic lanes into "fast" toll lanes and "regular" untolled lanes, each electronically monitored. Drivers in regular lanes would receive credits toward use of the fast lanes to compensate for loss of access to the converted lanes and the associated delay increase in the remaining regular lanes. It is claimed that FAIR lanes can increase freeway throughput, speed public transit service, let drivers bypass congestion, and cure gum disease. Any semblance to diamond lanes is likely inadvertent [ web ].

FAR

In zoning, the Floor Area Ratio expresses the total floor space of a building as a fraction of the total area of a site. FAR combines horizontal (e.g., setback) and vertical (e.g., height restriction) dimensional limits into a single parameter that correlates well with site traffic impact measures (trips, parking demands, etc.).

Fare

The price paid by a user to utilize a public mode of transportation.

Fare Elasticity

The elasticity of transit demand D with respect to transit fare f, defined as the percent change in demand with respect to the percent change in fare, or eD|f = (dD/D)/(df/f), has been found to be about -0.30 in many studies. For small changes in fare, a corresponding linear change in demand can be estimated.

Farebox Recovery

The proportion of operating costs covered by passenger fare revenue.

Feasible Solution

A solution that satisfies problem constraints but may not be optimal. Many algorithms require an initial feasible solution for execution.

Federal Aid Highway Act

The series of federal transportation authorization bills signed into law over the past century, beginning with the Federal Aid Road Act of 1916 and commonly named Federal Aid Highway Acts from 1938 through 1987. The last three acts have been more whimsically named ISTEA (1991), TEA-21 (1998), and SAFETEA-LU (2005).

Federal Aid Highway Program

The generic name for the funding program between the federal government and local and state agencies.

Federal Aid Highway System

The National Highway System and the Dwight D. Eisenhower National System of Interstate and Defense Highways.

Federal Register

Daily federal publication that publicizes regulations and legal notices issued by various departments of the Federal government.

Federal Test Procedure

For vehicular emission models, emission factors are derived from laboratory experiments measuring vehicle emissions with a dynamometer for a standard Federal Test Procedure (FTP) driving cycle. The FTP was developed in the 1970s to capture real-world driving characteristics using a defined cycle of accelerations and decelerations.

Feedback

Using the output from a step in a modeling sequence as revised input to a prior step to re-execute the model sequence. In the last step of the Four Step Model, trip assignment has conventionally been equilibrated given a fixed trip table as input, with no feedback to prior steps. Most recent models use assigned link volumes to revise link travel times and feed them back to the minimum path algorithm to repeat trip distribution and mode choice with more consistent estimates of network travel times and costs.

FHWA

The Federal Highway Administration, part of USDOT, administers federal-aid highway programs that provide the states with funding and technical support for the design, construction, and management of highway infrastructure.

FIFO

First In First Out, a single-server queueing regime where service is provided in order of arrival in the queue.

First Mile / Last Mile Problem

The common public transit problem of system access and egress: how to get from home to the transit stop (the "first mile") and how to get from the transit system to your final destination (the "last mile").

Fixed Cost

Costs that remain constant, independent of the level of transportation operations (such as infrastructure costs). Also known as capital costs. Compare to variable cost.

Fixed Route

Public transportation service provided on a pre-defined route, with pre-defined access and egress points, and usually on a pre-defined schedule, typically associated with bus transit service but also applicable to other modes.

Fixed Time

A traffic signal timing plan with a set sequence of pre-determined phase lengths defined for a particular period of operation. The resulting fixed cycle length facilitates coordination, but there is no real-time connection between actual vehicle demands and signal operations.

Flexible Pavement

A pavement with a bituminous asphalt wearing surface and that is supported by and distributes loads to the subgrade (see rigid pavement).

Floating Car

A method of traffic data collection where a test vehicle drives with the traffic stream at the ambient speed in each direction to estimate link volume and speed. Floating cars have evolved into probe vehicles to estimate traffic stream charactistics with advanced location and communication technologies.

Floor Area Ratio

See FAR.

Flows

In Transportation Systems Analysis, the output of demand performance equilibration (trip assignment in the basic Four Step Model) is a set of flows, represented by a set of link volume and level-of-service measures. Flow is sometimes taken as only traffic volume (especially in traffic operations), typically measured in vehicles per hour (vph).

Floyd-Warshall Algorithm

An algorithm to find the minimum paths between all pairs of network nodes based on the iterative evaluation of potential intermediate nodes. Floyd's is a label-correcting path algorithm that operates on all nodes in a network, not only on centroids, thus it is not recommended for conventional transportation networks. See Dijkstra's and Moore's.

Focus Model

A focus model uses an existing regional transportation model as a base, and holds the activity system outside of the primary (or focused) study area constant or aggregates these data into larger zones (particularly as distance from the focus area increases). Within the focused area, the detail associated with the activity and transport systems is typically increased, allowing for a more detailed analysis and evaluation of local demand and performance. See also sub-area extraction. Compare with window model.

FONSI

Finding Of No Significant Impact: a document providing the justification for a planned action having no environmental impact that would require the preparation of an EIR/EIS.

Forecasting

Forecasting (or prediction) is the application of a calibrated or validated model or model system to estimate future system performance. In conventional travel forecasting, the process involves the sequential application of the four step model for each defined transportation system alternative, as a function of a specified future activity system.

Fossil Fuel

Oil, coal, natural gas, or other naturally-occurring but non-renewable hydrocarbons fuels produced by the decomposition of organic material over millions of years.

Four Step Model

The conventional model for travel forecasting, so named for the four major steps of the process: trip generation, trip distribution, mode choice, and trip assignment (AKA UTMS).

Fractal

A mathematical construct that exhibits properties of self-similarity (the same structure characteristics emerge at different spatial scales) that has been applied to models of urban growth and development.

Frank-Wolfe Algorithm

A general, quadratic programming algorithm commonly used in iterative trip assignment to search for optimal weights of a convex combination of successive linear approximations (i.e., AON assignments).

Fratar Method

Essentially the same as the Furness method, Fratar's (1954) method is a growth factor model to forecast a future trip tables based on zonal growth factors and a base trip table.

Free Assignment

A trip assignment algorithm that is not capacity restrained, such as All-or-Nothing or Dial's multipath assignment.

Free-Flow Speed

Same as free mean speed.

Free Mean Speed

The average speed of vehicles traveling unimpeded on a section of roadway, (AKA free-flow speed). The equivalent travel time measure is base or free-flow travel time, the minimum time required to traverse a section of roadway.

Freight Analysis Framework

Freight Analysis Framework, a FHWA freight analysis tool based on commodity flow surveys, includes weight and value estimates for commodity movements by origin, destination, and mode; 30-year forecasts; and a network database [ web ].

Freight Demand Modeling

Travel forecasting for freight, encompassing (a) forecasting volumes and level of service for current and planned freight transportation facilities and services, (b) evaluating the associated benefits and costs, and (c) developing plans for system operations. Freight networks are usually integrated with other modal networks and freight shipments often involve multiple actors, increasing model complexity.

Freight Transportation

A generic term for all movements of goods via the highway and other modal networks. Freight transportation, also known as goods movement, has increased in importance in most metropolitan areas with the increase in network congestion and air quality conformity requirements. The methodological field is referred to as logistics.

FREQ

A macroscopic freeway corridor simulation model. Current version is FREQ12 includes models to analyze ramp metering and HOV lanes [ web ].

FRESIM

INTRAS, a freeway simulation model developed by KLD in the 1970s, evolved into FRESIM that, together with NETSIM (arterial simulation), formed the integrated software suite TRAF (now TSIS/CORSIM).

Friction Factor

Utilized in gravity trip distribution models as empirical measures of travel impedance (a function of travel time or generalized cost), friction factors (Fij) are a frequent alternative to inverse power, negative exponential, and combined (gamma) impedance functions. In practice, friction factors are mapped into one minute trip length categories related to the corresponding observed trip frequency.

FTA

The Federal Transit Administration, formerly UMTA, in the branch of USDOT responsible for federal funding for and technical support of the planning and development of deomestic public transportation systems.

FTP

Federal Test Procedure (see Federal Test Procedure).

Functional Classification

The classification of roads by design function. Road classes higher in the hierarchy place relatively more emphasis on service to through traffic and less emphasis on serving local land uses. Classes lower in the hierarchy place increasingly greater emphasis on access to local land uses. A sample hierarchy is freeways and other limited-access facilities, arterials (primary, secondary), collectors, and local streets.

Functionally Obsolete

Infrastructure (e.g., a bridge) that does not meet current design standards for geometrics (see structurally deficient).

Fundamental Diagram

The three fundamental traffic flow variables, volume V, density D, and space mean speed s, define the fundamental traffic flow relationship ( V = D s ). The fundamental diagram depicts this relationship graphically with the speed-density relationship defining the shapes of the volume-density and volume-speed relationships. This diagram provides a direct link between traffic engineering (speed-density models) and travel forecasting (performance functions).

Furness Method

A matrix modification method used to extrapolate trip distribution on the basis of zonal growth factors and iterative proportional fitting (see IPF). It can also be used to balance productions and attractions in the singly-constrained gravity model.

Fuzzy Logic

Zadeh's (1965) alternative model of uncertainty where classification is defined by rules that use subjective or approximate values applied to ambiguous objects or concepts, utilized in AI applications in traffic control.

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Game Theory

An approach to modeling interactions between agents facing strategic decisionmaking with imperfect information and varying incentives.

Gamma Function

In gravity models, a variety of function forms have been used for travel impedance in trip distribution, including inverse power, negative exponential, and combined gamma functions, as well as the conventional friction factors. The gamma function combines the inverse power and negative exponential functions:
f(tij) = μ0 tij-μ1exp(-μ2 tij)

Garin-Lowry Model

Garin's (1966) modification of Lowry's land use model, which introduced both a matrix formulation and explicit spatial interaction models for activity location.

Gasoline Tax

Taxes on gasoline and diesel fuel for vehicles using public roads include federal, state, and local components. The federal excise tax is 18.4 cents per gallon (independent of fuel price). In California, the state excise tax is 18 cents per gallon. There are also state and local sales taxes on fuel. In California, the state sales tax is 7.25 percent and several local counties have voted for additional sales taxes dedicted to transportation (1.0 and 0.5 percent in LA and Orange counties, respectively) [as of 1 January 2008].

GEH Statistic

A self-scaling measure designed to compare traffic volumes over a range of facility types, named for its developer Geoffrey E. Havers. It is defined as:
GEH = √ [ 2(Ea-Oa)2/(Ea+Oa) ] where Ea and Oa are estimated and observed volumes.

Gender

Traditionally used synonymously with biological sex, indicting male or female, but increasingly associated with social constructs such as role in the study of travel behavior.

General Plan

The long-range plan for a community's future development, a general plan (also known as a comprehensive plan or a master plan) defines goals and objectives, policies and standards, and constraints for the growth and development of the community. The plan provides a land use and zoning plan indicating planned land uses (e.g., residential, commercial, institutional) for districts and parcels, and addresses all planning elements including transportation infrastructure and services, the natural and built environment, and demographic trends.

Generalized Cost

A weighted combination of the perceived attributes of travel cost such as monetary cost, travel time (with component parts such as access, waiting, and in-vehicle time usually separated), and distance. May involve composite general costs over alternate modes but not indirect costs of externalities. See impedance.

General-Purpose Lane

A (freeway) lane open to all motor vehicles, unrestricted by tolls or occupancy requirements.

Generation

Generic term for trip generation.

Generic Variable

In discrete choice models, a generic variable appears in the utility function of each choice alternative with the same estimated coefficient (compare to alternative-specific variable).

Genetic Algorithm

Heuristic optimization algorithms that, borrowing from evolutionary biology, use replication, mutation, recombination, and natural selection processes operating on a parent population of potential solutions to generate offspring that are evaluated based on a defined fitness function to see which will be selected to form a new population to repeat the process and move toward an optimal solution. GAs have been applied to complex problems where evolutionary processes can move potential solutions away from local optimums.

Gentrification

The rehabilitation or redevelopment of deteriorated, typically low income neighborhoods with the resulting increased property values displacing the original residents by new residents with higher socio-economic status.

Geo-coding

The process of mapping activity locations reported in travel surveys to a geographical coordinate system (e.g., latitude and longitude) for use in travel forecasting. Geo-coding conventionally assigns activity locations by TAZ number. GPS automatically provides precise geo-coded location data and is increasingly used in travel surveys.

Geometric Design

Physical roadway design, including horizontal and vertical alignment as well as cross-sectional design, and incorporating basic relationships decribing the interaction between drivers, vehicles, and the roadway, to accommodate safe and efficient travel.

GEV

Generalized Extreme Value, a family of continuous probability distributions that includes the Gumbel (GEV Type I) distribution associated with the multinomial logit model.

GHG

GreenHouse Gas, (see GHG).

Gini Coefficient

A measure of inequality on some population characteristic, usually income, defined from 0 (perfect equality) to 1 (perfect inequality) and computed as the area between the Lorenz Curve and the diagonal, divided by the total area under the diagonal.

GIS

A Geographical Information System is an integrated spatial database, analysis, and graphic display tool (such as ArcInfo or TransCAD).

Global Warming

The increase in annual average temperatures at the earth's surface based on various factors, including the increase in greenhouse gases in the atmosphere due to the combustion of fossil fuels [ see Climate Change ].

GNP

Gross National Product is the value of all the goods and services produced in a country.

Goals and Objectives

The first stage of the transportation planning process is Problem Definition, which incorporates Goals, broad statements based on community values and the current environment, and Objectives, restatements of goals in specific and measurable terms. Problems may be defined when objectives, often defined in terms of performance measures, are not met by the current environment.

Goodness of Fit

A measure of statistical performance that indicates how well a set of data is described by a model estimated with that data.

Goods Movement

The transportation of freight rather than people, most commonly applied to movements within metropolitan areas rather than within or between states or countries.

GPS

The Global Positioning System is a system of satellites and surface-based receivers that can accurately determine location and time. Increasingly utilized in traffic engineering and transportation modeling applications, including in the conduct of travel surveys, since GPS automatically provides precise, time-stamped geo-coding.

Grade

The percentage change in elevation to change in distance for a roadway section.

Grade Separation

A transportation facility elevated above or trenched below another facility to separate traffic movements, increasing capacity and safety. May involve one or more transport modes.

Gradient Projection

An optimization algorithm used in path-based trip assignment with some computational and theoretical advantages over link-based algorithms such as Frank-Wolfe. Path-based algorithms identify and store utilized paths in each iteration, which can limit practical applications to smaller networks, but providing better estimates of turning movements.

Gravity Model

A trip distribution model that represents trip interchanges as a function of trip productions, trip attractions, and interzonal impedance (a function of interzonal travel cost, usually defined by travel times). Singly and Doubly constrained versions are in common use.
[ DCGM: Tij = ai bj Pi Aj f(tij) ] [ SCGM: Tij = ai Pi Wj f(tij) ]

Greedy Algorithm

Any heuristic algorithm that makes locally optimal choices at each step in search of a global optimum.

Green

An increasing subjective buzzword implying concepts, products, processes, services, or technologies that increase the efficient use of energy and other resources and minimize the associated negative environmental impacts.

Green Book

AASHTO's "A Policy on Geometric Design of Highways and Streets", the standard reference on highway geometric design [ web ].

Green Time

The portion of a cycle for which signal indications display green to permit a particular movement (see also amber, red, and effective green).

Greenberg Model

A non-linear single-regime traffic stream model developed by Greenberg (1959), relates space mean speed to density, derived from the macroscopic hydrodynamic flow analogy, and provides a bridge to microscopic car-following models.
Greenberg: u = um ln(dj/d) where um is speed at maximum flow and dj is jam density.

Greenhouse Effect

Ambient temperature increase resulting from solar energy changing to heat energy and captured within the earth's atmosphere.

Greenhouse Gas

Any gas, such as carbon dioxide, methane, and nitrous oxide, that occurs naturally or through human activity, and contributes to the greenhouse effect where heat is trapped in the atmosphere and contributes to global warming.

Greenshields Model

A linear single-regime traffic stream model developed by Greenshields (1935), relates space mean speed to density and leads to a parabolic flow/density plot of the fundamental diagram.
Greenshields: u = uf [ 1 - (d/dj) ] where uf is mean free speed and dj is jam density.

Grid Cell

The basic geographical unit of a regional air quality dispersion model, analogous to (but not necessarily corresponding to) zones in the regional transportation model.

Grid Network

A network design pattern dominated by orthogonal streets, often hierarchically designated, and typically with 4-way, right-angled, at-grade intersections.

Gridlock

Strictly defined as occurring on a grid network where traffic heading in one direction is blocked from proceeding through an intersection by queued cross traffic, traffic which itself may be blocked by queued cross traffic at the next downstream intersection. Queues spillback to block upstream intersections, "locking" the grid. The term has become synonymous with "traffic jam", or a high level or duration of traffic congestion, regardless of the presence of a grid. Note: An alternative definition places gridlock as a synonym to deadlock, a stalemate where no progress can be made, thus, to be stuck in place.

Ground Truth

An in situ measurement of an observed quantity for use in evaluation or validation of an estimate, model, or method. First used in military applications and then applied in remote sensing, the term has been used in the evaluation of sensors for intelligent transportation systems and increasingly, and perhaps inappropriately, in transport modeling and simulation, where ground truth is often defined rather than measured.

Group Quarters

Residential units housing groups of unrelated individuals such as military barracks or dormitories. Typically serves as a separate input in trip generation analysis.

Growth Factor Model

A trip distribution model that represents trip interchanges as a function of trip productions, trip attractions, and a base trip table (in place of a matrix of interzonal travel times such as in the gravity model). The base trip matrix is factored by estimated productions and attractions. Growth factor models do not include LOS variables (e.g., travel time), so practical applications are generally limited to modeling external trips which have unknown total trip lengths. An example is the Fratar model.
[Example: a doubly-constrained growth factor model: Tij = ai bj Pi Aj Tijobs ]

Growth Management

Local government activities to minimize the negative impacts of development by controlling the density, timing, and siting of new development to match available transportation infrastructure and services.

GUI

Graphic User Interface.

Guideway, Fixed

A transportation facility with an exclusive mode- and grade-separated guideway.

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Habit

A recurrent pattern of behavior resulting from frequent repetition, such as the commute to work where the mode, departure time, and route may no longer be a conscious choice.

Hagerstrand

Swedish geographer who formalized the field of Time Geography and the representation of human activity with space-time prisms, the three dimensional depiction of activity and travel over time and space that reflected the capability, coupling, and authority constraints that confine if not define movement.

Hansen's Accessibility Model

Hansen (1959) developed a model to allocate residential growth constrained by total regional growth and directly proportional to vacant land and employment accessibility [2] (defined for each residential zone as the sum of the ratios of employment to the square of distance from the residential zone).

HAR

Highway Advisory Radio (see HAR).

Hazard Model

In general, a statistical model relating an item's failure to time in service, used in time use and activity-based analysis to model activity duration.

Hazardous Material

Any material legally designated as capable of posing a risk to health, safety, and property when stored or transported.

HazMat

See Hazardous Material.

HBNW

Home-Based Non-Work is equivalent to Home-Based Other ( HBO ).

HBO

Home-Based Other (also known as Home-Based Non-Work, or HBNW): a classification for trips that either begin or end at a trip maker's residence (home) and have the other end NOT at the trip maker's work location (see HBW).

HBW

Home-Based Work: a classification for trips that either begin or end at a trip maker's residence (home) and have the other end at the trip maker's work location (see HBO).

HCM, HCS

Highway Capacity Manual (see HCM), Highway Capacity Software (see HCS).

Headway

[1] The time interval (in seconds) between successive vehicles passing a point on a section of roadway, usually averaged over a defined counting period. Headway h is the inverse of volume V: h = 3600/V in seconds/vehicle (see spacing).
[2] The time interval between successive arrivals of transit vehicles at a fixed location.

Herbert-Stevens Model

A land use allocation model developed by Herbert and Stevens (1960) for the Penn-Jersey Transportation Study using a linear programming framework.

HERS

The Highway Economic Requirements System ( see: HERS ).

Hessian Matrix

The matrix of all second-order partial derivatives of a specified function, used in optimization search routines with the Jacobian matrix of first derivatives.

Heuristic

Problem solving via replicable, experimental techniques such as trial and error, rule-based systems, and various search methods (including hill-climbing and genetic algorithms). Heuristic approaches do not guarantee a optimal solution, thus, they are usually selected only when formal mathematical models can not be applied.

Hierarchical Logit Model

See nested logit model.

High-Speed Rail

Low emission, energy-efficient, high-technology rail systems. Planning in the US envisions systems with average speeds of 100-150 mph (or higher) operating on dedicated or shared, grade-separated track in defined intercity corridors.

Highway Advisory Radio

Low-powered radio stations operated by transportation agencies to provide local traveler information regarding incidents, congestion, construction, special events, and other delays.

Highway Capacity Manual

Standard guidelines and procedures for roadway and intersection capacity design and analysis, developed by TRB, AASHTO, and FHWA. Implemented in Highway Capacity Software (HCS) [ HCM web, HCS web ].

Highway Capacity Software

The software implementation of the Highway Capacity Manual (HCM), developed by TRB for roadway and intersection capacity design and analysis [ HCS web ].

Highway Economic Requirements System

The Highway Economic Requirements System (HERS) models the relationship between levels of investment in and the performance of the highway system. It is linked to the Highway Performance Monitoring System ( HPMS ). [ web ].

Highway Performance Monitoring System

The Highway Performance Monitoring System is a federally-mandated database consisting of facility design and performance data for highway links which is used to track the characteristics, performance, and condition of the nation's highway system [ web ].

Highway Trust Fund

The federal trust fund that collects and allocates highway user taxes and fees such as the federal motor fuel tax. The fund is used to provide federal support to both highway and transit projects.

Hinterland

The area defined by the sphere of influence associated with an activity center. See Central Place Theory and von Thünen's model.

Hitchcock's Algorithm

An efficient solution algorithm to the Transportation Problem that utilizes the dual of the linear programming formulation to obtain a single-commodity, uncapacitated, minimum costs flow solution.

HLFM-II

Highway Land Use Forecasting Model developed by Horowitz (1993) based on a Garin-Lowry framework and sharing model components and utilities with QRS-II. Similar land use software packages include MEPLAN, MUSSA, PECAS, TRANUS, and UrbanSim.

Hogertrafikomlaggningen Day

September 3, 1967, the day that Sweden changed from driving on the lefthand to the righthand side of the road, to match neighboring countries.

Home

Home is an individual's residential housing unit and location. Most travel surveys are home-based, sampling households by residential location, in part since most trips are home-based. Home (or Return Home) was often used as a trip purpose in travel surveys.

Home-based

A classification for trips that either begin and/or end at a trip maker's residence (home). Regardless of the origin and destination, the home location is always the production for home-based trips; the other end is always the attraction (see Non-home-based).

