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10 Info USA is a mailing list company that tries to maintain a often used along with gravity expressions. The following equa- complete list of businesses in the United States. Claritas pro- tion shows a composite impedance expression, tij, for two modes vides demographic data. All three companies are possible (1 and 2) between origin zone i and destination zone j. The sources of data on employment at the workplace. empirical constant, , is usually of similar size to the in-vehicle time coefficient from a logit mode-split expression, provided the impedance has units of minutes. Regional Economic Model, Inc. 1 tij = ln [ etg1 + etg 2 ] Regional Economic Model, Inc. (REMI), essentially has two products, Policy Insight and TranSight. Policy Insight is Composite impedances are especially important for statewide designed to forecast the economic impacts of major govern- and intercity models, where the travel times by various modes mental policy initiatives. TranSight specifically forecasts the can differ radically, but trip distribution must be accom- economic impacts of transportation projects. plished ahead of (before knowing) mode split. DEFINITIONS OF COMMON TECHNICAL TERMS Dynamic all-or-nothing assignment--See "all-or-nothing USED TO DESCRIBE STATEWIDE TRAVEL traffic assignment." Trips are assigned within small intervals MODELS of time so as to track the progress of packets of vehicles over time between their origins and destinations. The principle ad- Technical Concepts vantage of dynamic traffic assignment for statewide models All-or-nothing traffic assignment--A model step where all is an ability to determine the amount of traffic that occurs traffic between an origin and destination is assigned to during peak hours within urban areas. the shortest path between that origin and destination and no traffic is assigned to any other path. An all-or-nothing Dynamic equilibrium traffic assignment--An application traffic assignment is unresponsive to delays caused by traf- of equilibrium principles (see static equilibrium traffic as- fic. Historically, many statewide models have used the all- signment) where trips are also assigned within small inter- or-nothing traffic assignment because volume-to-capacity vals of time, so as to track the progress of packets of vehicles ratios were difficult to determine for 24-h forecasts and over time between their origins and destinations. A single networks in urban areas were sketchy. Many freight compo- dynamic equilibrium traffic assignment requires several nents still use all-or-nothing assignment to preload trucks to dynamic all-or-nothing assignments (see "dynamic all-or- a highway network. nothing assignment"). BPR curve--A simple expression that computes travel Four-step model--A modeling paradigm that has become time as a function of volume, originally developed at the standard practice in urban areas and involves the major steps Bureau of Public Roads (BPR). A BPR curve has two para- of trip generation, trip distribution, mode-split, and traffic as- meters, and , that can be varied by functional class: signment. A common variation is a three-step model that eliminates the mode-split step. A four-step model may in- v volve minor steps, including time-of-day and automobile-oc- t = t0 1 + cupancy calculations. c where t0 is free flow travel time, v is the assigned volume, and Fratar factoring--A popular empirical technique for fore- c is the capacity. The BPR curve is used within a traffic as- casting origin-to-destination trip patterns by applying row signment step to provide loaded travel times so that traffic and column factors to an existing origin-destination (OD) can be placed on the shortest path. table. Fratar factoring can also be applied to a production-to- attraction trip table. Commodity group--A grouping of similar commodities that can be analyzed and forecasted together. The groupings GPS (Global Positioning System)-based survey--Use of are often based on the Standard Classification of Transported the GPS to trace the location of a traveler or vehicle over Goods or the older STCC. Standard Classification of Trans- time, which would be linked to a travel diary. ported Goods codes are of up to five digits, organized such that adding a digit increases the precision of the commodity Gravity expression--Sometimes called a "gravity model," description. which determines the production-to-attraction trip pattern as a function of the number of productions and attractions in Composite impedance (or composite disutility)--A measure each zone and measures of proximity between zones. Grav- of the separation between an origin and a destination (often as a ity expressions can be either singly or doubly constrained. A function of travel time, travel cost, and convenience) that takes singly constrained gravity expression holds productions by into consideration the accessibility of more than one mode zone constant, allocating trips to other zones on the basis of between the origin and destination. Composite impedances are a measure of their zonal attractiveness. A doubly constrained