Home-based Other

A trip with one end at home and the other end at a non-work location (see HBO).

Home-based Work

A trip with one end at work and the other end at home (see HBW).

Home Interview Survey

A legacy term for a household travel survey, conducted by interviewers in the home of the survey respondent. Largely replaced by CATI interviews and self administered surveys.

Homeland Security

A reactionary term created and applied after the September 11th attacks referring to the consolidated efforts to protect the US from perceived threats to national security (also refers to the newly created cabinet department).

Horizontal Alignment

The design of a roadway alignments in the horizontal plane, including tangent sections, circular curves, and spiral transition curves, and reflecting topography, safety, facility type, speed, and other design concerns (see vertical alignment and cross-sectional design).

HOT Lanes

A High Occupancy Toll Lane is a managed lane reserved for high-occupancy vehicles (HOVs) and for non-HOVs that pay a toll to use the facility.

Hot-soak Emissions

Emissions that occur after a hot engine has been turned off.

Hot Start

A hot start occurs when a vehicle's engine is started after less than an hour from the previous period of operation (see cold start).

Household

A fundamental sampling unit for travel surveys, the household (HH) is a behavioral unit of one or more individuals who share a dwelling unit, resources (e.g., income, automobiles), and travel/activity responsibilities. Interaction among HH members influences travel and activity behavior. Most trip production models are household-based.

Household Interaction

Individual decision-making and the resulting activity and travel behavior is facilitated by household resources and constrained by responsibilities to the household. HH interactions are in part defined by HH life style and individual roles.

Household Travel Survey

A survey designed to measure household travel behavior and the characteristics of the household that are relevant to its travel behavior. The survey typically collects information on the household, household members, household vehicles, and a travel activity diary that records all activity and travel that occurs during the survey period. Typically conducted in metropolitan areas every 10 years.

Housing Unit

Same as dwelling unit.

HOV

High-Occupancy Vehicle, that is typically 2 or more (sometimes 3 or more) occupants (see SOV). HOV lanes are dedicted facilities for the use of HOVs only (with exceptions such as motorcycles or electric vehicles, or for off-peak periods in some regions).

HOV Lane

A High Occupancy Vehicle lane is more commonly known as a carpool lane.

HPMS

The Highway Performance Monitoring System ( see: HPMS ).

HSR

High Speed Rail (see HSR).

Hybrid Vehicle

A vehicle that combines two propulsion systems, typically an electric motor and an internal combustion engine, to improve efficiency and decrease emissions.

Hydrocarbons (HC)

Compounds containing carbon and hydrogen that are the primary components of fossil fuels as well as forming emissions from the incomplete combustion of fossil fuels, some HCs are major air pollutants.

Hydrodynamic Analogy

Kinematic wave theory, introduced by Lighthill and Whitham (1955) is a macroscopic theory of traffic flow based on analogy to continuous, incompressible fluid flow: ∂k/∂t + (∂q/∂k)(∂k/∂x) = 0 where ∂q/∂k is the speed of the shock wave and where k = density (vpm); q = volume (vph); t = time (min); and x = distance (miles).

Hypothesis

An hypothesis is a proposition for which a test of validity is to be performed. The null hypothesis (H0) is a testable hypothesis: one for which a sampling distribution can be defined. This is typically a proposition that no relationship exists between variables of interest. Hypothesis testing is the process of attempting to reject H0, thus accepting the alternative hypothesis that the estimated relationship is valid.

Hysteresis

[1] The failure of a system to return to its initial state after a temporary shock
[2] The system property of a time lag between a cause and an effect.

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IATBR

International Association of Travel Behavior Research, an international association promoting research on activity and travel behaviour, including the organisation of triennial conferences on travel behaviour research. [ web ].

IATUR

International Association of Time Use Research, an international forum to promote time use data collection, analysis, and dissemination of research results [ web ].

ICE

Internal Combustion Engine (see ICE).

ICT

Information and Communications Technology (see ICT).

ICU

Intersection Capacity Utilization, a simple technique used for measuring intersection capacity based on critical movements analysis. Designed for planning applications in site impact studies and roadway design, versus delay-based methods for operations and signal design (such as HCS and SYNCHRO).

IEEE

Institute of Electrical and Electronic Engineers, or "I-triple-E", is the professional society whose role in transportation is the advancement of information technologies and communication standards (e.g., 802.11).

IIA

Independence from Irrelevant Alternatives ( see ILD ).

IID

Independently and Identically Distributed.

ILD

Inductive Loop Detector ( see ILD ).

Impact

Intended or unintended effects on the natural and built environments as a result of operation or implementation of transportation infrastructure and services.

Impact Fees

Fees assessed by municipal or regional government and charged to developers to mitigate for the degradation in traffic performance caused by the impacts of a particular proposed development project. Fees are based on established performance standards and/or the cost of the mitigation strategy.

Impedance

Impedance, a computed measure of the disincentive to travel due to spatial separation, is a composite function of travel time (often split by access, in-vehicle, and egress time), travel cost, and/or distance. Commonly utilized as only a function of travel time. Also known as deterrence or generalized cost (see skims).

Incidence Matrix

An efficient means of storing network topology relationships, including node-link and link-path incidence matrices.

Incident

A nonrecurring event that affects transportation system performance, including traffic accidents, disabled vehicles, roadway maintenance activities, and other flow disruptions.

Incident Management

Application of ITS methods and technologies to detect, identify, respond to, and manage traffic incidents in real-time.

Inclusive Value

The log of the logit model denominator and equivalent to the expected utility of the choice being modeled. Also known by the descriptive term logsum, the inclusive value is a summary measure of the desirability of the entire choice set and is directly connected to conventional welfare measures. In nested logit, the inclusive value is a component of the utility of the composite alternative representing a particular nest in the higher level of the choice structure.

Incremental Assignment

A trip assignment algorithm that loads predetermined increments of a trip table onto the minimum paths, then recalculates minimum paths and assigns the next increment, accumulating increments of volume at each step. This technique can produce results approximating UE solutions but determining the best set of increments can be difficult.

Incremental Elasticity Model

Incremental elasticity analysis facilitates application of elasticities to "what if" scenarios, by expressing the change in demand "D" as a function of the change in service "s" level and the associated elasticity, or: [ ΔD = eD|s • D • Δs / s ]

Incremental Logit Model

A quick response application of logit models where an estimated model, current market shares, and the proposed change in a specific policy variable are used to directly estimate policy impacts in lieu of a full model application,

Independence of Irrelevant Alternatives

IIA is a mathematical property of logit models where the ratio of choice probabilities for any two alternatives in independent of the presence of any other alternatives (see Red Bus / Blue Bus problem).

Independent Variable

An exogenous variable hypothesized to influence the value of a dependent variable (i.e., an explanatory variable). The term independent draws from the OLS assumption that multiple exogenous variables must be uncorrelated with each other (i.e., independent).

INDEX

INDEX is a GIS-based software package that analyzes and graphically presents the impacts of alternative planning scenarios using a range of design measures (such as the 5 Ds). INDEX estimates travel impacts such as changes in vehicle-trip rates and VMT and can be applied at spatial scales ranging from neighborhoods to regions. It was developed by Criterion Planners [ web ] based on the public-domain PLACE3S.

Indication

The illumination of one or more signal displays, such as a green arrow and a green ball allowing for left turning and through movements on a given approach to proceed.

Indifference Curve

A graph of the relative demands for two goods for which the utility derived remains the same and thus for which the consumer has no choice preference (AKA iso-utility curve).

Indirect Elasticity

An elasticity defined as the change in market share of an alternative due to a change in an attribute of a competing alternative (e.g., the change in transit demand due to auto fuel price increase). Also called a cross elasticity. Compare with direct elasticity.

Induced Demand

Travel demand generated by increased capacity or decreased price. The induced demand may represent trips that have been made to other destinations, by other modes and routes, and/or at other times, now reflected in a new travel choice in response to improved local performance, as well as representing new (latent) demand on the network. New demand can be associated with socio-demographic growth or may simply be demand that was suppressed by prior costs of travel.

Inductive Loop Detector

A solenoid loop of wire embedded in roadway pavement and connected to a sensor that records vehicles breaking the magnetic field above the loop. For a defined counting interval, the sensor typically records the number of such activations (count) and the proportion of time that the activation occurs (occupancy).

Infill

An infill area contains vacant land parcels surrounded by developed parcels. Associated infill development may be planned to increase the density or land use mix to address policy goals such as smart growth.

Information and Communications Technology

The full range of technologies for the generation, collection, archiving, processing, and communication of information, including software and hardware systems, ICT can complement and/or substitute for transportation (see IT).

INFORMS

INstitute For Operations Research and the Management Sciences, the international professional society in operations research and management science [ web ]. The Transportation Science and Logistics section focuses on OR applications in these special areas [ web ].

Infrastructure

The built environment -- in transportation, the components of the modal networks, terminals, and control systems that comprise the physical transportation system.

Input Output Analysis

An economic analysis acounting system that extends the economic base model to depict the flow of goods and services (represented in monetary or employment terms) within a region (or between regions). IO traces the required inputs to a particular economic sector and the distribution of outputs (to intermediate and final demands) via a matrix approach to provide employment projections and economic impact assessments.

INRETS

Institut National de REcherche sur les Transports et leur Sécurité, the French nationally-sponsored institute for transport and safety research [ web ].

Institutional Interaction

The spatial nature of transportation infrastructure and services over multiple jurisdictions and/or agencies often creates problems of institutional coordination, with different stakeholders having different managerial, operational, and financial policies. Efficient transportation systems require meaningful interaction between stakeholders in planning and development, design and construction, and management and operations.

Integrated Urban Model

Urban Model was used synonymously with land use model, but as travel forecasting models were integrated into land use models, and as land use models evolved to better reflect land use economics, the resulting term became integrated urban model.

Integration

The trip-based traffic microsimulation model that addresses freeways and arterials, as well as traffic assignment, simulation, and optimization.

IntelliDrive

Buzzword for a group of Vehicle Infrastructure Integration (VII) initiatives that provide drivers with improved situational awareness of event and hazards within the vehicle's environment.

Intelligent

An increasing trite buzzword implying a process or technology that facilitates transportation planning, design, or management, or increases the efficiency of transportation operations. Similar terms include "smart" and "advanced".

Intelligent Infrastructure

Infrastructure with integrated sensors and adaptive controls that sense the environment, assess system status, and actively or passively manage and control.

Intelligent Transportation Systems

Advanced methods and technologies to improve performance of existing transportation capacity, including Advanced Transportation Management Systems ( ATMS ), Advanced Traveler Information Systems ( ATIS ), and Advanced Vehicle Control Systems ( AVCSS ) (ITS, formerly IVHS).

Intercept Survey

A location-based sampling frame and survey. External surveys sample drivers at external stations to obtain basic O-D data (either roadside interviews or recording license plates for later contact). Transit on-board surveys sample transit users on transit vehicles for transit use surveys.

Intercity

Between cities, versus "intracity" meaning within a city, often referring to intercity rail service operated in the US by Amtrak, distinguished from intracity rail transit operations serving urban corridors.

Intermodal

Transportation planning or operations that involve more than one mode of transportation. Sometimes taken as the operational aspect that involves direct interactions between modes (such as in passenger or freight transfers). See also multimodal.

Internal Capture

In traffic impact analysis for mixed-use developments, internal capture is the proportion of trips attracted to a particular parcel that are drawn from other parcels in the development, thus reducing the site's traffic impact on adjacent streets (see also pass-by trips). The design of sustainable communities is in part based on a land use mix that accommodates internal trips being made by public or non-motorized modes or with shorter trip lengths.

Internal Combustion Engine

The motive power of the majority of current vehicles is provided by the combustion of fuel and air within an engine, which transfers the resulting mechanical motion to the vehicle drive train.

Internal Rate-of-Return

In project appraisal, the effective annual compound interest rate that yields a net present value of zero (a benefit cost ratio of 1.0), providing a measure of investment efficiency.

Internal Trip

A trip with both its origin and its destination located inside of the study area. If either trip ends is outside the study area, it is an external trip. If both trip ends are outside the study area, it is a through trip.

Interoperability

The ability of multiple systems to efficiently exchange and utilize information, interoperability applies to general systems and organizations but has been most commonly referenced with respect to communication systems in transportation.

Intersection Capacity Analysis

Capacity analysis for uncontrolled, sign-controlled, or signal controlled intersections, a key stage in traffic impact analysis.

Interval

The time period during which all signal indications on all approaches remain the same.

Interval Scale

A scale used to measure a variable for which the intervals between values are defined but with an arbitrary zero point. An example in travel forecasting in departure time. See measurement scales.

Intervening Opportunities Model

An infrequently applied alternative trip distribution model that probabilistically estimates destination choice as a function of the number of activity options available between the trip origin and any given destination zone.

Interzonal

Between two different analysis zones (versus intrazonal, or within a zone).

Intrazonal

Within a single analysis zone (versus interzonal, or between zones).

Intrazonal Trip

A trip with both origin and destination in the same zone. In trip assignment, intrazonal trips are not loaded on the network.

In-vehicle

The portion of a trip that occurs while traveling in a vehicle as opposed to the out-of-vehicle portion that includes access, waiting, transfer, and egress times). Disutilities are in general lower for in-vehicle time.

In-Vehicle Navigation System

A way-finding technology for automobiles that combines a map database (often on a CD) with a graphic and/or audio user interface, typically integrated with a GPS to provide drivers with real-time directions, and potentially integrated with a cellular feed to provide real-time traffic conditions.

Inventory

A term used in the past for the process and results of a comprehensive cataloging of existing conditions in a study area, including transportation infrastructure and services; traffic volumes and level-of-service by mode; land use; and transport impacts (noise, emissions, etc.), which serve as basic inputs to the transportation modeling process.

Inverse Power Function

In gravity models, a variety of function forms have been used for travel impedance in trip distribution, including inverse power, negative exponential, and combined (gamma) functions, as well as the conventional friction factors. The inverse power function results from assuming a logarithmic total cost constraint in the entropy maximization derivation:
f(tij) = tij

IPF

Iterative Proportional Fitting (see IPF).

ISTEA

The Intermodal Surface Transportation Efficiency Act of 1991, the federal transportation authorization that continued the series of Federal Aid Highway Acts but incorporated significant revisions to metropolitan and statewide planning processes, emphasizing an intermodal approach and strengthening the role of metropolitan planning organizations in the overall planning process. ISTEA also implemented the Congestion Mitigation and Air Quality Improvement Program (CMAQ). ISTEA was followed by TEA-21, the Transportation Equity Act for the 21st Century (1998), and SAFETEA-LU, the Safe, Accountable, Flexible, and Efficient Transportation Equity Act -- A Legacy for Users (2005) [ web ].

IT

Information Technology is the design, development, implementation, and operation of computer-based information software and hardware systems.

ITE

The Institute of Transportation Engineers, an international association of transportation professionals directed primarily toward planning, design, implementation, and operations of transportation infrastructure and services [ web ].

ITE Trip Generation

The ITE Trip Generation report is a compendium of trip generation rates primarily used in traffic impact studies. Average rates and estimated models are provided by land use, time period, and a variety of explanatory variables.

Iterative

A procedure that repeats calculations in a loop but does not utilize self-reference as in a recursive procedure. Whereas a recursive procedure F(n) to compute factorial (n) would itself call F(n-1), an iterative procedure would call F(n) from a main program for each separate evaluation of n. Both imply a cascading process where the output of one step is the input to the next.

Iterative Assignment

A family of trip assignment algorithms that solve for optimal flows via iterative application of an elementary loading algorithm (e.g., All-or-Nothing Assignment). The results of two successive iterations are combined to find the volumes and corresponding travel times for the next iteration (e.g., see Frank-Wolfe or MSA).

Iterative Proportional Fitting

A technique to generate population cross classification cell frequencies, given marginal population frequencies, consistent with sample cross classification cell frequencies. IPF is used in the generation of synthetic populations for activity-based models.

ITLUP

Integrated Transportation and Land Use Package is a land use model software package, integrating the DRAM/EMPAL spatial interaction models, and applied in many metropolitan transportation studies over the past few decades (Putman, 1983). Other land use software packages include MEPLAN, MUSSA, PECAS, TRANUS, and UrbanSim.

ITS

[1] The Institute of Transportation Studies of the University of California [ web ]
[2] Intelligent Transportation Systems (see ITS) [ web ].

ITS America

National organization advocating the development and application of advanced technologies for surface transportation (e.g., Intelligent Transportation Systems, ITS), comprising private firms, public agencies, and academic institutions [ web ].

ITS Architecture

A structural framework defining the interrelationships between intelligent transportation system (ITS) components.

IVHS

Intelligent Vehicle Highway Systems, now referred to as ITS.

IVNS

In-Vehicle Navigation System (see IVNS).

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Jacobian

The vector of all first-order partial derivatives of a specified function (see Hessian).

Jam Density

A facility's maximum density (vehicles per mile), where spacing and space mean speed approach zero (when cars are "bumper to bumper").

Jitney

A form of paratransit characterized by smaller vehicles operating on a fixed route but stopping anywhere for boarding/unboarding and not running on a fixed schedule. Compare with conventional or demand-responsive transit operations.

Jobs-Housing Balance

The spatial distribution of employment relative to the distribution of workers (by residence) within a defined area. An area with a balance of jobs and housing would imply a greater likelihood that a worker would find a job nearby, minimizing commute trip length. Variations in job and worker types require a concise definition of balance as having complementary job and housing characteristics.

Joint Development

Coordinated public and private sector land development, typically around transit stations, with the public sector offering development incentives in return for the private provision of transit-compatible development.

Journey to Work

The trip to work (AKA commute). In the decennial census, the associated distributions of mode and travel time.

Jurisdiction

A level of government (city, county, state, or federal) or regulatory authority (local, regional, state, or federal) responsible for some or all aspects of the planning, implementation, operations, and maintenance of transportation facilities and services in a defined area.

Just In Time

An inventory control strategy where resources are delivered precisely when necessary, minimizing inventory costs, but potentially resulting in service problems when variation in demand is not fully anticipated.

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K Factor

[1] Adjustment factors sometimes applied to trip distribution gravity models in an attempt to replicate specific trip table cell-to-cell flows. These factors attempt to capture aspects of travel behavior that are not captured via productions, attractions, and impedance, but it is difficult to assess the impact of these factors in forecasting. Some sources recommend limited use of K-factors where needed, while other sources recommend a more liberal use to improve the fit of a base model.
[2] The design hour volume (DHV) expressed as a percentage of the annual average daily traffic (AADT).

Kalman Filter

A class of filters (recursive algorithms) for predicting or smoothing time series data, used with traffic detector data for travel time and OD estimation. A Kalman Filter estimates a linear process via feedback control with a state equation projecting system variables and a measurement equation providing feedback by incorporating new data in the estimation process. Non-linear state and measurement equations need an extended Kalman Filter.

Karush-Kuhn-Tucker

The necessary conditions for an optimal solution to a constrained non-linear program.

Kinematic Wave Theory

Introduced by Lighthill and Whitham (1955) and Richards (1956), kinematic wave theory is a macroscopic theory of traffic flow based on analogy to continuous, incompressible fluid flow, or: ∂k/∂t + (∂q/∂k)(∂k/∂x) = 0 , where ∂q/∂k is the speed of the shock wave and where k = density (vpm); q = volume (vph); t = time (min); and x = distance (miles) (also known as the hydrodyanamic analogy).

KKT

See Karush-Kuhn-Tucker conditions.

Knight, Frank

American economist (1885-1972) and a founder of the Chicago school who formulated early arguments for toll roads and a statement for traffic equilibrium in 1924 (which led to Wardrop's principles in 1952).

Knowledge-based System

A software system that incorporates quantitative and qualitative professional expertise and judgment in a database integrated with an inference engine to emulate the decision process of experts in a particular area ( also know as Expert System ).

Kriging

A geostatistics technique similar to least squares regression that interpolates values for unsampled locations from observed data at neighboring sampled locations.

Kth Shortest Path

A variant of the minimum path problem where the shortest path, the 2nd shortest, through the kth shortest are found. Algorithms include the Double Sweep and generalizations of Dijkstra's and Floyd's single path algorithms.

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Label-correcting

Minimum path algorithms assign node labels based on cumulative travel time from the origin node. Algorithms such as Moore's or Floyds correct node labels at each iteration as better paths are found (compare with label-setting algorithms). Labels are final only after all nodes have been precessed (some processed multiple times).

Label-setting

Minimum path algorithms assign node labels based on cumulative travel time from the origin node. At each iteration, algorithms such as Dijkstra's set the label for the node closest to the origin as permanent thus adding it to the minimum path tree (compare with label-correcting algorithms).

Labor Force

Workers defined by residential location, versus employment defined by job location.

Lagging Left Turn

A left-turn phase that begins after the start of the approach's thru movement (see leading left).

Lagrange Multiplier

A solution method for constrained, non-linear, optimization problems. Each constraint is multiplied by a scalar Lagrange Multiplier and is added to the original objective function, changing a problem with N variables and M constraints to a problem with N+M variables and no constraints. The multiplier indicates the amount the objective function will change with a unit change in the corresponding constraint (see shadow price).

Land Use

The primary activity for which a parcel of land is used (residential, commercial, industrial, open space, undeveloped, etc.). Municipal zoning and general plans identify legal designations of current and planned land use.

Land Use Data

Description of the amount of land for specified land use designations, defined spatially by various zoning systems (e.g., parcels, TAZs). Amounts may be represented in areal units (e.g., acres), by activity descriptors (e.g., number of housing units), or densities (e.g., population per square mile). In travel forecasting, land use data includes these quantitative and quantitative attributes as well as demographic and socio-economic data that describe the population utilizing the land in question. Together these define the activity system.

Land Use Model

One of an extensive range of quantitative models and procedures that describe land use patterns (the activity system) of a region. These models typically describe and predict the spatial distribution of population and employment, and the corresponding land consumed. While transportation networks are typically incorporated in the activity location sub-models, historically, land use models were not fully integrated with travel forecasting models. However, recently developed land use models such as MEPLAN, MUSSA, and UrbanSim are integrated land use transportation models.

Lane-miles

One lane-mile corresponds to a mile-long segment of a single lane. Total lane-miles is a measure of total road infrastructure capacity. Compare to center-line miles.

Latent Demand

Travel demand is a relationship between the price of travel and the quantity of travel demanded. Travel that corresponds to that part of a demand relationship that is not being realized due to limited capacity and/or high price is considered latent demand (observed travel may increase as these constraints are relaxed). See also induced demand.

Lead

A criteria pollutant, lead (Pb) is a stable, heavy metal that had been used extensively (e.g., in paint, batteries, and pipes) but is highly poisonous to humans and animals.

Lead Lag

A signal phasing plan with a leading left turn opposed by a lagging left turn.

Leading Left Turn

A left-turn phase that begins before or simultaneously with the approach's thru movement (see lagging left).

LED

Light Emitting Diode (see LED).

LEED

Leadership in Energy and Environmental Design is a system for ranking infrastructure projects based on environmental sustainability.

Legacy

A generic term for older versions of software, technologies, etc.

LEV

Low Emission Vehicle: a vehicle with emission levels below a threshold based on standard vehicles.

Level of Service

[1] A set of quantitative or qualitative measures of transportation system performance. The Highway Capacity Manual defines levels of service (LOS) for traffic operations as ratings ranging from A (best) to F (worst).
[2] In Transportation Systems Analysis (TSA), performance functions model LOS as a function of system characteristics (such as speed and capacity) and link volume.

Licensed Driver

An individual possessing a legal license to drive a vehicle on public roads. The number of licensed drivers in a household is strongly correlated with the total number of generated trips; individual mode choice is strongly correlated with possession of a drivers license.

LIDAR

LIght Detection And Ranging, a remote sensing technology, locates objects on a principle similar to radar but using light from a laser.

Life Cycle

[1] Life style reflects an individual's or household's activity and travel preferences, needs, and constraints as a function of demographic, socio-economic, and geographic characteristics. The progression through various life styles is termed the life cycle (childhood, living alone, married couple, married with children, etc.) and role is the social position held by an individual in a particular life cycle stage (single adult, parent, etc.).
[2] The total productive time span of a system from construction to salvage, often reflecting the associated costs and benefits over that time period.

Life Cycle Cost Analysis

An engineering economics analysis tool to evaluate the lifetime economic consequences of mutually exclusive project alternatives.

Light Emitting Diode

An energy-efficient, long-lifed, micro-electronic device for traffic signal and CMS displays.

Likelihood

An estimated value of the likelihood function, for a given set of parameters, providing a measure of the probability that the model replicates an observed distribution.

Likelihood Function

A mathematical expression, usually expressed as a log likelihood, that indicates the probability that a model with a given set of parameters would replicate the observed choices of a sample, or: L(β) = Σn log f( xn | β). See maximum likelihood estimation.

Likelihood Ratio

A test of model significance in maximum likelihood estimation and analagous to the F-test in regression, the likelihood ratio compares the log likelihood of an estimated model [L(β)] with that for the null hypothesis that all model parameters equal zero [L(0)], with the resulting statistic, LR, being chi-square distributed: LR = 2 [ L(β) - L(0) ]

Likert Scale

Measures the degree to which a survey respondent agrees or disagrees with the question being asked.

Limited Access

In the functional classification of roads, limited-access facilities such as freeways, expressways, and tollways place primary emphasis on service to through traffic rather than local land access. Facilities are typically grade-separated and without traffic control (other than at access/egress points such as ramps). Interchanges are usually limited to other limited-access facilities and arterials. Arterials and limited-access facilities account for less than 10 percent of the lane-miles in the US, but almost 75 percent of the VMT. In travel forecasting networks, limited access facilities are virtually always coded, often with great detail regarding directional separation and interchange geometry.

Line Haul

The portion of a commute trip associated with the largest segment of total travel time or distance. For an individual who drives a short distance to a Park and Ride lot, then takes and express bus 30 miles to an activity center and walks 5 minutes to a work place, the express bus portion is the line-haul portion. Typically refers to mass transit systems such as commuter rail or express bus. See access and egress.

Line Sources

Point, line, and area sources are idealized zero-, one-, and two-dimensional locations that are sources for various pollutants. Roadway links are line sources for both noise and air quality pollutants.

Linear Program

A mathematical optimization problem with a linear objective function and linear constraints. In trip assignment, while the all-or-nothing formulation is a linear program, UE and SO formulations have non-linear objective functions and linear constraints.

Link

A fundamental element of a transportation network defined by a starting and an ending node and having attributes such as length, travel time and/or speed, and capacity. Represented in travel forecasting models by link performance functions.

Link Interaction

Conventional traffic assignment formulations assume that link performance functions are independent (a link's performance is dependent only on that link's volume), but interactions occur at intersections as well as with "rubbernecking" on freeways. Algorithms available in conventional software do not reflect the associated effects.

Link-Path Incidence Matrix

A matrix of network topology containing M (links) rows and K (paths) columns (for one or more origin-destination pairs) with each element equal to one if link m is on path k for a given OD pair (and zero otherwise). A column k thus identifies all of the links on path k; a row m thus identifies all of the paths k that contain a given link m (see: incidence matrix ).

Linked Trip

A journey by transit from an origin to a destination may have multiple segments that are completed on different vehicles or modes. While each segment involves a transfer between transit vehicles or modes, the overall O-D travel is considered one linked trip even though it produces multiple transit boardings (and thus multiple unlinked trips ).

Link Performance Function

A volume-delay function, a functional performance relationship between level-of-service (e.g., travel time) and link volume for a specific facility. The classic example is the BPR link performance function.

Livability

The environmental quality of a community as perceived by residents, employees, and visitors. The environmental qualities include social, cultural, aesthetic, ecological, and other aspects of the community. From a transport perspective, livability reflects the quality of transportation alteratives available and the impacts of those alternatives on the community. See also person-environment fit.

Load Factor

For a transportation vehicle, route, or other service, the ratio of capacity utilized to total capacity.

Loading

The assignment of O-D demands to a network. A generic term for trip assignment.

Local Jurisdiction

A level of government or regulatory authority responsible for some or all aspects of the planning, implementation, operations, and maintenance of transportation facilities and services for a local area (e.g., city or county).

Local Model

The phrase local models (or travel models used at the local level) may be interpreted as: (a) models that are developed by city or county agencies and/or are applied at the city or county level (although increasingly reflecting regional model characteristics) and/or
(b) alternate models that are directed toward capturing traffic impacts of evolving land use policies at the local level (contrast with regional land use models). Local models provide inputs to micro-simulation analyses; to infrastructure and control system design; and to planning, investment, and operation decisions, including the review of the effects of local land use projects, general and specific plans, and other transportation system elements.

Local Streets

In the functional classification of roads, local streets provide access to local land uses. Local streets are low speed and low capacity and often lack signalized traffic control. Local streets account for about two thirds of the lane-miles in the US, but less than 10 percent of the VMT. In travel forecasting networks, local streets often are not coded but are indirectly represented by centroid connectors.

Locally Preferred Alternative

The specific project that is selected by the lead agency following the completion of a Major Investment Study (or any Alternatives Analysis process). Also known as the Locally Preferred Strategy.

Location Quotient

A sectoring method used in economic base analysis to estimate the proportion of basic employment by economic sector, assuming that local demands are met by a level of economic activity proportional to that in a larger, reference economy, with remaining activity being basic.

Logistic Growth

[1] While the assumption of unconstrained natural increase of a population yields the exponential growth model (Malthus, 1798), an assumption of finite resources yields the logistic growth model where growth is a linearly decreasing function of the current population (Verhulst, 1838), resulting in an S-shaped growth curve.
[2] A logistic map results from the discrete version of the logistic growth equation that exhibits chaotic behavior (the resulting bifurcation diagram is a fractal).

Logistics

The component of supply chain management involving the planning, implementation, and control of processes for product distribution. Supply chain management has taken on the broader definition that encompasses the full production chain from raw materials to final consumption, usually involving multiple actors. The term logistics is often used when referring to operations research and other tools used in the analysis and design of logistic systems.

Logit Model

A discrete choice model that assumes an individual maximizes utility in choosing between available alternatives. The logit model's utility function comprises a deterministic component (which is a function of measurable characteristics of the individual and of the alternatives in the individual's choice set) and a stochastic component (or error term) assumed to have an extreme value distribution (see also Probit Model; Nested Logit Model).

Log Likelihood

For logit models, the log likelihood function is the sum over all individuals of the log of each individual's estimated probability for the observed choice alternative. In maximum likelihood estimation, the log of the likelihood function is maximized by differentiating with respect to model parameters and employing an iterative process to update parameters.

Log-linear Model

Models formulated as a linear relationship between independent variables and the logarithm of the dependent variable.

Logsum

The log of the logit model denominator, equivalent to the expected utility of the choice being modeled, and thus used in nested logit models (see inclusive value).

Longitudinal

A survey of and data from a sample of observations collected at repeated points over a period of time. Compare with cross sectional surveys and data.

Long Range Transportation Plan

The Long Range Transportation Plan (LRTP) is a regional (or statewide) consensus plan that defines the region's (or state's) transportation system and planned improvements scheduled for funding over the next 20 years.

Long Term

In transportation planning, a time frame of generally 20 years, corresponding to the long range transportation plan horizon.

Look-Up Table

In travel forecasting software, link speed and capacity can be coded individually for each network link or by using a look-up table that provides default speed and capacity for links defined by facility type and area type.

Loop

Common name for an Inductive Loop Detector ( see ILD ).

Loose-ends Table

In network algorithms, a list of nodes which have been reached and processed. In minimum path algorithms, the list contains nodes reached but not permanently labeled.

Lorenz Curve

A graph of some cumulative population distribution, often income, showing the cumulative percent of the characteristic (income) versus the cumulative percent of households (see Gini Coefficient).

LOS

Level of Service (see LOS).

LOV

A Low Occupancy Vehicle is the complement of an HOV, or high-occupancy vehicle, and a more general term for a SOV, or single-occupant vehicle, as the definition of required HOW-occupancy evolves from 2 or more to 3 or more.

Lowry Model

One of the first large-scale urban land use models, the Lowry Model (1964) combined an economic base model with spatial allocation models to generate and allocate population and service employment, within constraints on land availability and compatible land uses (see the Garin-Lowry Model).

LPF

Link Performance Function (see LPF).

LPG

Liquid Propane Gas.

LRTP

Long Range Transportation Plan (see LRTP).

LUTRAQ

The Land Use TRansportation Air Quality study was commissioned by the 1000 Friends of Oregon to analyze alternative land use patterns and design standards, and to estimate the associated transportation impacts (e.g., mobility, air quality, energy consumption).

LUTRIM

Land Use Transportation Integrated Model, developed by Mann (1995), is a Lowry-type land use model integrated with a four-step transportation planning model.

LWR Model

A macroscopic theory of equilibrium traffic flow developed by Lighthill and Whitham (1955) and Richards (1956) based on analogy to continuous, incompressible fluid flow (also known as Kinematic wave theory).

Lyft

A "shared economy" technology firm that provides transport services for profit where the economic transaction ("sharing") is between the technology provider, a contracted physical provider, and a customer paying for the service. (see also Uber). The technology is a smart phone application that links customers seeking immediate transportation with affiliated drivers in the vicinity.

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Macroscopic Model

A model that describes system operations in aggregate terms (using TAZs in four step models or using platoons for traffic flows) (see Microscopic Model; see also aggregate).

Major Investment Study

A Major Investment Study (MIS) is an evaluation of the cost-effectiveness of alternative transportation investments in addressing stated goals and objectives. An MIS typically produces a ranking of proposed alternatives based on relative performance and cost.

Major Operating Segment

A section or operating component of a transportation project that essentially can function independently of the remainder of the project, such as the initial section of a planned regional rail transit system.

Managed Lanes

Designated traffic lanes that utilize a variety of traffic management strategies such as metering, pricing, or access control to more efficiently utilize existing capacity.

MAP-21

Moving Ahead for Progress in the 21st Century, the surface transportation authorization bill for 2013-2014, passed by Congress in July 2012 to replace SAFETEA-LU, with am emphasis on improving transportation investment decisionmaking through performance-based planning and programming.

Marginal Cost

[1] The change in total cost (see average cost).
[2] In travel forecasting, cost is usually taken as either travel time or generalized cost for network links or O-D pairs. In trip assignment, equilibrating marginal costs of link performance functions leads to a system optimal result.

Marginal Rate of Substitution

In a utility function, MRS is the ratio at which one choice alternative substitutes for another along an indifference curve, equal to the ratio of marginal utilities. Under a budget constraint, MRS equals the ratio of unit prices.

Market Clearing

A price or other condition where demand equals supply.

Market Segment

A group of users of a transportation system that share common travel or socio-economic attributes.

Market Share

The proportion of trips captured by a particular mode, such as bus transit capturing two percent of the total trips in a region.

Markov Chain

A stochastic process having the Markov Property where the future probability distribution is conditional on only the current state and not on the path of past states (a "memoryless" random process).

Mathematical Programming

The general case of a linear program where a general objective function is optimized subject to linear or non-linear, equality or inequality, and/or non-negativity constraints.

Maximum Likelihood

Maximum likelihood estimation (MLE) finds a model's parameters that will generate the observed choices with the greatest likelihood. More general than least squares estimation, MLE is used to estimate discrete choice models.

McTrans

The Center for Microcomputers in Transportation established by FHWA in 1986 for the distribution and support of software for transportation applications [ web ].

Mean Free Speed

The average speed of vehicles traveling unimpeded on a defined section of roadway.

Measurement Error

The difference between a true value and an observed or reported survey value due to factors such as the survey instrument, the observer, or other elements of the data collection process.

Measurement Scales

The four general metrics for measuring variables: nominal, ordinal, interval, and ratio. The measurement scale determines the type of mathematical operations that are possible (e.g., for central tendency, nominal scales can use only mode, ordinal scales can use mode and median, and interval scales can use mode, median, and mean). The distinction may often be subjective regarding these scales, depending on what the variable is actually measuring (e.g., increasing the number of cars in a household from 1 to 2 and from 2 to 3 represents the same numerical increase in the number of cars but not necessarily a similar increase in the level of mobility provided).

MEPLAN

A land use model software package developed by Echenique & Partners (1987), based on a spatial input-output framework. Similar packages include MUSSA, PECAS, TRANUS, UrbanSim, and ITLUP.

Mesoscopic Model

A model defined between conventional macroscopic and microscopic models, blending attributes of each. For example, in traffic simulation a mesoscopic model might represent individual vehicles but define movement in platoons (for example, see DYNASMART).

Methane (CH4)

A non-toxic greenhouse gas, but not a criteria pollutant, methane is the main component of natural gas (perhaps the most abundant fuel on earth), a contributor to degradation of the ozone layer, and a significantly more potent GHG than carbon dioxide (CO2).

Method of Successive Averages

An iterative assignment algorithm, MSA solves for optimal link flows through the iterative application of an elementary loading algorithm (e.g., Dial's for SUE or AON for UE). At each iteration, link volumes are averaged with the loading results of all previous iterations, and are used to determine link travel times for the next iteration. MSA, a feasible direction method with a predetermined move size, converges non-monotonically. SUE can not use Frank-Wolfe and thus uses MSA. In the four step model, MSA is used in feedback to trip distribution to achieve system convergence.

Metropolitan Area

Core areas of substantial centralized population and the adjacent economically and socially integrated areas (see census areas and metropolitan classification).

Metropolitan Classification

The standard urban/suburban/rural trichotomy expanded to include the hierarchy of urban, suburban, exurban, and rural, terms such as edge cities and conurbation, and various geographic scales at which travel forecasting is performed, including local, metropolitan, regional, and statewide. Note that these classification terms are often both subjective and relative with no fixed definition.

Metropolitan Planning Organization

The MPO is the local or regional agency designated by the state to coordinate federal transportation planning requirements, including growth management and air quality.

METROSIM

METROSIM is an urban land use simulation model developed by Anas (1998) that employs discrete choice models and market-clearing mechanisms for residential and commercial land use, emploment, and transportation flows.

ME2

Maximum Entropy Matrix Estimation, a technique for updating OD trip tables based on maximum entropy principles and using observed traffic counts.

Microscopic Model

A model that describes system operations in terms of individual system elements (such as individual vehicles in traffic microsimulation models or individual persons in activity microsimulation models) (see Macroscopic Model; see also disaggregate).

Microsimulation

A microscopic simulation focusing on the behavior of individual system elements, such as individual persons or vehicles.

Midday Period

The time period between the end of the AM-peak and the start of the PM-peak period.

Migration

The relocation of population from one region to another that, together with birth and death rates, are the determinants of population change.

Minimum Path

A path is a sequence of nodes and links in a network. A minimum path is the optimal (shortest) path, measured by travel time, distance, or generalized cost from a defined origin to a defined destination. In travel forecasting, algorithms (such as Dijkstra's) find the minimum path tree and are used to generate travel time matrices for trip distribution models and as an integral part of trip assignment models.

Minimum Path Tree

A tree is the set of minimum paths from a specified origin to all destinations.

Minimum Requirement

A sectoring method used in economic base analysis to estimate the proportion of basic employment by economic sector, assuming that local demands are met by a level of economic activity equivalent to the minimum of comparable, reference economies, with remaining activity being basic.

MinUTP

A software package for travel forecasting and transportation network analysis. Developed by Comsis based on the original UTPS and now supported by Citilabs [ web ]. Other travel forecasting packages include Cube, EMME/2, QRSII, T-Model, Tranplan, and TransCAD.

MIS

Major Investment Study, see MIS.

MIST

Management Information System for Transportation, an ITS system that integrates communications, data and video systems, incident detection, and traffic control devices in a comprehensive transportation management system [ web ]

Mitigation

To reduce the impact of a proposed project by developing counter-measures to restore the impacted area to prior or otherwise acceptable conditions. Perhaps most commonly linked with traffic impact studies where the traffic impacts of a proposed development are estimated and a plan to ameliorate the impacts is proposed and funded by the developer, evaluated by the appropriate govenment agency, and implemented as part of the project. Some jurisdictions charge impact fees which may fund a portion of future network improvements based on the TIS.

Mitigation Measures

Specific design commitments made as part of a traffic impact study or environmental impact study that serve to reduce the impacts of a proposed project. These measures may include planning and development commitments, environmental measures, and/or right-of-way improvements.

MITSIM

A microscopic traffic simulation software developed at MIT [ web ].

Mixed-Flow

Traffic flow characterized by or facilities defined for utilization by single and multiple occupant automobiles, trucks, buses, and motorcycles sharing traffic lanes.

Mixed Logit

Relaxes standard logit by allowing respondents individual inter-related deterministic and random utility components for each choice alternative (e.g., utility function coefficients can vary over respondents). Mixed logit can approximate any other choice model.

Mixed Use

A land use classification characterized by a variety of compatible land uses, such as a mix of residential, commercial, and public land uses found in historical downtowns and some New Urbanist development.

MOBILE

EPA's software package for calculating motor vehicle emissions. Mobile 6.2 is the current version (see MOVES EMFAC) [ web ].

Mobile Sources

Mobile sources are non-fixed sources of pollution including on-road vehicles such as cars, trucks, and buses, and off-road sources such as trains, planes, and construction equipment. Mobile source pollutants include NAAQS criteria pollutants such as carbon monoxide (CO), hydrocarbons (HC), nitrogen oxides (NOx), and particulate matter (PM) (see stationary source).

Mobility

Mobility is a measure of the degree to which the demand for personal travel is achieved, measured by a variety of system performance indicators (see also accessibility).

Mode

A means of conveyance between origins and destinations, modes are motorized (cars and other private vehicles, buses, rail transit) and non-motorized (walking, bikes). In the US, about 90 percent of all travel is by private vehicles, with the remainder by other modes (percentage varies over activity and transportation characteristics and over geographic areas).

Mode Choice

Mode Choice (MC) is the third step in the conventional four step model of travel forecasting. MC is the process by which a traveler chooses a transportation mode for a trip, given the trip's purpose, origin, and destination (the results of the first two steps of the four step model); characteristics of the traveler; and characteristics of the modes available to the traveler. Mode choice typically follows trip distribution in the four step model sequence. Historical use of aggregate mode choice has given way to the multinomial logit as the preferred mode choice formulation.

Mode Split

The market share of trips by each of the transport modes serving an area. The historical application of aggregate mode share models was often referred to as "modal split".

Model

An analytical, typically mathematical, abstraction of reality used in transportation as a tool to describe, predict, or optimize travel and activities, land use and economic activity, traffic flow and performance, and associated environmental impacts. A model of a transportation system comprises a set of rules that relates defined inputs to outputs.

Model Specification

The process of defining model structure, functional form, variables, and parameters (see estimation, calibration, validation, and application).

Modified Areal Unit Problem

A bias associated with arbitrary spatial zoning systems where the associated data may be artifacts of the zoning delineation rather than the underlying spatial behavior of the population.

MOE

A Measure of Effectiveness is a variable that is designed to assess system performance. Also know as a Performance Measure.

Monocentric Model

The traditional model used to analyze the spatial structure of cities and characterized by continuously declining density gradients from a central population and employment core. This simplified structure facilitates presentation, analysis, and underestanding of urban theory. Compare with polycentric model.

Monte Carlo Simulation

A simulation method that use random numbers rather than deterministic algorithms to model complex system behavior.

Moore's Algorithm

An efficient algorithm to find the minimum path tree for a given origin based on the iterative updating of alternative paths to network nodes. Alternate algorithms (e.g., Dijkstra's) differ primarily in the sequence that nodes are evaluated for inclusion on the minimum path. Moore's is a label-correcting path algorithm.

Most Probable Distribution

In Entropy Maximization, the solution to maximizing the combinatorial objective function subject to appropriate constraints is a trip matrix that is the most probable distribution that could have given rise to the observed constraints.

Motorized

Transportation modes that are powered by motors, either ICE or electric, such as automobiles, trucks, and most forms of public transit.

MOU

Memorandum Of Understanding: a document defining the general responsibilities of two or more entities to achieve defined goals and objectives.

MOVES

EPA's MOtor Vehicle Emission Simulator to estimate mobile source emissions for state implementation plans and transportation conformity analyses [ web ].

MPO

Metropolitan Planning Organization (see MPO).

MSA

[1] Metropolitan Statistical Area (see census areas).
[2] Method of Successive Averages (see MSA).

Multiclass Assignment

Network assignment that provides for the loading of multiple user classes, which can have differential access to portions of the network (e.g., HOV lanes), different link performance functions, and separate trip tables.

Multimodal

Transportation planning or operations that involve more than one mode of transportation. Sometimes taken as a coordinated planning focus on two or more modes of transportation. See also intermodal.

Multinomial Logit

A logit model for choice among more than two alternatives (referred to as "binary logit" when the choice is between only two alternatives).

Multipath Assignment

A trip assignment process that utilizes stochastic loading, resulting in O-D flows being assigned to multiple paths (rather than the shortest path as in deterministic assignment).

Multiple Classification Analysis

MCA is a statistical modification of category analysis used in trip generation. When an insufficient number of data points exist to compute reliable trip rates for all categories, MCA utilizes row, column, and overall means to estimate rates for all cells.

Multiple Nuclei Model

A descriptive model of urban development developed by Harris and Ullman (1945) that retains the central business district (CBD) as well as other elements of the Concentric Ring and Sector models but reflects non-CBD activity centers serving as additional centers of evolving urban patterns.

Multiplier

Defined indices that measure the direct and indirect economic effects of employment and income changes by sector of the economy.

MUSSA

A microeconomic land use model software package developed by Martinez (1996) that represents the equilibrium behavior of land use supply and demand. Similar packages include MEPLAN, PECAS, TRANUS, UrbanSim, and ITLUP [ web ].