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11 gravity expression, often used in statewide models, allocates expressions are preferred for activity allocation within land trips between zones while also holding trip productions and use components of integrated models. Logit expressions can trip attractions constant. A typical gravity expression finds replace gravity expressions for trip distribution within a tra- the number of trips, Tij, between production zone i and ditional four-step model. When doing so, the number of trip attraction zone j: attractions in a zone is calculated by the expression rather than given as an input. Therefore, logit expressions are more Tij = Pi Xi AjYj f (tij) sensitive to changes in policies and infrastructure. There is where Pi is the number of productions in zone i, Aj is the no consensus as to when logit expressions are preferred over number of attractions in zone j, and f(tij) is a measure of gravity expressions for trip distribution. proximity between zones i and j, as a function of impedance, tij, between zones. The measure of proximity, f(tij), is often Microscale traffic simulation--Sometimes called traffic called a friction factor. Xi and Yj are balancing factors that are microsimulation, this is traffic simulation that tracks the lo- set such that the numbers of productions and attractions, re- cation and performance of individual vehicles. Microscale spectively, are conserved in each zone. Some implementa- traffic simulations can be used as post-processors for output tions of the gravity expression have a term, kij, which are from a statewide travel forecasting model, so as to provide called "k-factors" or "socioeconomic adjustment factors." better estimates of delay from the assigned traffic. k-factors are empirical adjustments to the gravity expression based on household travel surveys or screenline counts to Monte Carlo simulation--A technique that is used within provide a better fit between the model and base-year data. microsimulation that can generate random events, such as households of given characteristics, trips, start times, modes, Household sectors--Groups of households within an eco- and vehicles. The probability of an event is taken from his- nomic or land use model, usually organized by economic or torical information or from theory, such as a logit expression. life-cycle status. Multiclass assignment--A method of traffic assignment Industrial sectors--Groups of similar businesses, usually that separately accounts for different vehicle classes. Differ- organized by type of product or service. Industrial sectors are ent vehicle classes may be assigned to different routes if the often defined according to North American Industry Classi- link impedances vary across vehicle types. Multiclass assign- fication System or the older Standard Industrial Classifica- ments may take many different forms, static or dynamic and tion codes. North American Industry Classification System all-or-nothing or equilibrium. Multiclass assignments also codes are of up to six digits, organized such that adding a account for the differential impact heavy vehicles have on the digit increases the precision of the industry description. traffic stream. Multiclass assignment can be used to distin- guish automobiles from trucks and buses, single-occupant Input-output (IO) model--A type of economic model that automobiles from multiple-occupant automobiles or low- tracks flows of revenue (or sales) between industries and income drivers from high-income drivers. households in a national or regional economy. An IO model is organized by industrial sectors. A single cell in an IO table Nested logit algorithm--The use of two or more logit ex- would list the amount of revenue gained by a producing sec- pressions to determine the number of people who will make tor from sales to a consuming sector. a particular choice when the decision process is assumed to consist of a sequence of preliminary choices. Nested logit al- Logit expression--Sometimes called a "logit model," this gorithms are organized as a hierarchy, such that modes be- is a method for determining the number of people who will come more specialized in the lower parts of the hierarchy. make a particular choice (such as mode or destination) given Similar modes tend to be grouped together into "nests." the "utilities" of each alternative. A logit expression deter- Travelers are assumed to make decisions between nests be- mines the proportion of people, pi, who choose an option, i: fore making decisions about the individual modes within nests. A utility for a nest is created as a composite of utilities eU1 of all modes within a nest (see composite impedance). pi = eUk Origindestination (OD) table estimation from ground k counts--A method of determining the OD patterns of vehi- where Ui is the utility of option i, and an option can be either cles by primarily using observations of ground counts. OD a mode or a destination, or both. Utility is usually taken to be table estimation usually requires a good guess as to the OD a linear combination of travel time, travel costs, and mea- patterns, often referred to as a "seed" or "prior" table. The es- sures of convenience, such that Ui becomes more negative or timation algorithm tries to make limited improvements to the less positive as trip lengths increase. seed table, so that the assigned volumes will be closer to ground counts. There are many mathematical formulations to When used for destination choice, a logit expression is the OD estimation problem, and the various formulations a form of a singly constrained gravity expression. Logit will result in different OD tables using the same data. For