MUTCD

FHWA's Manual of Uniform Traffic Control Devices defines standards for the installation of traffic signals and signs, pavement markings, and other traffic control devices [ web ].

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NAAQS

National Ambient Air Quality Standards are the Federal standards that set allowable concentrations and exposure limits for criteria pollutants, including ozone, carbon monoxide, nitrogen oxides, sulfur oxides, particulate matter, and lead. Developed by the EPA under CAA. See current standards [ web ].

NAFTA

North American Free Trade Agreement (see NAFTA).

NAICS

North American Industry Classification System, the North American industry clasification taxonomy that replaces the Standard Industrial Classification (SIC) [ web ]

Naive Aggregation

An aggregation technique that uses a group's average values for variables in a disaggregate model. Valid for linear models, but introduces aggregation bias in non-linear models (see sample enumeration).

NARC

National Association of Regional Councils, a national public interest organization established as an advocate for Councils of Government (COGs) and Metropolitan Planning Organizations (MPOs) [ web ].

Nash Equilibrium

In game theory, a Nash equilibrium (after Nobel laureate John Nash) exists when no player can unilaterally benefit by changing strategy. Equivalent to UE trip assignment.

National Highway Institute

The National Highway Institute is part of the FHWA that provides transportation-related training [ web ].

National Highway System

Approximately 160,000 miles of roads deemed critical to the economy, defense, and mobility, including the Interstate system.

National ITS Architecture

A common framework for planning and integrating ITS interoperability, defining the system functions, physical components, and corresponding information flows. The architecture is maintained by USDoT [ web: NITSA; Version 6.1 ].

National Transportation Atlas Database

A BTS product comprising a set of domestic geographic databases of transportation facilities, transportation networks, and associated infrastructure [ NTAD 2012 ].

Natural Sources

Although anthropogenic sources of pollution are significant, about half of all air pollutants are from natural sources, including soils and plants, forest fires and volcanic activity, and decomposition of organic matter. Natural sources are dispersed, thus their concentration in metropolitan areas is often minimal.

NBER Model

National Bureau of Economic Research land use model was developed by Ingram (1972) to simulate housing markets and policies.

NCHRP

National Cooperative Highway Research Program, sponsored by the Transportation Research Board (TRB), supports applied research that develop near-term, practical solutions to problems facing transportation agencies [ web ].

NCHRP 187

TRB's NCHRP Report 187 Quick-Response Urban Travel Estimation Techniques and Transferable Parameters (1978) documented "transferable parameters, factors, and manual techniques for simplified travel forecasting". See NCHRP 365.

NCHRP 365

TRB's NCHRP Report 365 Travel Estimation Techniques for Urban Planning (1998) updated NCHRP 187.

Negative Declaration

A lead agency's determination that a proposed action will not result in any significant adverse environmental impacts, ending the environmental review process. A positive declaration states that the proposed action is likely to have a significant adverse impact and that an environmental impact study (EIS) will be required.

Negative Exponential Function

[1] In gravity models, a variety of function forms have been utilized for travel impedance in trip distribution, including inverse power, negative exponential, and combined (gamma) functions, as well as the conventional friction factors. The negative exponential function results directly from the standard entropy maximization derivation: f(tij) = exp(-μtij)
[2] In traffic flow theory, the distribution of headways is negative exponential with the corresponding arrivals Poisson distributed.
[3] In the monocentric land use model, the population density gradient is theorized as a negative exponential distribution.

Neo-traditional Design

A neighborhood design philosophy derived from traditional community characteristics such as mixed land uses in relatively close proximity. It's also known as Traditional Neighborhood Development. Often used interchangeably with New Urbanism and Transit-Oriented Development, Neo-traditional Design is more correctly an architectural design philosophy.

NEMA

National Electrical Manufacturers Association. The Transportation Management Systems and Associated Control Devices Section develops technical standards and guidelines for vehicular transportation systems and related elements [ web ]

NEMA Controller

NEMA's functional standard traffic controller. Controller operations are based on the standard single- or dual-ring phase diagrams.

NEPA

The National Environmental Policy Act (1969) established national policy requiring projects using Federal funds to examine the environmental impacts of the proposed and alternative projects prior to Federal approval or support (see also EIS).

Nested Logit

Hierarchical formulation of the multinomial logit model. When some choice alternatives are correlted with others (e.g., transit modes are more similar to each other than to private vehicle modes), the error term independence assumption is violated. Nested logit involves the grouping of similar alternatives into one or more "lower level" logit models, with the "higher level" choice between the "nests" of similar alternatives.

Net Present Value

In project appraisal, the discounting of all current and future benefits and costs, to be incurred over the project lifetime, to a net current value.

NETSIM

UTCS-1, the arterial network simulation model developed in the 1970s, evolved into NETSIM that, together with FRESIM (freeway simulation), formed the integrated software suite TRAF (now TSIS/CORSIM).

Network

A graphical and/or mathematical representation of a region's transportation infrastructure and services, comprising links and nodes and their corresponding characteristics. Also refers to the actual networks of highways, transit, and other modes.

Network Coding

The process of developing the transportation networks for a travel forecasting model. Today, coding is most likely enhancing networks from existing models or developing networks from aerial photographs or remote surveillance technology. The level of detail for which network characteristics such as links and nodes will be included varies based on the scale and scope of the model and the policies to be tested. A rule of thumb is to include a functional classification level one lower than the level at which policy decisions will be made.

Network Design Problem

A broad class of OR problems that addresses network topology and/or characteristics of network components (e.g., links and nodes) to achieve some specified objective. A common application is the addition of capacity to a network (via added links, lanes, or services) while minimizing total cost or maximizing user benefit. Related to Minimum Spanning Tree and Facility Location problems.

Network Equilibrium

A system state where fixed O-D demand and network performance are in equilibrium. In the four step model, this is achieved in equilibrium trip assignment. Network flow is in equilibrium when no traveler can unilaterally change route and be better off, thus there is no incentive to change. See system equilibrium.

Network Simplex

A streamlined version of the Simplex algorithm to solve network minimum cost flow problems. As with transportation simplex, the algorithm exploits the special structure of network problems.

Neural Network

A family of computational models based on an analogy to biological neural networks and used to model complex non-linear systems. The basic structure comprises: an input layer, an output layer, and one or more hidden layers, with each layer comprising two or more neurons, and links between any or all neurons in adjacent layers. Learning is the process of calibrating link weights to optimize a defined cost function.

New Start

A federal program to provide discretionary support for capital funding of new fixed guideway transit systems or extensions, based on projected system performance and the level of local commitment.

New Urbanism

An urban design philosophy derived from traditional community characteristics such as grid street layouts, higher densities, and mixed land uses that increase the relative accessibility of non-automotive modes of travel. By reducing automobile travel and land consumption, New Urbanism seeks to minimize impacts on the built and natural environments. Transit-oriented development and neo-traditional design are often components of New Urbanism development strategies.

NGO

Non-Government Organization (see NGO).

NGSIM

FHWA's Next Generation Simulation Program [ web ].

NHB

Non-Home-Based -- a classification for trips which neither begin nor end at a trip maker's residence (home). The origin of a NHB trip is also the production; the destination of a NHB trip is also the attraction.

NHTS

National Household Travel Survey is a comprehensive survey of household travel behavior in the US. The sample is drawn from urban, suburban, and rural areas and is expanded to provide national estimates of household and travel characteristics. NHTS combines the prior NPTS and a long distance travel survey.

Nitrogen Oxides

Oxides of nitrogen are criteria pollutants produced from atmospheric nitrogen and oxygen when combustion occurs at high temperatures. NOx are major air pollutants, components of acid rain and smog, and a precursor of ozone.

No Build Alternative

In alternatives analysis, the No Build Alternative represents the option of no additional transportation improvements beyond what has already been planned and programmed prior to the current study. Serves as a baseline for comparison of various alternatives to improve or expand infrastructure and services or to manage demand.

Node

A point joining two or more links in a transportation network, and having attributes such as turn penalties and turn prohibitions in addition to spatial coordinates.

Node Label

An attribute of a node on a transportation network providing the cumulative travel time from a specified origin on a minimum path tree.

Node-Link Incidence Matrix

A matrix of network topology containing N (nodes) rows and M (links) columns with each column containing a single -1 and a single +1 corresponding to the start- and end-nodes of the link in question, with all other cells equal to zero. This is also the A-matrix of constraint coefficients in the mathematical program for the Transshipment Problem. (see: incidence matrix ).

Nominal Scale

A scale used to measure a variable whose values are simply names and no meaning is implied relative to rank order or intervals (also called categorical variables). Examples in travel forecasting include gender and occupation. See measurement scales.

Non-Attainment

NOT meeting NAAQS standards for criteria air pollutants (including ozone, carbon monoxide, particulate matter, and nitrous oxides [ web ].

Non-Attainment Area

An EPA-designated area whose air quality does not meet one or more NAAQS standards.

Non-basic Employment

See service employment (compare with basic employment).

Non-compensatory Model

Whereas compensatory models are characterized by the simultaneous evaluation of choice attributes so a greater value on one attribute can compensate for a lesser value on another, non-compensatory models choice structures (including hierarchical, sequential, and lexicographic) are such that alternatives with attributes not meeting a threshold are eliminated from further consideration.

Non-Government Organization

A formal group, independent of any level of government, created for operational or advocacy support of particular policy issues.

Non-Home-Based

A classification for trips which neither begin nor end at a trip maker's residence (home). The origin of a NHB trip is also the production; the destination of a NHB trip is also the attraction (see HB).

Non-inferior

In multi-objective evaluation, an alternative that ranks higher on at least one but not all attributes so that a dominant solution can not be idenfified.

Non-motorized

Transportation modes that are not powered by motors, either ICE or electric, including bicycles and walking.

Non-parametric

In statistical modeling, nonparametric indicates that the number and nature of model parameters are not fixed in advance (distribution free).

Non-proportional Assignment

A trip assignment algorithm where the proportion of OD flow loaded on a link depends on link flows (e.g., equilibrium assignment algorithms explicitly reflecting congestion). See Proportional Assignment.

Non-recurrent Congestion

Congestion resulting from independent occurances of atypical events such as incidents, special events, or weather.

Normalize

To scale a vector of data to match some known or desired value:
[1] In trip generation, trip productions and attractions, independently estimated, are scaled so that total attractions are balanced with total productions.
[2] In statistics, a variable is said to be normalized when it is scaled to have a mean of zero and a variance of one.

North American Free Trade Agreement

NAFTA is the 1994 agreement between the United States, Canada, and Mexico that addresses tariffs and regulatory barriers of trade.

NOx

Oxides of nitrogen are criteria pollutants produced from atmospheric nitrogen and oxygen when combustion occurs at high temperatures. NOx are major air pollutants, components of acid rain and smog, and a precursor of ozone.

NPTS

National Personal Transportation Survey was a comprehensive survey of household travel behavior conducted approximately every seven years from 1969 to 1995. NPTS was replaced in 2001 by NHTS and a long distance travel survey.

NTAD

National Transportation Atlas Database (see NTAD ).

NTCIP

National Transportation Communications for ITS Protocol is the family of standards providing the communication protocols and vocabulary necessary for the interoperability of electronic traffic control equipment [ web ].

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O(n)

See Big O Notation.

Objectives

The first stage of the transportation planning process is Problem Definition, which incorporates Goals, broad statements based on community values and the current environment, and Objectives, restatements of goals in specific and measurable terms. Problems may be defined when objectives are not met by the current environment.

Objective Function

A mathematical function of decision variables that is to be optimized, often subject to specified constraints.

Object-oriented Program

A modeling approach featuring data structures that allocate objects to hierarchical classes, that allow them to inherit characteristics of higher levels.

Occupancy

[1] loop occupancy is the proportion of a counting interval (typically 30 seconds) during which a vehicle is detected by a sensor;
[2] vehicle occupancy is the number of persons per vehicle.

OCTA

The Orange County Transportation Authority [ web ].

O-D or OD

An origin / destination pair.

OD Estimation

The inverse of traffic assignment, where a network and observed traffic counts are used to estimate or update a trip table that most likely produced the counts (note that counts alone are insufficient to infer a trip table).

OD Matrix

A trip table in origin / destination format (see PA Matrix).

Off-peak

A period of relatively low traffic volume and density (see peak). Applications of travel forecasting increasing develop two separate off-peak models corresponding to midday (from the AM peak to the PM peak) and evening (from the PM to the AM peak) periods.

OLS

Ordinary Least Squares (see regression analysis).

OPAC

Optimized Policies for Adaptive Control, a comprehensive traffic signal control algorithm that provides both distributed intersection control and network coordinated control (see RT-TRACS).

Open Loop System

A control system without feedback of measured output to adjust a control strategy. In a fixed-timed control system, a control strategy is implemented via field controllers with no feedback of system performance data. The advantage of open loop control is speed and simplicity of operation (compare with Closed Loop System).

Operating Costs

Recurring costs of operating and maintaining infrastructure and service over a given period. Compare with capital costs.

Operations

All activity performed to ensure the effective functioning of a transportation system, including: data collection, archiving, and processing; analysis, modeling, and simulation; control of system components and traffic flows; incident management; and coordination within the operations agency and with other institutions and jurisdictions.

Operations Research

The application of mathematical methods and models to analysis, optimization, and decision-making for complex real-world systems. The term Management Science is often used synonymously with OR, and OR techniques are common tools in civil, environmental, industrial, and systems engineering. In transportation, application areas include Facility Location, Minimum Path, Minimum Spanning Tree, Network Design, and Vehicle Routing problems.

Opportunity Cost

The economic value that is foregone when an alternative course of action is taken.

Optimal

A solution that is deemed best based on pre-specified criteria.

Optimal Control Theory

Methodology to solve for the optimal levels of control variables over a defined time interval. The control variables describe system state variables via a set of differential equations.

OR

Operations Research (see OR).

Ordered Probit

A type of probit analysis where the dependent variable is ordinal, providing estimates of the explanatory variables and the cut-points that defined the ordinal categories (logit counterpart is ordered logit).

Ordinal Scale

A scale used to measure a variable whose values are in terms of rank order (e.g., 1st, 2nd, ...) with no meaning associated with the size of the interval between values. Examples in travel forecasting include attitudinal variables. See measurement scales.

Origin

The location or zone where a trip begins (also used for the trip itself: a zone is a origin for N trip origins). The destination is where a trip ends. Compare with production and attraction.

Origin-based Assignment

An efficient alternative to Frank-Wolfe (link-based) and path-based assignment algorithms to solve the UE trip assignment problem, defining decision variables as origin-based approach proportions for each link into a node and providing a direct conversion to route proportions and associated convergence measures.

Origin-Destination Study

A survey of trip origins and destinations (and usually purpose, time of day, and selected demographic data). Household travel surveys utilize travel/activity diaries to gather this data. Many O-D surveys are corridor-based and conducted as intercept surveys at key network locations (e.g., on and off ramps on a freeway corridor to obtain freeway O-Ds).

Out-of-vehicle

The portion of a trip that involves accessing, waiting for, transferring to, and/or egressing from a vehicle, as opposed to the in-vehicle portion. Disutilities are in general higher for out-of-vehicle time.

Ozone (O3)

The O3 form of oxygen, a regulated criteria pollutant. In the troposphere, O3 is produced photochemically from hydrocarbons, oxides of nitrogen, and sunlight, and becomes a major component of smog; in the stratosphere, ozone protects the earth from ultraviolet radiation.

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P3

Public-Private Partnership (aka PPP).

PA Matrix

A trip table in production / attraction format (see OD Matrix).

Panel Data

Data from a sample for which repeated measurements are made.

Panel Survey

A survey for which repeated measurements are made of the same respondents(e.g., a travel survey of the same households at a regular interval).

Paradigm

A body of theory, concepts, assumptions, and empirical evidence that constitute a formal framework for analyzing some phenomena of interest.

Paradyn

A hybrid traffic network simulation model integrating components of the mesoscopic Dynasmart [1] model with the microscopic Paramics model.

Paradox

In transportation analysis, most likely an unexpected rather than an impossible outcome (e.g., Braess' Paradox).

Parallel Processing

The concurrent use of two or more processors (CPUs) by a single program application. Problems and algorithms that lend themselves to parallelization can realize faster execution times (in multi-tasking where a single CPU performs multiple operations).

Parameter

A constant describing a population property utilized in a model development process. Often used interchangeably but incorrectly with estimator, a calibrated value for a parameter.

PARAMICS

A comprehensive traffic microsimulation software package that features Application Programming Interfaces (APIs). Developed by Quadstone [ web ]. Other traffic microsimulation packages include AIMSUN, CORSIM, Cube Dynasim, SimTraffic, TransModeler, and VISSIM.

Paratransit

[1] Alternate forms of conventional transit operations, designed for specific market segments and not restricted by vehicle type, fixed routes, or fixed schedules. Includes jitneys, taxis, demand-responsive transit, and other specialized transit modes.
[2] Demand responsive transit such as door-to-door bus, van, and taxi services used for special markets such as elderly and disabled riders.

Parcel

A legally-defined, single ownership, single land, use contiguous area of land, parcels form the basic disaggregate land use unit.

Pareto Optimal

In multiobjective optimization and decision-making, a solution is Pareto Optimal if no alternative improves at least one objective without reducing the value of another objective.

Pareto Efficient

Same as Pareto Optimal.

Parking Generation

Estimation of the demand for parking spaces based on land use and time of day. The ITE Parking Generation report is the parking demand equivalent of ITE's Trip Generation.

Parking Management

Strategies to increase the efficiency of current parking supply, including pricing, use restrictions, and preferential parking for carpools.

Particulate Matter (PM)

Regulated criteria pollutants associated with adverse health effects and reduction in visibility, PMs consist primarily of dust from roads, agriculture, and construction and not significantly from motor vehicle emissions.
[1] PM2.5 includes "fine particles" of less than 2.5 micrometers found in smoke and haze, which can also be formed from chemical reactions in the air;
[2] PM10 includes "inhalable coarse particles" of 10 micrometers or less.

Pass-by Trip

In traffic impact analysis, pass-by trips are diverted to the site from traffic already utilizing adjacent streets. These trips are part of the trip ends estimated for the site, but they do not increase the traffic volume on the adjacent street other than via the turning movements entering and exiting the site (see also internal capture).

Passenger-mile

One passenger transported one mile.

PASSER II

The Progression Analysis and Signal System Evaluation Routine is a traffic signal optimizer that maximizes 2-way arterial progression (bandwidth) and minimizes signal delay, computing equlvalent fixed-time phasing, cycle length, and offsets. (see PASSER III and PASSER IV)

PASSER III

A diamond interchange signal optimization program (see PASSER II and PASSER IV)

PASSER IV

Maximum bandwidth optimization for a traffic signal network (see PASSER II and PASSER III)

PATH

Partners for Advanced Transit and Highways, the University of California's Caltrans-sponsored research center for intelligent transportation systems [ web ].

Path

A sequence of links and nodes connecting an origin to a destination in a network. Used interchangably with route.

Path-based Assignment

The state-of-the-practice in trip assignment is link-based assignment, using algorithms such as Frank-Wolfe or MSA to find the UE solution that is unique in link flows but not in path flows. Path-based algorithms such as gradient projection identify and store utilized paths at each iteration and produce path-based flows.

Path Enumeration

The prior identification of paths in a network. Traffic networks typically have a large number of paths so algorithms that either obviate path enumeration or generate paths automatically (see column generation) are preferred.

Pavement Design

The structural design of roadway pavements, including thickness and load-carrying capacity, material composition, and drainage design.

Pavement Management System

A integrated process and/or database that records, analyzes, and evaluates pavement condition for all roadway surfaces in a defined area. The information is used in planning and programming pavement maintenance and rehabilitation.

PCE

Passenger Car Equivalent (see PCE).

Peak

The period of maximum traffic volume and density. The distribution of trip frequency by time-of-day typically shows morning (AM peak) and afternoon/evening peak (PM peak) periods. Travel forecasting models have conventionally focused on problems associated with peak period flows (such as congestion). A behavioral response to increased congestion in peak periods has been the increase in duration of the peak period, deemed peak spreading (see off-peak).

Peak Hour

For a transportation facility or network, the hour of the day during which the maximum traffic volume occurs. Often a longer "peak period" is the salient operational period for analysis.

Peak Hour Factor

[1] In the planning context, PHF is the ratio of traffic volume in the peak period to the facility's average daily traffic (also known as "peaking factor").
[2] In the operational context, PHF is a measure of peaking within a peak period, usually expressed as the ratio of peak hour traffic volume to four times the traffic volume in the highest volume 15 minute period.

Peak Spreading

The lengthening of the peak period caused by earlier and later departure times of travelers attempting to avoid increased peak period congestion by traveling in the peak shoulders (effectively temporal sprawl).

PECAS

Production Exchange and Consumption Allocation System is a comprehensive land use model developed by Hunt and Abraham based on the TRANUS model.

Pedestrian

A person not utilizing a motorized or other transportation vehicle. Typically, an individual who is walking in, across, or along a transportation right-of-way.

Peer Review

The process of having other agencies or parties review the structure and performance of a travel forecasting model and make recommendations for model improvement.

PEF

Pedestrian Environment Factor is a qualitative index of the degree that an area is accommodating to travel by walking.

P-E Fit

Person-Environment Fit is a measure of the degree that an individual's activity requirements are met by the current physical and social environments.

PeMS

Caltrans' Freeway Performance Measurement System containing real-time and historical freeway detector data for California freeways to compute performance measures [ web ].

Performance

A general term for the output level of service for a transportation infrastructure element. For roadways, a performance function represents the relationship between input variables such as free speed, capacity, and volume and the output performance measure of speed or travel time. Performance, rather than supply, is a more appropriate term for assessing level-of-service for transportation facilities and, thus, for determining system equilibrium.

Performance-based Infrastructure

California's program for public-private partnerships with the public and private sectors jointly responsible for the providing public infrastructure and services [ web ].

Performance Function

In transportation systems analysis, a performance function reflects characteristics of the Transportation System that together with demand functions (which reflect characteristics of the Activity System) determine network flows (volumes and travel times).
A performance function relates level-of-service to a function of characteristics of a given facility and the associated volume, or: LOS = P( Tsys, volume )

Performance Measures

Output measures of system effectiveness, either measured or modeled. Often expressed in relative terms of output (usage) to input (cost). Also known as Performance Indicators or Measures of Effectiveness (MOE).

Persistent Traffic Cookie

A time and location stamp written by a roadside controller to a vehicle's on-board memory via local, wireless communications, which can be read by subsequent controllers to identify the vehicle's route history.

Person Category Model

A trip production model based on a category analysis of individuals rather than households.

Person-mile

One person traveling one mile. Total person-miles traveled is an indicator of system performance measuring total mobility in a region based on total distance traveled (see vehicle-mile).

Person-Trip

A single trip by a single person. The output of trip generation is measured in person-trips. On the other hand, a vehicle-trip is a single trip by a vehicle, regardless of the number of occupants in the vehicle. A vehicle with three occupants on the same trip equals one vehicle-trip or three person-trips.

Phase

A set of signal indications that permit one or more movements to proceed simultaneously (e.g., two opposing left turns comprise a phase that ends when at least one left turn indication ends and is replaced by a phase comprising the other left turn and the corresponding through movements (or both through movements)).

PHF

Peak Hour Factor (see PHF).

Pivot Point

Pivot point modeling applies a demand function to estimate changes in demand using only current market shares and proposed performance changes. Incremental logit is perhaps the most commonly utilized.

PLACE3S

PLAnning for Community Energy, Economic, and Environmental Sustainability is a planning method developed to facilitate forecasts of energy use for alternate land use plans. Land use assumptions are specified via a GIS interface. The distributions of households and employment are then estimated and alternate land use plans are compared in terms of transportation impacts such as generated VMT and vehicle-trips. The transport component was developed by Fehr & Peers. [ web ]. See INDEX.

Planned Unit Development

Land development permitted and planned on multiple parcels with a compatible mix of land uses. PUD permits developers with greater flexibility in meeting density and land use goals. This variation from fixed lot size "grid" development often led to sprawling, automobile-dominated development patterns.

Planning

The practice and process of developing, implementing, and evaluating formal plans, programs, and policies to provide infrastructure and services to achieve defined goals and objectives.

Platoon

Vehicles that effectively move through a traffic network, real or simulated, as a group. Platoon dispersion refers to the spatial separation of vehicles in a platoon that occurs as they progess downstream from a traffic signal.

Platoon Dispersion

The separation of vehicles in a platoon that occurs as they progess downstream from a traffic signal or other upstream queueing.

PM, PM2.5, PM10

Regulated criteria pollutants associated with adverse health effects and reduction in visibility, PMs consist primarily of dust from roads, agriculture, and construction and not significantly from motor vehicle emissions.
[1] PM2.5 includes "fine particles" of less than 2.5 micrometers found in smoke and haze, which can also be formed from chemical reactions in the air;
[2] PM10 includes "inhalable coarse particles" of 10 micrometers or less.

PM-peak

The time period corresponding to the highest afternoon traffic volumes. The PM-peak hour is the highest volume hour within the PM-peak period, which is typically defined as a 2 to 4 hour long period (see midday, AM-peak, and off-peak periods).

PMS

Pavement Management System (see PMS ).

Point Sources

Point, line, and area sources are idealized zero-, one-, and two-dimensional locations that are sources for various pollutants. Large, fixed facilities such as power plants, chemical processing industries, and petroleum refineries are point sources of a range of air pollution emissions (AKA stationary sources).

Pollution Sources

Sources of air and noise pollutants, including mobile, stationary, area-wide, and natural sources.

Polycentric Model

Models used to analyze the spatial structure of cities and characterized by multiple centers of employment and population. Compare with monocentric model.

Population

[1] The number of persons living in a zone, region, or other defined spatial unit, often categorized by socio-economic characteristics or housing type. An endogenous, variable in land use models and an exogenous variable in travel forecasting models, especially trip generation where population is critical in most aggregate trip production models (household size appears for population in disaggregate trip production models).
[2] In sampling, the population is the entire group from which a sample is drawn and may be defined based on any desired characteristics.

Population Pyramid

In a cohort component model, a graphical depiction of cohorts defined by age on the vertical axis and gender horizontally) yielding a pyramid-shaped distribution for populations with consistent natural increase.

Post-Processor

Models or procedures applied to refine or extend core model output. Applications in travel forecasting include link speed estimation and air quality analysis. See pre-processor.

PPP

Public-Private Partnership (aka P3).

Pre-emption

See signal pre-emption.

Prediction

A general term for the estimation of the value of a variable of interest in an unknown situation. Often used synonymously with forecasting, a term that is more often used in the estimation of values at a future time (such as for travel forecasting).

Preliminary Engineering

Initial technical phase of project development to determine location, feasible alternatives, and design options, as well as all relevant constraints.

Pre-processor

Models or procedures that preceed core model application, typically to process data required in the core model. In travel forecasting models, this might be to develop auto ownership models or to estimate joint distributions of socio-economic and demographic data for trip generation. See post-processor.

Presence

In traffic signal control, a binary indicator that a vehicle has activated a sensor at an intersection stopline signaling the controller that at least one vehicle is queued for service. Most stopline detectors (e.g., loop detectors) provide only a presence indication and not a count.

Pre-timed Control

Same as fixed-time control.

Price

The amount charged by a provider and paid directly by a user for consuming a good or service. Price is usually defined to include only monetary amounts, but overall cost can also include time and other elements.

Pricing

A fundamental economic strategy to charge users for the use of a transportation systems based on system demand. Also known as congestion or value pricing.

Privatization

The process of converting infrastructure and/or services, in whole or part, from public sector ownership and/or operations to private sector ownership and/or operations (see public-private partnerships).

Probe Vehicle

A means of traffic data collection that obtains measurements from an instrumented vehicle inserted in the traffic stream (see floating car).

Probit Model

A discrete choice model, conceptually similar to logit, but with multivariate Normal error terms and an arbitrary covariance matrix and thus not limited by the IID assumption of logit. The generality of this formulation presents a significantly greater computational burden in estimation than does logit (compare with nested logit).

Problem Definition

A stage in the transportation planning process that defines gaps between the desired or expected performance of the transportation system, based on local values, goals and objectives, and the current or projected level of system performance.

Production

The location or zone responsible for a trip occurring but also used for the produced trip itself: a zone is a production for N trip productions). A household generates N productions that may be split as home-based or non-home-based. Home-based trips, by definition, have their production in the zone containing the household, regardless of the origin and destination of the trip. The productions for non-home-based trips must be allocated to the NHB trip's origin zone (see reallocation model). Contrast with attraction.

Production Function

In Transportation Systems Analysis, a Production Function finds the set of minimum cost resource input combinations for a specified output, given current technology, and serves as an input to supply procedures (comparable to the microeconomic production function that maximizes output for fixed inputs).

Programming

The process of prioritizing and committing funds to transportation projects and programs for a defined time period based on overall effectiveness and available resources.

Proportional Assignment

A trip assignment algorithm where link flows are proportional to OD flows (e.g., free assignment algorithms where there are no congestion effects).

Proposition 1B

California's "Highway Safety, Traffic Reduction, Air Quality, and Port Security Bond Act of 2006" was one of a package of California legislative bond acts that were submitted to voters and approved in November 2006. Prop 1B provides $20 billion for transportation infrastructure, funds the Corridor Mobility Improvement Account (CMIA), and requires Corridor System Management Plans (CSMP) for corridors.

Ps and As

Productions and Attractions, a common abbreviation for the results of trip generation.

PS&E

Plans, Specifications and construction Estimates.

PSR

Project Study Report summarizes the analysis of feasibility and preliminary costs of project alternatives.

PTC

Persistent Traffic Cookie (see PTC).

Public Participation

Active involvement of the general public as a stakeholder in the planning and development of transportation projects and programs.

Public-Private Partnerships

Private sector provision of public infrastructure, such as roadways, with the public sector providing a long-term franchise under which private investors would recoup their investment, such as from toll revenues (see Performance-based Infrastructure).

Public Transit

Passenger transportation services available to the general public. Conventional forms of public transit typically operate on a scheduled basis, with defined routes, stops, and fares, and with vehicles designed to accommodate multiple passengers (compare to paratransit).

Public Works

Infrastructure that is funded, built, operated, and/or maintained by public agencies to provide transportation and other services to meet public demands.

PUD

Planned Unit Development (see PUD).

PUMA

A Public Use Microdata Area is a Census-defined area for which PUMS data is available.

PUMS

The Public Use Microdata Sample contains complete household records (with identifying information removed) from the US Census for a 1-5 percent sample. This disaggregate data can be used to synthesize population-level household data.

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QA/QC

Quality Assurance / Quality Control (see QA/QC).

QRSII

Quick Response System: a software package for travel forecasting, incorporating transferable models and parameters compiled from NCHRP187. Extensively expanded from the original QRS software by AJH Associates [ web ]. Other travel forecasting packages include Cube, EMME/2, MinUTP, T-Model, Tranplan, and TransCAD.

Quadratic Programming

A linear constrained optimization program with a quadratic objective function. In trip assignment, Frank-Wolfe is a QP algorithm that solves a non-linear program with linear and equality constraints (e.g., UE Assignment) via a sequence of linear approximations (e.g., AON Assignment).

Qualitative Analysis

An analysis that measures the nature of the characteristics of a sample (e.g., an average person described in qualitative terms as tall, thin, and middle-aged). See Quantitative Analysis.

Qualitative Choice

A modeling approach, also known as discrete choice modeling, representing a decision-maker's choice among a finite set of distinct choice alternatives (such as the choice of alternate modes among car, bus, or walk).

Quality Assurance
Quality Control

Quality Assurance (QA) is a formal process to ensure confidence in the satisfactory performance of a transportation facility or service, from planning, through design, to management, operations, and maintenance. Quality Control (QC) represents the components of a QA process that actively control the quality level of the facility or service in question.

Quality of Life

A subjective, multi-objective assessment of the well-being of individuals or populations.

Quantal Loading

A form of incremental trip assignment where all trips for a specified origin zone are assigned to the minimum path tree with link costs updated prior to the assignment of trips from the next origin. Quantal Loading is fast but an equilibrium solution is not guaranteed.

Quantitative Analysis

An analysis that measures the number or size of characteristics of a sample (e.g., an average person described in quantitative terms as 6 feet tall, weighing 180 pounds, and 50 years old). See also Qualitative Analysis.

QueensOD

Software for estimating origin-destination demand using observed link traffic counts, turning movement, and travel times [ web ].

Queue

A line of vehicles or pedestrians waiting to be served. Queueing Theory is the systematic analysis of queueing behavior under various assumptions. In conventional travel forecasting, queueing is not explicitly reflected: queues stack vertically so there is no spill-back upstream.

Quick Response

A compilation of transferable parameters, factors, and manual techniques for simplified transportation planning analysis (see NCHRP 187, NCHRP 365, and QRSII).

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Ramp Metering

Application of traffic control meters on freeway on-ramps to regulate entry volume at a set rate or as a function of upstream traffic volumes.

Random Utility Model

A disaggregate behavioral travel demand model that defines choice among discrete alternatives by individuals seeking to maximize utility, with utility defined as comprising a measurable (deterministic) component and a random component.

Random Walk

A stochastic path consisting of a sequence of random steps, defined by direction and step size.

Raster

A non-scalable graphic data format, with data represented as a rectangular grid of pixels (bitmap or compressed gif or jpg format), in contrast to vector format (comprising points, lines, and polygons). Raster formats are used for photographic images.

Ratio Scale

A scale used to measure a variable for which the intervals between values are defined and with an absolute zero point. Examples in travel forecasting include zonal income or the number of trips. See measurement scales.

RDI

Route Directness Index is an accessibility index defined by the ratio of Euclidean (straight-line) distance to actual travel route distance.

REACT!

A web-based self-adminstered survey instrument to collect, process, and archive geo-coded household travel/activity diaries, REACT! is integrated with TRACER, an in-vehicle GPS tracking device with wireless communications with the REACT! server (based on HATS and CHASE).

Real-time

Collecting and processing data as it occurs, in a simulation or real operating environment.

Re-allocation Model

Household trip production models are based on household surveys and estimate total household trips, including non-home-based trips. NHB trips have productions that are not generated at home and thus must be re-allocated to the zone of production via models that are similar in structure and composition to trip production models. An alternative approach assumes that the distribution of non-home attractions is correct (after balancing to total non-home productions) and equates non-home productions to non-home attractions for each zone.

Reasonableness Checks

The objectives of model validation are to verify that the model can replicate observed system performance and that the model is sensitive to proposed changes in model variables, parameters, and constraints. Reasonableness checks represent a battery of basic "reality checks" on all model components, including basic transportation and activity system inputs, model structure and parameters (compared to legacy models or averages for comparable areas), and model outputs (compared to observed data). Does the model "behave" in a logical and consistent manner? Also see model sensitivity tests.

Recurrent Congestion

Congestion reflecting persistent conditions where volumes repeatedly exceed capacity at a particular location and time period.

Recursive

A recursive procedure calls itself, necessitating definition of a base case (e.g., a procedure F(n) to compute the factorial (n!) would call F(n-1), which in turn would call F(n-2), until a defined base case F(0) is reached). An iterative procedure repeats calculations in a loop but does not utilize self-reference. Such a procedure is typically less elegant but often more efficient than a recursive procedure. Both imply a cascading process where the output of one step is the input to another.

Red Bus / Blue Bus

The prototypical example of the IIA property of logit models. Given equal utilities for two mode alternatives, car or red buses, equal logit choice probabilities result. If half of the red buses are painted blue, the resulting logit choice probabilities will be equal for all three modes. Nested Logit and Probit models can be structured to address this issue.

Red Time

The portion of a cycle for which signal indications display red to prohibit a particular movement (see also amber and green).

Region

A large but well-defined geographical area comprising multiple jurisdictions with significant social and economic interactions, for which appropriate levels of planning and development are typically coordinated (see metropolitan classification).

Regional Model

The phrase regional models may be interpreted as models that are developed by regional agencies and/or are applied at the regional level. These models reflect regional economics and demographics that interplay at the regional level. Land use models have typically been built at the regional level while historically most travel forecasting models have been built at the local level. Increasingly, local travel models have evolved into regional-scale models, corresponding to the scale of land use models.

Regional Science

An interdisciplinary field addressing the analysis and modeling of spatial phenomena at all geographic scales.

Regional Transportation Plan

The long range consensus plan that defines a region's transportation system and planned improvements scheduled for funding over the next 20 years, a basic requirement to obtain state and federal funds.

Regional Transportation Planning Agency

State of California statutorily created agencies charged with developing the Regional Transportation Plan and the RTIP, primarily created in rural areas in parallel to MPOs.

Regression

A general statistical technique for exploring relationships among variables. It is used extensively in the transportation field, in general, and for trip attraction and automobile ownership modeling, in particular. A curve (often linear) is fit to the relationship between one or more explanatory variables and a dependent variable, typically to minimize an error term to determine the best fitting function (see also Category Analysis).

REID

A traffic detection system that allows for a vehicle to be RE-IDentified downstream of an initial detection and identification. This anonymous tracking is based on matching unique electronic "signatures" generated by a vehicle passing over non-intrusive sensors.

Reilly's Law

Reilly's (1929) Law of Retail Gravitation, an early application of accessibility [2] concepts, specified a market area boundary by equating the ratios of population size to the square of distance from the boundary for a pair of competing retail centers.

Relative versus Absolute Change

Absolute change is defined as a simple difference whereas relative change is defined as proportional to a base level. For example, in comparing changes in mode choice, an area can show an absolute increase in transit use but a relative decrease, if the proportional increase in total transit trips is less than the proportional increase in total trips by all modes.

Relative Gap

In iterative trip assignment, a convergence parameter defined as the relative difference between the total travel time for the current solution and the total travel time via the current minimum path.

Reliability

The degree of certainty or predictability of a measure of system performance, such as travel time, in a transportation system.

Remote Sensing

The process of and the technologies for recording and measuring terrestrial features and activities at a distance from the surface (such as from aircraft or satellites).

Renewable Energy

Energy obtained from replaceable or essentially inexhaustible sources such as solar, hydro, wind, or geothermal, typically characterized by lower environmental impacts of production and use.

Request for Proposal

A announcement from a public agency or private entity requesting potential consultants or contractors to submit a proposal, cost estimate, and qualifications for professional engineering, planning, and/or related services.

Residential Location

A household's choice of where to live, a function of neighborhood and regional characteristics, proximity to work locations, household characteristics (e.g., household size, income), and the distribution of housing opportunities.

Retail Employment

A component of non-basic (service) employment characterized by the sales of products directly to consumers, retail employment is a common variable in trip generation models.

Return Home Trip

Travel surveys historically recorded a trip from a non-home destination back to the traveler's home location as a return home trip. Increased interest in trip purpose with activity-based models has led to this trip designation being replaced with the activity that occurs after returning home. If home is taken as a default location, however, the return home trip does not necessarily have a designated purpose other than returning to this default location.

Return on Investment

The economic and other benefits that accrue from an investment.

Returns to Scale

An index of how fast system costs rise with increasing system output, equal to the ratio of average cost to marginal cost. If the marginal cost is less than the average costs, then there are increasing returns to scale, also called economies of scale.

Revealed Preference

A individual's preference that is identified via observation of an actual choice and associated choice attributes. Compare with stated preference.

Revenue Passenger-mile

One fare-paying passenger transported one mile.

Revenue Vehicle-mile

One vehicle transporting one or more fare-paying passengers one mile.

Reverse Commuting

Travel oriented opposite the primary direction of commuting (e.g., commuting from the city to the suburbs).

RFID

Radio Frequency Identification Device technologies are small devices attached to objects that transmit identification and tracking data to a receiver.

RFP

Request for Proposal (see RFP).

Rho-squared

A contrived measure of model goodness-of-fit for discrete choice models estimated via maximum likelihood, roughly analagous to R-squared in least squares estimation, defined as one minus the ratio of the final to initial log likelihood [ ρ2 = 1 - L(β)/L(0) ].

Ridesharing

In a general sense, having multiple person-trips for each vehicle-trip, via carpools, vanpools, and similar modes. In a narrower sense, actively arranging for additional passengers in an existing vehicle-trip, often a commute to work.

Right of Way

Land legally designated for use by a transportation facility(ies).

Rigid Pavement

Pavements with a concrete wearing surface (see flexible pavement).

Ring

[1] A geographic ring is spatially defined as a portion of a metropolitan area approximated by an annulus centered on a central business district (CBD) and associated with land uses that decrease in terms of relative value of land as distance increased from the center (see also sector [2]).
[2] A ring road is equivalent to a beltway, a circumferential route around a city, CBD, or other developed area.

RITA

The Research and Innovative Technology Administration of the US Department of Transportation is dedicated to the advancement of research and innovative technology in transportation.

RMSE

Root Mean Squared Error is a goodness-of-fit measure equal to √[Σi(esti-obsi)2/n].

Road Diet

The rechannelization of multi-lane roadways replacing a through lane in each direction with a single, two-way, median left turn lane. Remaining ROW can be used for bicycle lanes, on-street parking, or a buffer between motorized and non-motorized traffic.

Road Pricing

An economic concept involving direct charges for the use of roads. Depending on the meaning of "direct", such charges may include fuel taxes, licence and parking fees, in addition to tolls (fixed, variable, or dynamic). Road pricing has at least two objectives: controlling traffic volumes (and thus congestion) and revenue generation (to fund both transportation infrastructure and services). Pricing has been increasingly promoted as a means of travel demand management on selected, congested facilities.

Roadway

A general term for transportation infrastructure designed for motor vehicle use, including highways, arterials, and other facility types.

Robertson's Platoon Dispersion

A macroscopic model of platoon dispersion developed by Robertson (1969) at TRRL and incorporated in TRANSYT, SCOOT, and other traffic operations software packages.

ROG

Reactive Organic Gases are volatile organic compounds (VOC) or hydrocarbons (HC) that contribute to smog.

ROI

Return On Investment (see ROI ).

Rolling Horizon

A hierarchical traffic control strategy where a control decision (e.g., to change or extend a phase) is executed based on current system status and conditions projected over a fixed time horizon that is "rolled" forward to reflect the most recent control decisions, sensor data, and projections.

Route

A path through a network; a series of links and nodes connecting an origin and a destination. Used interchangably with path.

Route Choice

Route choice is often used synonymously for trip assignment, the fourth major step in the four step model sequence, but it is sometimes reserved for the application of stochastic route choice models (compared to the deterministic models used in most trip assignment). In this context, Route Choice is based on the theory that a traveler will choose the route that maximizes utility.

Route Guidance

Realtime provision of location and directions using in-vehicle systems; also occasionally associated with the provision of transit routing and scheduling information.

ROW

See Right Of Way (ROW).

R-squared

R-squared is the short name for the coefficient of determination. It is a goodness-of-fit measure defined as the ratio of explained to total variance in a regression model.

RTIP

The Regional Transportation Improvement Program lists the transportation projects that a region proposes for funding, compiled from priority lists submitted by local jurisdictions and agencies. RTIP projects must be consistent with the regional transportation plan (RTP). RTIPs are combined with state-level projects in the State Transportation Improvement Program (STIP).

RTOR

Right Turn On Red permitted after a full stop.

RTP

The Regional Transportation Plan (RTP), prepared by the designated Metropolitan Planning Organization (MPO), comprises policies, programs, and specific projects to meet long-range transportation needs. The RTP is updated every three years and must reflect funding constraints and air quality regulations.

RTPA

Regional Transportation Planning Agency (see RTPA).

RT-TRACS

Real-Time TRaffic Adaptive Control System is a cooperatively-developed system for executing multiple traffic control algorithms.

Rule-based System

A knowledge-based system utilizing a set of "if/then" rules on which decisions are based.

RUM

Random Utility Maximization (or Model) (see RUM).

Rural

Settled areas not part of metropolitan areas and characterized by very low population densities (see metropolitan classification).

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Safe Routes to School

(A program that fosters) Designated walking and cycling paths that link residential areas and neighborhood schools.

SAFETEA-LU

The Safe, Accountable, Flexible, and Efficient Transportation Equity Act -- A Legacy for Users, the federal transportation authorization enacted in 2005 continuing the series of Federal Aid Highway Acts, addressed metropolitan and statewide planning by modifying a range of requirements to address flexibility and efficiency of the planning process. SAFETEA-LU was preceded by ISTEA, the Intermodal Surface Transportation Efficiency Act of 1991, and TEA-21, Transportation Equity Act for the 21st Century [ web ].

Sample Enumeration

An aggregation technique that averages individual choice probabilities from a disaggregate model over the part of the sample that made that choice. Assumes the sample used in estimation is representative of the population (see naive aggregation).

SASI

Self-Administered Survey Instrument (see SASI).

Satisficing

The assumption due to Herbert Simon that decision-makers select a satisfactory rather than a strictly optimal alternative from a choice set, perhaps due to limitation in the choice alternatives considered or the complexity of the decision process. Compare with bounded rationality and utility maximizing approaches.

SATURN

Simulation and Assignment of Traffic to Urban Road Networks is a combined simulation and assignment model designed to analyze travel behavior resulting from network changes [ web ].

SB 375

"Redesigning Communities to Reduce Greenhouse Gases", the 2008 California law linking land use planning and greenhouse gas reduction (GHG), requires MPOs to prepare a "Sustainable Communities Strategy" (SCS) to meet GHG targets by reducing vehicle miles traveled (VMT) or other strategies (see AB 32) [ web ].

SCAG

The Southern California Association of Governments, southern California's six-county MPO (all of southern California except San Diego County) [ web ].