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12 example, a simple generalized least-squares approach at- origin and destination, with the shortest path usually getting tempts to minimize this expression: the largest share. 2 A N N N N Transshipment--Goods shipment with multiple legs of min P = w a V a - s pij Tij + z (Tij* - sTij ) a 2 a =1 i =1 j =1 i =1 j =1 the journey, with short-term storage between the legs, either in warehouses or at terminals. Alternatively, transshipment where can refer to importing goods from one country (e.g., China) Va is a ground count for link direction a, that pass through another country (e.g., Canada) on its way Tij is the number of trips between origin i and destination to the destination (e.g., United States). j to be estimated, * Tij is the seed trip table, a p ij is the proportion of trips between zones i and j that use Notes About Terminology Used in This Synthesis link direction a (as determined by an equilibrium traffic as- signment), There is a tendency among those involved in building travel N is the number of zones, forecasting models to use the word "model" to describe var- wa are link weights, ious pieces of a model as well as the whole modeling frame- z is the trip table weight, and work. For example, planners often refer to a trip distribution s is a single factor that is either set to 1 or selected by soft- technique as a "gravity model" or a mode-split technique as ware to scale the trip table to produce the correct average a "logit model." To help distinguish between various parts of traffic count. a model, the following terms are used herein. Each direction of a two-way link, a, may have a separate Algorithm--A series of expressions or computational ground count. Tij is constrained to be no smaller than zero processes that produces a specific result within a step. (i.e., cannot be negative), otherwise Tij is unbounded. An example of an algorithm is path building. Component--A collection of steps that leads to a particu- It is readily apparent that there are as many variables in lar result. Most statewide models have two components: the constrained minimization problem as there are cells in the passenger and freight. OD table. Thus, computation times can be long for large net- Expression--A single equation that yields a single answer. works. Many formulations and algorithms have been pro- For example, mode-split steps might be built around a posed to accelerate computation times. logit expression, which itself contains a utility expres- sion. Special generator--A business or other activity site that is Model--The whole modeling framework, including soft- so large or so specialized that it should not be included in ware, databases, components, steps, algorithms, and standard trip generation calculations for a traffic analysis expressions. A model excludes the personnel neces- zone. A special generator may have a separate zone in the sary to operate it or to interpret its results. These model or its trips may be added to those coming from more personnel would be included into the "modeling general land uses in a zone. process." Software--Models require software for their implementa- Static equilibrium traffic assignment--A method by which tion. There are three major classes of software: traffic is assigned such that travel times on links are consis- statistical estimation software, travel forecasting tent with volumes and volumes are consistent with travel modeling software, and GIS. Although different com- times. A "user-optimal" equilibrium traffic assignment mercial software packages have distinguishing fea- method, which has been implemented in statewide models, tures, many are sufficiently general and flexible to meet also routes each vehicle on its shortest path between an origin the needs of statewide travel forecasting. Thus, this syn- and a destination. The most common algorithms for static thesis avoids mentioning or endorsing specific software equilibrium traffic assignment require that the assigned vol- products. Software can also be custom written for a umes from several all-or-nothing assignments be averaged. model. Step--A series of expressions or algorithms that represents Stochastic multipath assignment--Traffic between an ori- a behavioral process within a component. An example gin and destination is divided across many paths between that of a step is mode split.