Scalability

An algorithm's or a software's ability to continue to perform, or perform efficiently, when the dimension of the problem increases significantly. The ability of a digital graphic format to change scale without distorting the original image.

SCAQMD

South Coast Air Quality Management District, the AQMD for southern California.

SCATS

Sydney Coordinated Adaptive Traffic System) ...

SCOOT

Split, Cycle and Offset Optimization Technique, an adaptive control system developed by TRL based on the TRANSYT optimization and analysis model.

Barnes Dance

A traffic signal phase where red is displayed to all vehicle movements, allowing pedestrians to cross in all directions, including diagonally, simultaneously. Also known as a Scramble Phase. Can lead to greater delay for vehicular and pedestrian traffic by eliminating overlap phasing but may be appropriate when there are large pedestrian volumes.

Scramble Phase

A traffic signal phase where red is displayed to all vehicle movements, allowing pedestrians to cross in all directions, including diagonally, simultaneously. Also known as a Barnes Dance after Henry Barnes, an early pioneer in traffic operations and control. Can lead to greater delay for vehicular and pedestrian traffic by eliminating overlap phasing but may be appropriate when there are large pedestrian volumes.

Screenline

A control line for which estimated and observed performance measures are aggregated and compared for model validation or for performance assessment, a screenline splits a study area into two parts, capturing performance measures for flows from one part of the study area to the other. While screenline is usually defined on a regional scale, a cutline usually refers to a local area. With definition of spatial scale somewhat arbitrary, these terms are often used interchangeably (see also cordon line).

SCS

Sustainable Communities Strategy (see SCS).

Seamless Transportation

Transportation facilities and services fully integrated both physically and operationally to allow users to efficiently change modes and/or service areas.

Sector

[1] An economic sector is a basic component of an economy that is defined in terms of primary economic activity, either as a specific industry or in fundamental terms such as extraction, manufacturing, or services.
[2] A geographic sector is spatially defined as a wedge-shaped portion of a metropolitan area extending radially from a central business district (CBD) (see also ring).

Sector Model

A descriptive model of urban development developed by Hoyt (1939) based on a modification of the Concentric Ring Model with sectors defining different land uses extending radially along major transportation corridors to capitalize on accessibility. Compare with the Multiple Nuclei Model.

Selective Availability

An error introduced in GPS signals to prevent non-military receivers from obtaining highly accurate position data (deactivated in 2000).

Self Administered Survey

A evolving travel survey format designed to reduce cost and time expenditures, and to control respondent burden while increasing data quantity and quality (e.g., CHASE). A web-based instrument (e.g., REACT!), can be accessed by the respondent when convenient, integrates data quality checking, and transmits recorded data to a server at the end of each survey period.

Self-selection

A bias present in statistical modeling where individuals in a sample exhibit behavior associated with a priori preferences rather than as resulting from hypothesized explanatory variables. For example, individuals who prefer to use transit and not car may select to live in a transit-oriented development based on this preference, so the resulting behavior is not determined by (but is accommodated by) characteristics of the TOD.

Semi-actuated Control

A traffic control system where selected signal indications are determined by detection of vehicle or pedestrian arrivals at a sensor. When all movements are controlled by detection, the control is fully-actuated.

Sensitivity Tests

The process of investigating a model's response to small, defined changes in model variables, parameters, or constraints. A component of both model specification and model validation, sensitivity testing defines the ability of the model to respond to relevant policy variables that describe the current and planned transportation and activity systems. Ideally, sensitivity analysis should be used for all components of the modeling process (see also reasonableness tests).

Sensor

A device for identifying the presence or passing of a vehicle at a specified location, often used interchangeably with detector (see inductive loop detector). A sensor is a transducer that converts a signal, such as the inductance in an ILD, to a digital measure (such as count or occupancy).

Sequential Model

A model system where the output of one component serves as the input to a subsequent component (for example, the Four Step Model). Compare with simultaneous model.

Service Employment

Non-basic employment, defined as employment directed toward serving local demands, thus supporting population and other local employment (see basic employment).

Shadow Price

The implicit value associated with a resource constraint, equal to the amount the optimal solution will increase by relaxing the constraint by one unit. In the transportation algorithm, if the difference in relative shadow prices between a demand and a supply location (an opportunity cost) exceeds the transportation cost, then the solution can be improved (see LaGrange Multiplier).

Shared Economy

A buzzword with two distinct meanings. The original shared economy reflected individuals sharing resources in the sense of lending items, labor, or expertise to other individuals at no cost other than a willingness to return the favor. This concept was adapted by technology firms to sell primarily services for profit where the economic transaction ("sharing") is between the technology provider, a contracted physical provider, and a customer paying for the service. Transportation examples include Uber and Lyft, which are only marginally different in function and the service provided from taxi and other conventional paratransit services.

Sharrows

A shared-lane marking used on roadway travel lanes that may be simultaneously utilized by various modes (cars, bicycles, and other non-motorized vehicles). The term sharrows combines the conventional terms "shared lane" and "arrow".

Shift Share Analysis

An economic activity analysis and prediction method that models growth as a function of total national growth, a proportional shift due to a national sector's performance relative to the national economy as a whole, and a differential shift due to a local sector's performance relative to the national sector. The constant share approach assumes that regional sectors maintain a constant share of national sector growth. The constant shift approach compares regional sector changes to corresponding national sector changes to determine differential growth rates.

Shock Wave

The moving discontinuity between two roadway sections with different speed-density characteristics, represented in continuum flow models and observed in the field (see web).

Shortest Path

See Minimum Path.

Shoulder

[1] the less congested periods before and after the congested peak periods of travel, providing commuters the opportunity to depart earlier or later to reduce commuting times (producing peak spreading).
[2] the portions of a roadway adjacent to the travel lanes reserved for a wide variety of safety-related functions (including use by emergency vehicles and non-vehicular traffic).

SIC

Standard Industrial Classification (see SIC).

Sidra

SIDRA Intersection is a micro-analytical traffic software package for the evaluation of intersection capacity and performance. Developed by Akcelik and Associates [ web ].

Signal Coordination

The coordination of a series of traffic control signals to optimize the progression of traffic flow along an arterial or designated route by setting signal cycle length, phasing, and offsets so that traffic approaching a signalized intersection receives a green phase prior to slowing to minimize stops and delay (AKA signal synchronization ).

Signal Preemption

An interruption in the normal operation of a traffic control signal to immediately serve a demand on a particular movement (e.g., to allow an emergency vehicle to pass). The control system and the vehicle must be able to communicate to coordinate the preemption (see signal priority).

Signal Priority

Where signal preemption interrupts programmed signal operations (e.g., for emergency purposes), signal priority only alters signal timing, such as by extending the current green phase to accommodate an approaching transit vehicle (e.g., for schedule adherence). The control system and the vehicle must be able to communicate for priority treatment.

Signal Progression

Same as Traffic Signal Coordination.

Simplex Algorithm

The standard algorithm to solve linear programs, developed by Dantzig (1947), which uses a standardized tableau format for the objective function and constraints and systematically searches corner points from an initial basic feasible solution.

Simpson-Curtin Rule

An often cited, rule-of-thumb, transit fare elasticity value of -0.33, based on data from a 20 year period (Curtin, 1968).

SimTraffic

A traffic microsimulation software package linked to SYNCHRO, software for traffic systems operations and analysis. Developed by Trafficware [ web ]. Other traffic microsimulation packages include AIMSUN, CORSIM, Cube Dynasim, Paramics, TransModeler, and VISSIM.

Simulation

An abstraction of real behavior via a simplified, manageable process that models that behavior. Typically computer-based and often stochastic, simulation provides experimental results for complex real behavior. A distinction increasingly has been made between a simulation that is designed to replicate a system's behavior (necessarily dynamic) versus a simulation that is designed to produce similar outputs (typically static).

Simultaneous Model

A model where all outputs are determined at the same time (for example, most direct demand models). Compare with sequential model.

Singly Constrained Gravity Model

A gravity model used for trip distribution derived via the Most Probable Distribution (Maximum Entropy) approach where only trip productions are constrained to match those input from trip generation. Until recent years, such models were predominant in travel forecasting applications (see Doubly Constrained Gravity Model).

SIP

A State Implementation Plan, required of states with areas that do not achieve conformity with CAAA requirements, documents the proposed regulations and process to be implemented to achieve conformity.

Site Access

Access and egress points from a land development site to the adjacent transportation network. Site access design is a key component of traffic impact studies, influencing the directional distribution of traffic to and from the site and thus traffic impacts and options for mitigation.

Site Impact Analysis

Same as Traffic Impact Analysis.

Sketch Planning

Sketch planning is the application of simplified methods of analysis to provide approximate performance estimates in the initial screening of alternatives. With advances in computing, traditional sketch planning methods are of less benefit relative to full model applications. However, for smaller area analysis, "What if?" models that integrate GIS, databases, and graphic user interfaces have become popular tools in public participation forums such as charrettes.

Skims

Skimming a network is the process of systematically identifying network paths (typically) based on the minimization of travel time, distance, or generalized cost. The resulting matrices of zone-to-zone impedances are called skims.

Slugging

Ad hoc ridesharing combining elements of hitchhiking and carpooling to enable SOV drivers to utilize HOV facilities by picking up "sluggers" waiting at designated locations.

Smart

An increasing trite buzzword implying a process or technology that facilitates transportation planning, design, or management, or increases the efficiency of transportation operations. Similar terms include "advanced" and "intelligent".

Smart Card

A credit-card-sized authentication device with an imbedded microchip containing personal identification information that can be read via proximity or contact with a sensor/reader. Used in car sharing systems to provide vehicle access to only the reservation-holder during the reservation period.

Smart City

The interconnection of municipal systems (e.g., public services, energy generation and distribution, transportation) via information and communications technologies. Also known as the Connected City.

Smart Grid

The evolution of the domestic power generation and distribution system that can respond to dynamic electricity demands in real time.

Smart Growth

A planning concept focused on increased density and diversity, circulation continuity, alternative travel modes, and a better sense of neighborhood scale. By reducing automobile travel, land consumption, and the need for new transportation infrastructure, smart growth seeks to minimize impacts on the built and natural environments. Also known as New Urbanism, neo-traditional design, and transit-oriented development.

Smart Growth Index

A GIS sketch planning tool for comparing alternative and use and transportation scenarios and evaluating outcomes using community and environmental performance indicators. Developed by Criterion Planners for USEPA in 2002 [ web ]. See also INDEX.

Smart Places

A version of PLACE3S from the Electric Power Research Institute [ web ].

SMSA

Standard Metropolitan Statistical Area (see census areas).

Smog

A generic term for visibly poor air quality, smog is an historical contraction of smoke and fog. Today's photochemical smog is characterized by a stagnant haze produced by chemical reactions of sunlight and various air pollutants including nitrogen oxides and volatile organic compounds.

SO

System Optimal (see SO).

Soak Time

The time interval since the end of a vehicle's last trip, a key factor in determining engine emissions and cold starts.

Socio-Demographic Data

A combination of socio-economic and demographic characteristics of a population.

Socio-economic Adjustment Factor

Another name for K Factors, calibration parameters that provide a means for cell-by-cell adjustments in gravity models, and which typically are related to socio-economic variables to lend credibility in forecasting.

Socio-Economic Data

Characteristics of the population, including income, employment status, and auto ownership, usually defined at the household and individual levels. While often used interchangeably with the term demographic data, socio-economic data are perhaps best viewed as time-varying attributes that define status within a state (such as auto ownership or income) whereas demographic data correspond to fixed characteristics of population that define the state (such as age and gender). Socio-economic and demographic data, together with land use data, are key inputs to trip generation.

Sojourn

A distinct, non-home activity on a trip chain. Multi-purpose activities occuring at the same location (a single stop on a trip chain) typically have been considered as a single sojourn (although this may be a survey limitation).

SOV

Single Occupant Vehicle (see HOV).

SOx

Oxides of sulfur are criteria pollutants produced in the combustion of sulfur-containing fuels.

Space Mean Speed

The average speed of all vehicles occupying a defined section of roadway at a point in time (the harmonic mean speed). With volume and density, space mean speed defines the fundamental diagram of traffic flow. Compare with time mean speed.

Space-Time Prism

The three dimensional representation of human activity and travel developed by Hagerstrand depicts the space that an individual can access, in the time available and reflecting the speed of movement through the defined space, as a prism. In idealized space, the prism is a cone expanding radially in 2-D space as it extends vertically in time from a fixed location, intersecting a second cone collapsing to an alternate location in space and time that the individual is constrained to visit.

Spacing

The distance (in feet) between successive vehicles traversing a section of roadway, usually averaged over all vehicles on a defined section. Spacing s (distance headway) is the inverse of density d: s = 5280/d in feet/vehicle (see also headway).

Spatial

Pertaining to dimensions of space such as location, distance, direction, elevation, and area. Often used synonymously with "geographic".

Spatial Interaction

Theory and models that address the interdependence between geographical areas.
Trip distribution utililizes models of spatial interaction.

Spatial Mismatch

The geographical separation of employment and housing, paricularly for low income or minority groups.

Special Generator

A location or zone that exhibits trip rates or patterns that can not be captured by a study area's trip generation model. Separate surveys and models are used to estimate productions and attractions for special generator. Examples include airports, college campuses, and military bases.

Specific Plan

Part of a comprehensive plan corresponding to a defined function and/or spatial area and containing development standards and criteria that supplement those of the comprehensive plan.

Specification, Model

The process of defining model structure, functional form, variables, and parameters (see estimation, calibration, validation, and application).

Speed

Speed is distance traveled per unit time (the inverse of unit travel time). In traffic operations, two measures of average speed are space mean speed and time mean speed. SMS is the average speed of all vehicles occupying a defined section of roadway at a point in time. TMS is the average speed of all vehicles passing a point on a roadway for a defined period of time. With volume and density, space mean speed defines the fundamental diagram of traffic flow (which provides a direct link to performance functions). Speed and travel time are standard performance measures in system evaluation.

Speed Bump

A raised section of pavement across a travel lane designed to reduce vehicle speed. Speed bumps are more pronounced than speed humps, thus, humps have become more common on roads (see also traffic calming).

Speed/Capacity Table

See look-up table.

Speed-Density Model

A traffic stream model that depicts the relationship between speed and density, such as Greenshields' or Greenberg's models.

Sprawl

An often pejorative term referring to the expansive spatial growth of a metropolitan region, often at lower development densities. The development of most American suburban areas is considered sprawl.

Stakeholder

Any individual or group that holds some intrinsic interest in and may be impacted by a specific plan, policy, or project.

Standard Industrial Classification

Employer groups identified by primary economic activity to standardize data collection and analysis (see sector [1]), since replaced by NAICS.

State Implementation Plan

Part of the overall transportation planning process, the SIP is a statewide plan mandated by the CAA and containing procedures to monitor, control, maintain, and enforce compliance with NAAQS.

State-of-the-Art

The highest level of research and development of a specific technology or concept at the current time (see state-of-the-practice).

State-of-the-Practice

The level of general deployment of a specific technology or concept at the current time (see state-of-the-art).

State Plane Coordinates

A geographical coordinate system based on distance from a defined origin. SPC systems may vary by state to minimize distortion. GPS utilizes latitude and longitude.

Stated Preference

A individual's preference expressed in response to a set of hypothetical choices. Compare with revealed preference.

Statewide Model

A travel forecasting model developed for an entire state to address statewide planning issues (such as freight modeling) and to coordinate input to and improve consistency of regional models, increasing common in the post-ISTEA era.

Static Assignment

Network assignment with fixed (versus dynamic) demand (see dynamic assignment).

Stationary Sources

Large, fixed facilities such as power plants, oil refineries, and chemical processing industries (also known as point sources). See also mobile sources and natural sources.

Station Car

A car sharing system linked to rail transit systems where a reserved shared-use vehicle is accessed at the end of a rail trip using a smart card and is later returned to the station to make the return rail trip. A balanced system has a shared-use vehicle that is returned to the station at the end of the work day to be picked up by a commuter returning home to use the car until the next morning (e.g., ZEVNet).

STEP

Surface Transportation Environment and Planning Cooperative Research Program, part of SAFETEA-LU.

STIP

The State Transportation Improvement Program combines RTIPs and state-level projects and determines which transportation projects will be funded. All STIP projects must be consistent with the long-range transportation plan (LRTP). The STIP typically covers a five year period and is updated every two years (see TIP).

Stochastic

A probabilistic function or process; having a random component (see deterministic).

Stochastic Assignment

A network loading process that assumes a distribution of perceived travel times and thus produces a multipath assignment (e.g., Dial's Algorithm). Compare to Wardrop's Principles, both which assume perfect knowledge of all path travel times and lead to deterministic assignments.

Stochastic User Equilibrium

An equilibrium trip assignment that satisfies a stochastic version of Wardrop's 1st Principle that states that no traveler can improve his perceived travel time by unilaterally changing routes. An SUE is obtained using MSA with stochastic loading (e.g., Dial's ) whereas UE typically uses Frank-Wolfe and deterministic loading (e.g., AON ).

Stop

[1] A designated location on a transit line where a transit vehicle stops to allow passenger boarding and unboarding.
[2] A vehicle coming to a complete stop, due to traffic control, congestion, of other factors, contributing to the overall delay on a trip.

Structural Equation Model

A linear-in-parameters multivariate statistical technique for estimating causal relationships. SEM, which includes regression, path analysis, simultaneous equations, and factor analysis as special cases, is confirmatory rather than exploratory. It combines a structural model with measurement models for exogenous and endogenous variables which allows for the estimation of latent variables.

Structurally Deficient

Infrastructure (e.g., a bridge) characterized by deteriorated conditions and a reduction in structural load-carrying capacity (see functionally obsolete).

Study Area

A defined region within which estimates of travel demand and system performance are desired. A corridor is a linear study area focused on one or more transportation facilities along the corridor.

Sub-area

A portion of a defined study area for a regional travel forecasting model that is selected for focused analyses, a process that often involves extraction of a sub-area network and a sub-area trip table.

Sub-area Extraction

The process of extracting a sub-area model from a larger (typically regional) model that involves defining the spatial extent of the sub-area and extracting a sub-area network and the corresponding sub-area trip table. The external network and travel data is removed, with the resulting model serving essentially as a window on the regional model (compare with a focus model).

Suburban

Characterized as predominantly residential areas adjacent to cities and usually with significant social and economic interaction. Geographically, the part of a metropolitan area outside of a center city (see metropolitan classification).

Suburbanization

The movement of population from urban areas to the development periphery (or suburbs), classically characterized by lower density, automobile-dominated housing.

Structural Equation Model

A linear-in-parameters multivariate statistical technique for estimating causal relationships. SEM, which includes regression, path analysis, simultaneous equations, and factor analysis as special cases, is confirmatory rather than exploratory. It combines a structural model with measurement models for exogenous and endogenous variables which allows for the estimation of latent variables.

Success Story

A buzzword for a typically public-funded project characterized by achievement of one or more expected objectives within the expected budget, when similar projects have not met objectives or have incurred cost overruns (compare with boondoggle).

SUE

Stochastic User Equilibrium (see SUE).

Sulfur Oxides

Oxides of sulfur (SOx) are criteria pollutants produced in the combustion of sulfur-containing fuels.

Superelevation

The degree of banking of a roadway on a curve to counter the centrifugal force on a vehicle traversing the curve.

Supply

The physical extent of transportation system infrastructure and services provided. Supply is often referenced as the broadly defined counterpoint of demand, but performance is utilized when level-of-service characteristics are relevant.

Supply Chain Management

The overall, performance-based, production and distribution management of a commodity from obtaining raw materials through intermediate and final consumption.

Supply Function

In Transportation Systems Analysis, the supply function represents the relationship between transportation infrastructure and services that a supplier (often a government agency) is willing to provide and the state of the current system, the costs of system alternatives, and expected system behavior. In travel forecasting, supply procedures typically are not specified analytically; rather, they are where the analyst "breaks" and controls the overall TSA process and evaluates alternative system options from a "what if?" approach. Supply alternatives are typically specified exogenously, then performance and demand functions are equilibrated to determine flows for the specified alternative.

Supressed Demand

Travel demand that was not previously observed due to high costs of travel but when capacity is increased or price decreased, is now observed on improved facilities. A more common term, induced demand, suggests that the "improvement" itself is be sufficient to increase demand rather than demand being determined by activity and lifestyle behavior and cost factors. Suppressed demand would not include that associated with socio-demographic growth.

Survey

The systematic collection of data representing the behavior of a defined population (see: cross sectional, Longitudinal, or Panel Survey; household travel, external travel, or origin-destination

Survey Coding

The translation of survey data into defined categories suitable for data processing and analysis.

Sustainability

The long-term stewardship of the natural and built environments, reflecting the environmental, economic, and social dimensions of resource maintenance and consumption.

Sustainable Communities Strategy

Under SB375, California MPOs must include a Sustainable Communities Strategy (SCS) that incorporates regional land use and housing strategies into the Regional Transportation Plan (RTP), with the objective of achieving targeted reductions in Greenhouse Gas (GHG) emissions from cars and light trucks (see also Alternative Planning Strategy).

Sustainable Development

[1] A land use pattern characterized by growth and development occurring in a manner supported by infrastructure and financial resources, and proportional to the preservation of the current built and natural environments.
[2] a general social, economic, and environmental perspective toward addressing current needs without compromising future needs.

Sustainable Transport

A component of sustainable development but otherwise not consistently defined beyond transportation systems supported by sustainable infrastructure as well as financial, natural, and other resources to meet mobility needs. Sustainable mobility, also called "new mobility", commonly extends the definition to more formal operational terms.

Swarm

A general agent-based modeling software platform developed by the Sante Fe Institute [ web ].

SWITRS

StateWide Integrated Traffic Records System is a database of traffic incidents maintained by the California Highway Patrol (see also TASAS ).

Synchro

A comprehensive software package for traffic systems operations and analysis, linked to SimTraffic, a traffic microsimulation software package. Developed by Trafficware [ web ].

Synthetic Population

Disaggregate household level data that is synthesized from population-level marginal demographic distributions and small-sample joint distributions.

System

A collection of independent but interrelated components, real or abstract, acting in form and/or function as a integrated whole.

Systems Analysis

The comprehensive approach to defining, formulating, and solving problems associated with complex systems, such as transportation systems, and applied to planning, design, implementation, operations, and management tasks.

System Equilibrium

A system state where overall demand and system performance are in equilibrium. In travel forecasting, this would corresponds to full equilibration of trip frequency, destination, mode, time-of-day, and route. In the four step model, only route choice is in equilibrium, unless system feedback is applied. See network equilibrium.

System Optimal

A system optimal (SO) trip assignment satisfies Wardrop's 2nd Principle that states, for a given origin-destination pair, that the total travel time is a minimum. An SO assignment is optimal but is not an equilibrium result, since individual travelers could improve their travel times by unilaterally changing routes (see also user equilibrium).

T[ back to top ]

t Test

A test of variable significance in least squares and other estimation methods, a t statistic is computed as the ratio of a variable's estimated coefficient to the standard error of that coefficient. If the calculated value is greater than the critical value, then the null hypothesis that the variable's actual coefficient is equal to zero is rejected. In binary regression, the t statistic is the square root of the F statistic.

TAC

Technical Advisory Committee, often associated with travel forecasting and related modeling efforts to provide peer review during model development and application.

TASAS

The Traffic Accident Surveillance and Analysis System is a database that integrates traffic incident data from CHP's SWITRS with a Highway Data Base maintained by Caltrans.

TAZ

Traffic Analysis Zone (see TAZ).

TCM

Transportation Control Measure (see TCM).

TDA

Transportation Development Act, California state law (1971) that dedicates 0.25 percent on each dollar on retail sales to transit and non-motorized transportation.

TDM

Travel Demand Management (see TDM).

TEA-21

The Transportation Equity Act for the 21st Century, the federal transportation authorization enacted in 1998 and continuing the series of Federal Aid Highway Acts, continued the metropolitan and statewide planning requirements from ISTEA but consolidated the broad range of factors to be considered in the planning process. TEA-21 was preceded by ISTEA, the Intermodal Surface Transportation Efficiency Act of 1991 and followed by SAFETEA-LU, the Safe, Accountable, Flexible, and Efficient Transportation Equity Act -- A Legacy for Users (2005) [ web ].

Technical Advisory Committee

A standing committee organized and utilized by MPOs and transportation authorities to provide regular review of technical planning and modeling activities, typically comprising engineers, planners, and similar professionals from local and state agencies.

Technology Transfer

Activities that foster the dissemination and application of a innovative technologies and methodologies.

Telecommuting

Replacing a commute trip by performing a work activity at home (or a telecommuting center closer to home than the workplace is), creating a potential trip reduction (at least in the peak commute period). A substitution of telecommunications for travel.

Telecommunications

The conveyance of information via electronic means, whereas transportation is the conveyance of people or freight via physical means. Substituting telecommunications for travel can affect trip rates and the associated system impacts.

Terminal

A transportation component of the built environment that includes the interface facilities that allow for transfers between different modes.

Terminal Time

The excess time incurred at the origin and destination of a trip, accessing or egressing an activity and beginning a trip's primary travel component. Terminal time is often added to skim tree results to better reflect full point-to-point travel time.

TEU

A Twenty-Foot Equivalent Unit container, the standard unit of measure to quantify container traffic flows. One (8 foot by 8 foot by) 20-foot long shipping container equals one 20-foot Equivalent Unit.

Third Party Logistics

An independent provider of transportation, warehousing, and logistics services to buyers and/or sellers.

Through Trip

A trip with both its origin and its destination located outside of the study area (see also internal trip and external trip).

TIGER

Ttransportation Investment Generating Economic Recovery (part of ARRA).

TIGER Files

Topographically Integrated Geographic Encoding and Referencing files are digital maps produced by the US Census Bureau.

Time-based

A simulation where all variables are updated at a fixed time increment (e.g., every second); in an event-based simulation, variables are updated with the occurance of specified events (e.g., arrivals or departures for a queue).

Time Budget

The average amount of time an individual allocates for daily travel. Time budget theory postulates that people with similar socio-economic and life style characteristics tend to have similar time budgets. Empirical evidence supports population averages over a wide range of infrastructure and land use characteristics.

Time Geography

A conceptual framework for the analysis of activity and travel that reflects the simultaneity of time and space and the constraints that define possible actions, typically expressed in a three dimensional graphical representation (see Hagerstrand ).

Time In Motion

A time in motion study produces the distribution of trips from a travel survey based on reported trip start and end times, providing the proportion of total trips actually in motion at a specified time.

Time Mean Speed

The average speed of vehicles passing a point on a road over a defined period of time (the arithmetic mean speed). Compare with space mean speed.

Time-of-Day

In the conventional Four Step Model, Time-of-Day (TOD) modeling can be considered a 5th step, often executed at a point prior to trip assignment to convert generated daily trips to those trips occurring during a particular period of analysis (such as the peak hour).

Time-of-Day Factor

The proportion of daily vehicle-trips occuring in a designated period, such as a peak-hour or a peak-period. In travel forecasting, these parameters are based on travel survey data and are used to convert daily PA trip tables to period-specific OD trip tables prior to assignment.

Time Series

Data defined and ordered for specified (usually regularly distributed) points in time, as well as methods and models used to describe and forecast with such data.

Time-Space Diagram

A plot of vehicle trajectories over time and space, with space defined along an arterial or other roadway and time reflecting movement and queueing.

Time-Space Prism

See space-time prisms.

Time Use

How individuals allocate their time to activity and travel over defined time periods.

Time Value of Money

The concept that an amount of money today can be invested and earn interest by a future time, forming the basis of project appraisal.

TIP

Transportation Improvement Program (see TIP).

Title VI

That portion of the Civil Rights Act of 1964 that ensures environmental justice in the location of transportation infrastructure and for improvements in system performance.

TLFD

Trip Length Frequency Distribution (see TLFD ).

TMC

Traffic Management Center (see TMC ).

TMIP

USDOT's Travel Model Improvement Program is designed to foster the development and application of improved transportation modeling methods and capabilities to meet local, state, and federal planning requirements [ web ].

T-Model

A software package for travel forecasting and transportation network analysis. Developed by T-Model Corporation and owned by PTV [ web1 ; web2 ] Other travel forecasting packages include Cube, EMME/2, MinUTP, QRSII, Tranplan, and TransCAD.

TOC

Traffic Operations Center, same as TMC.

TOD

[1] Time-of-Day (see TOD[1] )
[2] Transit Oriented Development (see TOD[2]).

TODF

Time-Of-Day Factor (see TODF)

Toll

A fixed or distance-based fee charged to users of certain roadways and bridges. As federal and state revenues are declining from fuel excise taxes, increased pressure has been placed on alternative revenue sources such as tolls, including as part of congestion pricing where tolls can serve to regulate demand as well as generate revenues.

Tour

A trip chain -- a sequence of trips and activities, typically starting and ending at home.

Tour-based Model

A variation of the conventional trip-based travel forecasting model that uses tours rather than individual trips and thus can account for the linkages between individual trips and activities (and thus better represent the underlying connectivity of travel behavior). Compare with trip-based and activity-based models.

TP+

A software package for travel forecasting and transportation network analysis that evolved from TranPlan. Marketed by Citilabs [ web ]. Other travel forecasting packages include Cube, EMME/2, MinUTP, QRSII, T-Model, and TransCAD.

TRAF

An traffic simulation software suite integrating NETSIM for arterials and FRESIM for freeways, that evolved into TSIS/CORSIM.

Traffic Analysis Zone

A defined zone for travel forecasting and traffic simulation studies, represented in the network by a centroid. See also census tract.

Traffic Assignment

The process of loading vehicle traffic onto a network, traffic assignment is often used interchangeably with trip assignment, which more broadly defines the process of loading trips, vehicle-trips or person-trips, to appropriately defined modal networks.

Traffic Calming

Local street design techniques that reduce traffic speeds and discourage traffic incursion in residential neighborhoods to improve local street safety and neighborhood quality of life. Techniques include physical traffic barriers (e.g., diverters, chokers, speed humps), revised street alignments (e.g., traffic circles, chicanes), and traffic speed enforcement.

Traffic Congestion Relief

A State of California program enacted in AB 2928 (2000) that designates funding for specific projects to address traffic congestion.

Traffic Control

The control of traffic via any of a number of passive rules or signs (including travel way delineations, rights-of-way and other rules-of-the-road, and traffic markings and signs) or active human agents or control devices (police officers and traffic signals), to optimize safe and efficient flows.

Traffic Counts

Observed volume counts on network links, often collected annually for a 24-hour period. Detectors such as ILDs are becoming more widely deployed to provide continuous traffic data (see PeMS), replacing the need for annual field studies using mechanical detectors such as pneumatic tubes. In travel forecasting, counts are used in model validation as well as for updating trip tables, particularly for real time traffic management.

Traffic Engineering

The Institute of Transportation Engineers (ITE) defines Traffic Engineering as "the phase of transportation engineering that deals with the planning, geometric design, and traffic operations of roads, streets, and highways; their networks, terminals, abutting lands, and relationships with other modes of transportation."

Traffic Flow Theory

Established and experimentally verified principles that describe mathematically the interactions between vehicles, drivers, and transportation infrastructure.

Traffic Impact Analysis

The process of completing a Traffic Impact Study.

Traffic Impact Study

A Traffic Impact Study (TIS) is conducted to assess the effects on traffic flows of changes in land development. The analysis often follows the general steps of the four step model but while regional travel forecasting models may be used with large scale development changes (often as part of an EIS), various heuristic methods are often used for smaller scale land use changes. The TIS focus is the quantification of traffic impact and the development of mitigation measures and impact fees.

Traffic Intensity

In queueing theory, the ratio of the mean arrival rate to the mean service rate (must be less than 1.0 for most queueing relationships to apply).

Traffic Management Center

A hub for management, operations, and control of traffic networks, incorporating real time data collection via loops, CCTV, and other sensors; communication technologies such as VMS and HAR; traffic analysis capabilities such as expert systems, microsimulation, and control optimization software, and visual display capabilities. A TMC addresses special events, incidents, and public relations, and the day-to-day management of traffic systems.

Traffic Signal Optimization

Optimizing the performance of an isolated traffic signal, with regard to signal phasing and timing, or a series of signals, with regard to phasing, timing, and coordination.

Traffic Signal Synchronization

The coordination of a series of traffic control signals to optimize the progression of traffic flow along an arterial or designated route by setting signal cycle length, phasing, and offsets so that traffic approaching a signalized intersection receives a green phase prior to slowing to minimize stops and delay (AKA signal coordination ).

Traffic Stream Model

Macroscopic speed-density models that describe basic uninterrupted traffic flow, including the Greenshields, Greenberg, and Underwood single-regime models, and multi-regime models such as Edie's.

TRAFFIX

Integrated software for intersection capacity analysis and traffic impact analysis. Developed by Dowling [ web ].

TranPlan

A software package for travel forecasting and transportation network analysis. Evolved into TP+. Marketed by Citilabs [ web ]. Other travel forecasting packages include Cube, EMME/2, MinUTP, QRSII, T-Model, and TransCAD.

TransCAD

A software package for travel forecasting, incorporating an integrated GIS. Developed by Caliper [ web ]. Other travel forecasting packages include Cube, EMME/2, MinUTP, QRSII, T-Model, and Tranplan.

Transferability

The degree to which a model developed for a particular time and location can be successfully applied to a different time and/or location.

TRANSIMS

A third generation travel analysis software package, incorporating microsimulation and activity-based approaches to provide more accurate assessment of travel impacts, especially regarding performance and air quality assessment. Developed by Los Alamos National Laboratories ( LANL ), a commercial version was marketed by IBM Business Consultants [ web ] and an open-source version is available via FHWA [ web ].

Transit

See public transit.

Transit Dependent

An individual lacking alternative modes of transportation and thus dependent on public transit to meet private mobility needs.

Transit Line

A defined route along which a transit vehicle travels, usually on a defined schedule and stopping at designated transit stops.

Transit Oriented Development

Residential, commercial, or mixed use developments designed to maximize access to and usage of public transport. New urbanist or neo-traditional development designs typically are more amenable to transit-oriented development than are conventional urban development approaches, but none of these design alternatives require transit to be integrated. Transit-Oriented Developments (TODs) may be designed jointly with new transit systems. The growth in domestic rail transit systems initially was accompanied by expectations for joint development where TOD accessibility benefits could be taxed to help pay for the rail project; today, however, TODs may receive tax concessions and other financial incentives to encourage development supportive of transit use.

Trans-Loading

The transfer of a commodity from one form of transportation (carrier, mode, container type) to another that explicitly involves unloading and reloading of the commodity with the goal of reducing overall transportation cost. Compare to transshipment.

TransModeler

A traffic microsimulation software, linked to TransCAD. Developed by Caliper [ web ]. Other traffic microsimulation packages include AIMSUN, CORSIM, Cube Dynasim, Paramics, SimTraffic, and VISSIM.

Transponder

An electronic tag attached to a vehicle and scanned by a roadside device that identifies the vehicle and takes appropriate action (e.g., charge a toll).

Transportation Control Measures

Various strategies to reduce private vehicle usage and/or improve traffic flow to reduce vehicle emissions. Examples include ridesharing programs, freeway service patrols, and improved transit programs.

Transportation Demand Management

Same as Travel Demand Management. Programs and projects to reduce transportation demand, including ridesharing, transit service enhancements, and parking management. Transportation Systems Management is the corresponding supply-side process.

Transportation Economics

The interdisciplinary development and application of economic concepts and methods to the planning, analysis, and operation of transportation systems.

Transportation Engineering

The Institute of Transportation Engineers (ITE) defines Transportation Engineering as "the application of technology and scientific principles to the planning, design, operation, maintenance, and management of systems and facilities for any mode of surface transportation in order to provide for the safe, rapid, comfortable, convenient, economical, and environmentally compatible movement of people and goods."

Transportation Geography

A sub-discipline of geography that studies the spatial patterns of movement of people, goods, and information.

Transportation Impact Fee

A fee levied on a developer by a jurisdiction to compensate for the expected transportation impacts of the development, usually determined as part of a traffic impact study. See impact fees.

Transportation Improvement Program

The Transportation Improvement Program (TIP) is a plan for state and federal funds to be allocated to local and regional transportation projects. In California, the TIP is cooperatively prepared by local governments, local and regional transportation agencies, and Caltrans. It covers a 5-7 year period and is updated every 2 years. A Regional TIP lists highway and transit projects from priority lists developed by local jurisdictions. The state TIP is submitted to the California Transportation Commission and incorporates elements of the regional TIPs.

Transportation Management Association

Not-for-profit organizations that coordinate and/or provide transportation services in defined employment centers, TMAs are often public/private partnerships.

Transportation Planning

The "continuing, comprehensive, and collaborative" (3C) process to develop and maintain a transportation system to meet needs for the effective and efficient movement of people and goods reflecting social, economic, and environmental constraints. The Transportation Planning Process involves defining goals and objectives, monitoring system performance, identifying problems, generating and analyzing alternatives, evaluating, selecting, and implementing a preferred alternative.

Transportation Planning Model

A general term for models used in transportation planning, usually implying models used for travel forecasting but also including land use and other models used in planning.

Transportation Planning Process

The Transportation Planning Process is the system-analytic application of continuing, comprehensive, and cooperative planning. Often represented as five broad stages of (1) problem definition, (2) solution generation, (3) solution analysis, (4) evaluation and choice, and (5) implementation and monitoring. Travel forecasting occurs during the third stage, solution analysis.

Transportation Problem

A fundamental problem in operations research that seeks the single commodity, minimum cost flow for N supply points directly connected to each of M demand points. Optimal versus descriptive, it is directly applicable to freight distribution problems but not to conventional travel forecasting (see Hitchcock Transportation Algorithm).

Transportation Research Board

The Transportation Research Board, a branch of the National Academy of Science, promotes innovation and progress in transportation through research and facilitates the sharing of information on transportation practice and policy [ web ].

Transportation System

In the Manheim/Florian framework for transportation systems analysis, the Transportation System constitutes the networks (modes, links, nodes, etc.) connecting a region's Activity System. The Transportation System in defined by supply functions and is a key element in defining performance functions.

Transportation Systems Analysis

Transportation Systems Analysis (TSA) as formalized by Manheim and later expanded by Florian (1986) is a framework for the systematic analysis and evaluation of transportation systems. The framework is flexible to allow components to be specified exogenously or endogenously based on the scale and scope of the models and policies being studies. Basic components include demand and performance procedures, as well as activity location and supply procedures.

Transportation Systems Management

Programs and projects to increase performance of the transportation system via lower cost improvements in the design, maintenance, and operations of existing facilities rather than investing in major increases in infrastructure. Travel Demand Management is the corresponding demand-side process.

Transshipment

The movement of freight from one carrier and/or mode to another carrier and/or mode. Compare to transloading.

Transshipment Problem

An generalization of the Transportation Problem that seeks the single-commodity, minimum cost flow for networks with intermediate (transshipment) points. The system optimal formulation of the trip assignment problem is a multi-commodity transshipment problem.

TRANSYT

The TRAffic Network StudY Tool, an optimization and analysis model for traffic operations, was developed by TRRL to optimally determine fixed-time traffic control (cycle length, offsets, and splits) [web: UK version ; US version ].

TRANSYT-7F

The official FHWA version of TRANSYT combines signal optimization with macroscopic simulation, incorporating platoon dispersion and queue spillback. The objective function can include signal progression, delay and stop reduction, fuel consumption variables to obtain an optimal fixed-time signal plan [web: US version ].

TRANUS

A land use model software package developed by de la Barra (1990), based on a spatial input-output framework. Similar packages include MEPLAN, MUSSA, UrbanSim, PECAS, and ITLUP [ web ].

Travel Activity Diary

A primary component of household travel surveys, the travel and activity diary records all trips and activities for all household members for one or more days. In the past, diaries recorded daily travel behavior by trips and associated characteristics (including trip purpose). Now, most diaries record activities and associated characteristics (including travel attributes). While in theory the same data is collected, results suggest that activity diaries provide a more complete recording of all travel and activity. Recent develops include the use of GPS and the application of computer-based diaries to complement or replace manual recording.

Travel Behavior

The travel and associated activities in which individuals engage over space and time (see activity analysis and time use).

Travel Cost

Travel cost can refer to the actual monetary expenditure on a trip or to a more general measure of travel disutility. Regarding monetary expenditure, travel cost can refer to actual out-of-pocket costs on a trip or a more complete cost accounting of total cost. Regarding general measures, in travel forecasting models, travel time is often individually used for travel cost or as a component of generalized cost.

Travel Demand

The derived demand to access locations via the transportation system to perform activities (see demand).

Travel Demand Forecasting

The process of estimating, calibrating, validating, and applying models of travel demand and performance to estimate the impacts of alternative transportation systems. Perhaps better expressed as travel forecasting since inputs and outputs include elements of both demand and supply.

Travel Demand Management

Programs and projects to reduce transportation demand, including ridesharing, transit service enhancements, and parking management. Transportation Systems Management is the corresponding supply-side process.

Travel Demand Model

A (typically) mathematical abstraction of travel behavior, based on statistical and/or algorithmic representations of observed travel patterns. The objective is to forecast outcomes of alternate transportation and activity system scenarios as a function of current transportation and activity systems, empirical distributions of travel patterns (by frequency, destination, mode, time-of-day, and route), and projections of future socio-demographic characteristics. A misnomer since most such models reflect both demand and supply of transportation in a particular area at a particular time.

Travel Forecasting

The process of estimating, calibrating, validating, and applying models of travel demand and performance to estimate the impacts of alternative transportation systems. Also referred to as travel demand forecasting (or travel demand modeling). Although the basic model structure (the four step model) appears relatively unchanged since its introduction in the 1960s, model components and software have evolved significantly. Changes have included a move from aggregate to disaggregate models and data, a greater behavior basis, and more sophisticated equilibration algorithms. The evolution continues with tour-based, activity-based, dynamic, and microsimulation approaches.

Travel Survey

The primary mechanism to collect data on travel behavior are travel surveys. A Household Travel Survey provides the primary data for regional travel forecasting model development. Other surveys include external, transit on-board, and commercial vehicle surveys. Travel data complements non-travel data from census, land use, and employer surveys.

Travel Time

Travel time is a key determinant of travel behavior, frequently used as the sole measure of travel cost in conventional travel forecasting. In Transportation Systems Analysis, travel time is an output of performance functions and an input to demand functions, both of which are equilibrated to solve for system flows. In the four step model, travel time is a key variable in the trip distribution, mode choice, and trip assignment steps, and the basis for feedback in the model system. Mean travel time (overall, by time-of-day, by trip type, etc.) is a key performance measure in alternatives analysis and system evaluation. See value of time.

Travel Time Function

A mathematical function of travel time used in spatial interaction models, with standard forms including inverse power, negative exponential, and combined gamma functions, as well as friction factors.

Travel Time Reliability

The degree of certainty or predictability of travel times in a transportation system.

Travelling Salesman Problem

A classic problem of operations research that seeks the minimum cost route between multiple network locations, visiting each location only once and returning to the starting point. An NP-Hard problem solved via dynamic programming, heuristic algorithms, and a variety of other approaches.

TRB

The Transportation Research Board, a branch of the National Academy of Science, promotes innovation and progress in transportation through research and facilitates the sharing of information on transportation practice and policy [ web ].

Tree

See Minimum Path Tree.

Trend

A regular pattern evident in data that can be associated with an hypothesized system behavior and thus can be used to predict future states of the system (see trend projection).

Trend Projection

Applying statistical methods to identify and extrapolate trends in observed data, such as using a time-series model to predict a future state. Used for demographic projections (such as population projection via linear, exponential, or logistic models) and for forecasting other inputs to transportation models (such as auto ownership or labor force participation rates).

Trip

A movement by an individual from one location to perform an activity at a different location. The fundamental unit of analysis in conventional travel forecasting.

Trip Assignment

Trip Assignment (TA), the fourth step in the conventional four step model of travel forecasting, is a process by which trips, defined by time-of-day and mode, are loaded on feasible paths between an origin and a destination in a network. The output of assignment is the number of vehicle-trips (or passenger-trips) allocated to paths and links on a modal network, as well as associated performance measures (such as VMT and travel time). Also known as traffic assignment or Network Assignment (see also route choice).

Trip Attraction

A trip generated (typically) by a household has both a production and an attraction. The number of trip attractions in a zone is proportional to the level of activity (land use) in that zone associated with the type of trip in question. If the trip maker's residence (home) is one end of the trip, then the other trip end is the attraction. If the trip maker's residence (home) is at neither trip end (i.e., a NHB trip), then the attraction is the same as the trip destination. Trip attractions are typically represented as aggregate regression models using data pooled from zones into districts (since travel surveys are residential-based, there are usually not enough observed attractions in all zones to estimate zonal attractions directly. See also production or origin.

Trip Attraction Model

In trip generation, separate models are estimated for trip productions and trip attractions, for each defined trip purpose, as a function of activity (and in theory LOS characteristics). Trip attraction models are usually regression models expressed at the zonal level, often as a function of employment variables.

Trip Balancing

In trip generation, trip productions and attractions are independently estimated, thus, for a given trip purpose, total productions will not equal total attractions. Since productions are typically considered to be more accurate estimates, zonal attractions are normalized so that total attractions are balanced with total productions for each trip purpose. Typically, trip balancing reflects both internal and external trips.

Trip-based Model

The predominant framework for travel forecasting models used in practice, trip-based models reflect the basic assumptions of the four-step model and focus on individual trips, not trips linked into tours nor the activities that generate these trips. Compare with tour-based and activity-based models.

Trip Chaining

An individual's process of linking trips and non-home activities into sequences without intervening trips returning to home. A trip chain (or tour) is defined such that the destination of the first trip becomes the origin of the second (a simple round trip is a trip chain with one non-home activity). From a policy viewpoint, multiple-activity trip chains are more efficient than a series of single-activity round trips.

Trip Distribution

Trip Distribution (TD) is the second step in the conventional four step model of travel forecasting. TD is the process of pairing generated productions and attractions (or origins and destinations) to determine the number of trips between all pairs of zones in the study area. The primary TD output are trip tables (typically 24-hour person trips, specified by trip purpose). TD follows trip generation in the four step model sequence, and is followed by mode choice or time-of-day factoring. The gravity model is the most common tool applied. In disaggregate terms, it is the process by which a trip's destination is selected, given the trip's purpose, origin, and travel cost to possible destinations.

Trip End

Either end of a trip. In trip generation, trips are represented as trip ends, with origins (or productions) modeled separately from destinations (or attractions). Trip end mode choice models were applied to the trip ends produced in trip generation rather than the more conventional trip interchange mode choice models that were applied after trip distribution.

Trip Frequency

The number of trips per unit time (typically, daily trips).

Trip Generation

Trip Generation (TG) is the first step in the conventional four step model of travel forecasting. TG is the process of estimating trip productions (or origins) and attractions (or destinations) for all zones in the study area. In regional travel forecasting studies, category or regression models are applied to estimate trip ends by trip purpose as a function of individual, household, or zonal socio-economic, land use, or accessibility characteristics (results are typically aggregated to the zone level). In traffic impact studies, land use-based trip rates (such as ITE) are applied at the project or parcel level in place of regional TG models. The outputs of trip generation analysis ("Os and Ds" or "Ps and As") serve as input to the second step of the four step process, trip distribution.

Trip Interchange

A cell of a trip table indicates a trip interchange, the total number of trips from an origin (or production) to a destination (or attraction). Interchanges are estimated in trip distribution.

Trip Length Frequency Distribution

A graphical or tabular display of travel times or trip distances, typically represented in discrete categories. TLFDs are used in the calibration of gravity trip distribution models.

Trip Production

A trip generated generally by a household as a function of household socio-economic or residential land use characteristics. If the trip maker's residence (home) is either end of the trip, then that location or zone is the production. If the trip maker's residence (home) is at neither trip end (i.e., a NHB trip), then the production location is the same as the trip's origin (compare with attraction and destination).

Trip Production Model

In trip generation, separate models are estimated for trip productions and trip attractions, for each defined trip purpose, as a function of activity (and in theory LOS characteristics). Trip production models are typically category models expressed at the household level, usually as a function of household characteristics.

Trip Purpose

The purpose of virtually any trip is the activity in which the trip maker will participate at the location of the end of the trip. The demand for the trip is derived from the demand for the activity. Conventional travel forecasting models often employ aggregate trip purposes in lieu of actual trip purposes. Such purposes usually identify both ends of the trip preceding the activity, such as home-work (or HBW), home-other (or HBO), or non-home-based (or NHB). Many larger metropolitan areas use additional purpose categories.

Trip Rate

For a specified land use or geographic area, a trip rate is the number of trips per unit time, typically scaled per areal or other unit. For example, the number of vehicle-trips entering a 7-11 store in a peak hour for every 1000 square feet of retail floor space. Trip rates may be expressed as mode-specific or by time of day. The Institute of Transportation Engineers (ITE) publishes a widely-used compendium of average trip rates for a variety of land uses and land use characteristics. Trip rates, in general, are modeled in the four step model in trip generation via techniques such as category analysis (which yields a trip rate model).

Trip Reduction

A decrease in the number of trips (typically vehicle trips in peak periods) resulting from policies to reduce congestion (such as alternative work schedules).

Trip Table

A matrix of trips from each origin (or production) to each destination (or attraction) in a region (also thus referred to as O-D or P-A matrices). Trip tables may be specified for a particular trip purpose, time period, or mode, and for person- or vehicle-trips. Trip tables are produced in trip distribution models (and modified in subsequent steps of the four step model). Trip tables may also be estimated or updated using traffic counts.

TRL

The UK's Transportation Research Laboratory, formerly the Transportation Road Research Laboratory (TRRL) [ web ].

TRRL

The UK's Transportation Road Research Laboratory, now renamed the Transportation Research Laboratory (TRL) [ web ].

TSIS

Traffic Software Integrated System, a traffic simulation software suite integrating CORSIM and support utilities [ web ].

TTI

Texas Transportation Institute, TTI's Urban Mobility Report is an annual assessment of traffic congestion in metropolitan areas.

Turning Movements

The trajectories of vehicles approaching an intersection. For a typical 4-way intersection, each approach has left-turn, thru, and right-turn movements. Field surveys provide these basic inputs to traffic signal optimization and intersection capacity analysis.

Turn Penalty

In standard travel forecasting, intersection movements are not explicitly modeled. Turn penalties and prohibitions are introduced to partially capture intersection behavior. A turn penalty is a specified increment added to the path travel time of traffic being assigned from an intersection approach link to an intersection exit link, corresponding to a turning movement that is subject to an average delay at the real intersection.

Turn Prohibition

In standard travel forecasting, intersection movements are not explicitly modeled. Turn penalties and prohibitions are introduced to partially capture intersection behavior. A turn prohibition is a flag that prevents traffic from being assigned from an intersection approach link to an intersection exit link, corresponding to a turning movement that is prohibited at the real intersection.

Two-Fluid Model

A macroscopic traffic flow model proposed by Herman and Prigogine (1979) based on analogy to molecular behavior in fluid condensation (Bose-Einstein), where the velocity of moving vehicles is related to the proportion of stopped vehicles, fs, or:
vr = vmax [ 1- fs ]n

Type I Error

The error of rejecting the null hypothesis (H0) when it is actually true. Level of significance is the probability of committing a Type I Error and defines the critical region for rejecting H0 (see Type II Error).

Type II Error

The error of accepting the null hypothesis (H0) when it is actually false. The power of the test is the probability of not committing a Type II error (see Type I Error).

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Uber

A "shared economy" technology firm that provides transport services for profit where the economic transaction ("sharing") is between the technology provider, a contracted physical provider, and a customer paying for the service. (see also Lyft). The technology is a smart phone application that links customers seeking immediate transportation with affiliated drivers in the vicinity.

UCTC

The University of California Transportation Center [ UCTC web ], one of 13 regional University Transportation Centers (UTC) [ UTC web ].

UE

User Equilibrium (AKA Deterministic User Equilibrium) (see UE).

UFOSNET

A GIS-based travel demand modeling system that integrates node-based delay and GPS data handling. Developed by RST [ web ]. Other travel forecasting packages include Cube, EMME/2, MinUTP, QRSII, T-Model, Tranplan, and TransCAD.

UMOT

Unified Mechanism Of Travel, a model proposed by Zahavi (1979) based on aggregate relationships involving travel time and money budgets, and other parameters claimed to be transferable over time and space.

UMTA

The former Urban Mass Transportation Administration, currently the FTA, in the US DOT.

Unbiased

An estimator is unbiased if its expected value is equal to the population parameter.

Underwood Model

A non-linear single-regime traffic stream model developed by Underwood (1961), relates space mean speed to density, addresses limitations in Greenberg's model, and is incorporated in Edie's two-regime flow model.
Underwood: u = uf exp(-d/dm) where uf is mean free speed; dm is density at max flow.

Unified Planning Work Program

An MPO's management plan to coordinate tasks in the overall transportation planning and programming process for the metropolitan area.

Unit Travel Time

Travel time per unit of distance, the inverse of speed, expressed in minutes per mile.

Unlinked Trip

A journey by transit from an origin to a destination may have multiple segments that are completed on different vehicles or modes. The overall O-D journey is one linked trip comprising multiple unlinked trip segments (and multiple transit boardings).

Upstream

Traffic or infrastructure that is located behind the current position, opposite the direction of movement (as opposed to downstream traffic or infrastructure that is ahead of the current position in the current direction of movement).

UPWP

Unified Planning Work Program (see UPWP ).

Urban

Developed areas such as cities and towns that exceed a defined population (e.g., 50,000 people within a defined boundary) or density (e.g., 1,000 people per square mile) threshold (see metropolitan classification).

Urban Form

The integrated spatial pattern of urban activity and transportation systems.

Urban Growth Boundary

A defined boundary around a metropolitan area intended to accommodate projected population and employment growth within a defined planning period. Such a smart growth strategy is intended to control sprawl beyond the boundary.

Urban Mobility Report

The Texas Transportation Institute's (TTI) Urban Mobility Report is an annual assessment of congestion in metropolitan areas [ web ].

Urban Renewal

Controversial urban land redevelopment programs, implemented in the post-war decades, that brought freeways and public housing projects to many cities.

Urban Sprawl

An often pejorative term referring to the expansive growth of a metropolitan region. The development of most American suburban areas is considered sprawl.

Urban Village

An urban design concept characterized by medium density, mixed use zoning and a strong orientation toward the use of transit and walking. A variation on Neo-traditional Design, New Urbanism, and Transit-Oriented Development.

Urbanized Area

A densely settled area that contains 50,000 or more people, as delineated by the Census Bureau.

UrbanSim

A comprehensive land use model software package developed by Waddell (2000) utilizing a micro-simulation approach to modeling transportation and land use interactions. Similar packages include MEPLAN, MUSSA, PECAS, TRANUS, and ITLUP [ web ].

USDOT

The United States Department Of Transportation, includes FHWA, FTA, and other transport agencies. All states have an equivalent agency, in most cases also named a DOT [ web ].

User Equilibrium

A user equilibrium (UE) trip assignment satisfies Wardrop's 1st Principle that states, for a given origin-destination pair, that travel times are equal on all paths actually utilized, and are less than or equal to the travel time on any other paths. A UE assignment is optimal for individual travelers and is also a Nash Equilibrium, since no traveler can improve his travel time by unilaterally changing routes (see also system-optimal).

UTC

[1] Urban Traffic Control, the general term for the hardware and software systems and algorithms for the coordinated operation of traffic signals in congested areas, historically by centralized control but increasingly with distributed localized control.
[2] The University Transportation Centers program, created to advance US technology and expertise in transportation through education, research, and technology transfer at university-based centers of excellence [ UTC web ].

UTCS-1

Urban Traffic Control System, an arterial network simulation model developed by FHWA in the 1970s to facilitate the development and evaluation of advanced, urban traffic control systems. UTCS-1 evolved into NETSIM that, together with FRESIM (which evolved from the freeway simulation model INTRAS), formed the integrated software suite TRAF (now TSIS/CORSIM).

Utility

The value to a decision maker of a particular choice alternative. A rationale decision maker is assumed to maximize utility when making a choice. A utility function expresses this unobserved utility as a function of characteristics of the alternatives in the decision maker's choice set as well as characteristics of the decision maker.

Utility Maximizing

Standard economic assumption that decision-makers always seek to maximize overall utility when faced with a choice situation. Utility maximizing models reflect both observed choice attributes and error terms to reflect differences in decision-maker perceptions. Compare with bounded rationality and satisficing approaches.

UTMS

The Urban Transportation Modeling System is a generic name for the formal application of conventional travel forecasting models. UTPS was the first computer implementation of, and became synonymous with, UTMS. Currently, a more common generic name is the four step model.

UTPS

The Urban Transportation Planning System, one of the first travel forecasting modeling packages, was developed in the 1970s by the US Department of Transportation. Its widespread use in the first travel forecasting led to its name becoming synonymous with the modeling process itself. Designed for mainframe computers, UTPS has been replaced by various PC or workstation-based software packages (e.g., see TransCAD).

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Validation

An independent test of a calibrated model's predictive capabilities using data that was not used in estimating or calibrating the model. The objectives are to verify that the model can replicate observed system performance and that the model is sensitive to proposed changes in model variables, parameters, and constraints. While model estimation and calibration might occur every 10 years, model validation should be completed prior to any forecasting application within that period. Validation is often linked with elements of calibration: if model predictions are no longer within acceptable error bands, then model parameters should be re-calibrated.
Note: In travel forecasting, model validation usually has involved only an assessment of the ability of the model to replicate observed system-level performance and often only the outputs of the final steps (e.g., trip assignment) are validated. Instead, a range of model sensitivity and reasonableness tests are conducted to assess the ability of the model to properly reflect policy changes in forecasting.

Value Added

The economic value of a final or intermediate output minus the value of all intermediate inputs, reflecting the contributions of the factors of production (labor, capital, land, and entrepreneurship).

Value of Time

The opportunity cost of travel time, value of time is the monetary amount that a traveler would be willing to spend to save time (e.g., the marginal rate of substitution between time and cost). VOT is a parameter used in benefit-cost analysis to quantify travel time benefits of transportation infrastructure and service improvements.

Value Pricing

A euphemism for congestion pricing, the operational policy of tolling facility use via fees that vary with the level of congestion or, by proxy, by time of day. Increasingly associated with managed lanes that parallel unpriced lanes, emphasizing the value tradeoff between travel time and cost.

Vantage

A video traffic detection system that automates the process of vehicle detection and traffic data collection in lieu of loops [ web ]. A similar systems is Autoscope [ web ].

Variable Cost

Costs that vary based on the level of transportation operations (such as fuel or labor costs). Also known as operating costs. Compare to fixed cost.

Variable Demand

An expression for demand that is sensitive to cost. In standard trip assignment, demand is fixed and represented by static trip tables. Variable demand can be modeled with feedback or by using a combined distribution/assignment procedure (such as Evan's). Same as elastic demand.

Variational Inquality

An alternative but equivalent mathematical formulation of the user equilibrium trip assignment problem.

Vector

[1] A scalable graphic data format comprising geometric objects such as points, lines, and polygons, in contrast to raster format (comprising a rectangular grid of pixels).
[2] in matrix algebra, a single column or row or any given length.

Vehicle-hour

One vehicle traveling for one hour. Total vehicle-hours traveled, or VHT, is an indicator of system performance measuring the total amount of vehicular travel in a region based on total time spent traveling.

Vehicle Infrastructure Integration

Development and application of communication between vehicles and roadside infrastructure to improve safety and efficiency of the transportation system (see IntelliDrive) [ web ].

Vehicle-mile

One vehicle traveling one mile. Total vehicle-miles traveled, or VMT, is an indicator of system performance measuring the total amount of vehicular travel in a region based on distance traveled (see person-mile).

Vehicle Routing Problem

A class of OR problems to find the routes that minimize the total cost of assigning fleet vehicles from one or more depots to multiple demand points such that each point is visited only once by a single vehicle, all routes start and end at a depot, and the total demand does not exceed total vehicle capacity. The Traveling Salesman Problem is a well-known special case. The Vehicle Routing Problem with Time Windows (VRPTW) has been used in activity-based modeling (see HAPP) as well as in numerous OR applications.

Vehicle-Trip

A single trip by a single vehicle, regardless of the number of occupants of the vehicle. A vehicle with three occupants on the same trip equals one vehicle-trip or three person-trips. The inputs to trip assignment are vehicle-trips for highway modes and person-trips for non-vehicular modes.

Vertical Alignment

The design of a roadway alignments in the vertical plane, including grade tangent sections and parabolic vertical curves, and reflecting topography, safety, facility type, speed, and other design concerns (see horizontal alignment and cross-sectional design).

VHT

Vehicle-hours traveled. VHT is a common performance measure for traffic flow indicative of the total amount of travel in a region (see VMT).

VII

Vehicle Infrastructure Integration (see VII).

Vine Building

An alternative to tree-based path algorithms, vine-based algorithms are designed to allow paths to pass through nodes more than once to accommodate turn-prohibitions.

VISEM

An activity-based travel forecasting software package, linked with VISUM, a comprehensive travel forecasting software package. Developed by PTV [ web ].

Visioning

[1] The process of defining and documenting values, goals and objectives, and plans.
[2] Software for displaying graphic representations of systems or system components.

VISSIM

A traffic microsimulation software package, linked with VISUM, a comprehensive travel forecasting software package. Developed by PTV [ web ]. Other traffic microsimulation packages include AIMSUN, CORSIM, Cube Dynasim, Paramics, SimTraffic, and TransModeler.

Visualization

Techniques used in the development and presentation of transportation programs, plans, and designs to analysts, stakeholders, and the public in an accessible, graphical format to improve comprehension and evaluation.

VISUM

A comprehensive travel forecasting software package, integrated with VISSIM, a traffic microsimulation software package, and VISEM, an activity-based travel forecasting software package. Developed by PTV [ web ].

VMS

Variable Message Sign (same as CMS).

VMT

Vehicle-Miles Traveled. VMT is a common aggregate performance measure indicative of the total amount of travel (in vehicle-miles) in a region.

VOC

Volatile Organic Compounds are a broad family of both human-made and naturally-occurring chemical compounds, including hydrocarbons (HC) and reactive organics (ROG) which are contributors to the creation of smog.

Volume

The number of units of flow (e.g., vehicles) passing a defined point (or over a uniform roadway segment) during a defined time period, typically measured in flow units (vehicles) per hour (vph) (see also LOS and capacity).

Volume Capacity Ratio

The ratio of volume to capacity for a given transportation facility. A common performance measure used in assessing network performance output from four step models and a key variable in modeling link and intersection performance.

Volume-Delay Function

A link performance function, a functional performance relationship between level-of-service and volume for a specific facility.

Von Thünen

Von Thünen's (1826) idealized land use model of a monocentric agricultural region introduced fundamental land economics to the relative location of land uses. Bid-rent functions reflecting production and transportation costs, together with commodity and land consumption demands, are used to determine land use, land rents, and market prices. Comparable urban models were developed by Wingo, Alonzo, and others (see also Central Place Theory ).

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WAAS

Wide Area Augmentation System (see DGPS).

Waiting Time

The portion of a trip, often by public modes, that is spent waiting for the arrival of a vehicle (during access/egress or transfer). Disutilities are in general highest for waiting time.

Walkable Communities

A compact development pattern of mixed land uses that supports pedestrian and other non-motorized transportation.

Walking Distance

The defined maximum distance that a pedestrian is willing to walk to access a bus stop (or other transit system) or an activity location.

Wardrop's Principles

Wardrop's (1952) First Principle is equivalent to user equilibrium assignment; Wardrop's Second Principle is equivalent to system optimal assignment.

Warm Start

Selected iterative trip assignment algorithms such as origin-based assignment can significantly reduce computation time when a base equilibrium solution is saved and used as a "warm start" in model feedback or forecasting.

Warrants

Minimum criteria necessary to authorize a roadway infrastructure or operational change such as installation of a traffic control device. Criteria are often based on performance measures such as average speeds, traffic volumes, or accident levels.

WCTRS

World Conference on Transportation Research Society, a international forum for the promotion of transportation research and practice, including the organization of triennial research conferences and publishing the journal Transport Policy [ web ].

Weigh-in-Motion

The estimation of vehicle and load weight while a truck is in motion.

Wi-Fi

Technologies for the interoperability of wireless, local area networks based on IEEE 802.11 standards.

WIM

Weigh-In-Motion (see WIM).

Window Model

A sub-area model extracted from a larger model by defining greater detail in the transportation and activity systems within the window and deleting external areas, a process equivalent to sub-area extraction. Compare to focus model.

What If?

A GIS-based software package designed to investigate alternate development patterns and associated land use, population, and employment projections [ web ].

Women's Transportation Seminar

An international organization to promote the advancement of women in the field of transportation [ web ].

WTS

Women's Transportation Seminar, known familiarly as WTS [ web ].

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ZEV

Zero Emission Vehicle. A vehicle with no tailpipe emissions (typically, an electric vehicle).

ZEVNet

An operational, research-oriented, shared-use, station-car program in Irvine, CA that utilizes electric vehicle, hybrid, and hydrogen-fueled vehicles [ web ].

Zipcar

A national car sharing company [ web ].

Zone

The basic geographical unit of analysis for conventional travel forecasting. All locations in a study area are contained in one and only one analysis zone, the number and size of which depend on the scale and scope of the modeling effort. Zones should be as homogenous as possible with respect to the associated travel behavior. Most often referred to as Traffic Analysis Zones (TAZs). See also census tract.

Zoning

The general term for land use regulation, with authorized land use designations defined spatially (in "zones"). Applied at the local level (city or county), zoning serves to control the compatibility of neighboring land uses as well as the overall distribution of land use. Zoning regulates land use density via requirements for setbacks, floor-area ratios, and height restrictions, as well as transportation attributes such as parking and site access. See Euclid v Ambler.

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20-Foot Equivalent Unit

The standard unit of measure to quantify container traffic flows. One (8 foot by 8 foot by) 20-foot long shipping container equals one 20-foot Equivalent Unit (TEU).

3C Planning Process

The Continuing, Comprehensive, and Cooperative transportation planning process, mandated in the Federal-Aid Highway Act of 1962 and representing the first legislation requiring a formal planning process as a pre-requisite to receiving federal funding.

3D Process

The 3D process is a method for assessing travel impacts relative to changes in measures of the built environment. The original 3D measures were density, diversity, and design (a fourth D was added as Destination accessibility). The methodology utilizes a compilation of elasticities of vehicle trip rates and VMT relative to the defined "D" measures. These impacts are typically not captured in the standard four step model. See also 4D Process.

4 Step Model

The conventional model for travel forecasting, so named for the four major steps of the process: trip generation, trip distribution, mode choice, and trip assignment.

4D Elasticities

The 4D process assesses potential travel impacts of proposed changes in the built environment by applying elasticities (drawn from case studies or estimated locally) that reflect expected changed in trip rates, VMT, or other performance measures for a percent change in planning variables that measure density, diversity, design, and destination accessibility.

4D Process

The 4D process is an extension of the 3D Process that reflects a fourth measure of the built environment, Destination accessibility).

5D Method

An extension of the 4D Process as implemented in the INDEX software. The fifth measure of the built environment is distance from a heavy rail transit station.

511

A nationwide 3-digit telephone number assigned by the FCC for traffic information [ web ].

85th Percentile Speed

The speed at or under which 85 percent of free-flowing traffic are traveling, representing a common standard for setting safe speed limits.

802.11

Section 802.11 (a, c, g) of the IEEE standards corresponding to local area communication standards, known familiarly as Wi-Fi.

Symbol representing the Activity System in the Manheim/Florian framework for transportation systems analysis, constituting the land uses and activities in a region, excluding only the Transportation System T.

Symbol representing the Transportation System in the Manheim/Florian framework for transportation systems analysis, constituting the networks (modes, links, nodes, etc.) connecting a region's Activity System.

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NOTE:
This glossary has been compiled over many years of teaching courses on travel forecasting. Several text books and other glossaries have served as additional sources of ideas and information, however, all entries in this glossary have been newly defined in an consistent structure oriented toward transportation planning and modeling, as practiced in the United States, in general, and in California, in particular.

Travel Forecasting Glossary [ back to top ]

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