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33 analyze the impact of changes in employment, modal utility, will use either a freight truck only or multiclass assignment trip patterns, and network infrastructure, they usually do not model. Rules-based assignments typically are not used for account for increases in labor productivity, or the interaction freight trucks since a highway network must already be avail- between industries. If the commodity model is integrated with able to create the zone-to-zone impedances needed for trip highway passenger trip tables in assignment, the different rout- distribution and mode split. If highway network is available or ing procedures for large freight trucks can be accounted for by can be developed but there are no passenger vehicle tables, a the use of passenger car equivalencies and separate volume- freight truck only assignment can be used. If a highway net- delay functions. work and other vehicle trip tables are available, a multiclass assignment can be used. Common Model Components Trip Generation Case Studies and References The four-step commodity model includes a set of annual Two case studies demonstrate the truck model: the Florida or daily trip generation rates or equations by commodity pro- Freight Model Case Study and the Indiana Freight Model viding annual or daily flows as functions of TAZ or county Case Study. These are described in Sections 8.9 and 8.8, population and disaggregated employment data. respectively. The Wisconsin Freight Model is a four-step freight forecast- ing model. The purpose of the latest Wisconsin effort in freight Trip Distribution forecasting was to determine the impact of new rail/truck in- Four-step commodity models typically use gravity models termodal facilities on highway truck volumes and on railroad for trip distribution. The commodity groups serve as trip tonnage.14 The Wisconsin Department of Transportation prin- purposes and are distributed separately. The unit of flow in cipally used the method of factoring trip tables and traffic the distribution table is typically annual tons shipped, inde- assignment to accomplish its impact analysis. pendent of mode. The distribution of freight is to a national Cambridge Systematics developed a complete freight fore- system of zones, recognizing the large average trip lengths casting model as part of the Vermont Statewide Freight that govern the development of friction factors. Study.15 This model follows a variation of the classic four-step model. Mode Split Four-step commodity models may use any of the mode 6.5 The Economic Activity Model split models, developing highway modal utility information Description from their highway component and using this information to approximate the utilities by other modes even in the absence As shown in Figure 6.5, economic activity models use the of other modal networks. Because developing mode split trip generation, trip distribution, mode split, and assignment models for commodity models is very complex, a simple model components to produce freight forecasts for trans- application of existing mode share or qualitative adjustments portation facilities. The economic forecasts that serve as input of mode shares using market segmentation or other ap- to economic activity models are modified as a result of the per- proaches may be used. If a mode split model is developed, it formance determined by the model. Since the performance of typically uses an incremental or pivot point method to vary the highway/truck freight system depends on the demand and existing mode shares. usage of passenger autos, freight economic activity models are Commodity truck tonnage is converted to daily freight truck usually integrated with passenger forecasting models. trips by applying payload factors. Commodity flow tonnage is Economic activity models are the freight equivalent of converted to vehicles based on commodity-specific factors integrated land use-transportation models used in urban pas- (tons per truck) developed from state-specific sources or from senger travel; both use an economic/land use model as a step the national VIUS database. While conversion of rail freight to before the traditional four steps. Economic activity models carloads is not commonly done, there are exceptions. require special data concerning the availability of land and the rules governing the development and location of certain industries, and an understanding of the interdependencies of Network Assignment industries. This information is often unavailable to a state There are a number of options for assigning a trip table. The department of transportation, and is usually obtained in part- rail, water, and air assignments typically follow the rules-based nership with a state economic development agency that can assignment process. The assignment of truck freight typically explicitly account for changes in labor productivity.
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34 Figure 6.5. The economic activity model. Economy and Land Use Transportation Component Structure of the economy Trip Generation Network Location of the activity Costs Distribution Mode split Network assignment Figure 6.6. Integrated model interaction. Economic activity models formulate the flows of com- activity, identifying economic relationships between origins modities between economic sectors and between zones. The and destinations in the corridor. In future years, the spatial key assumptions in the economic activity models are that allocation of economic activity, and thus trip flows, is influ- the zonal employment or economic activity is not directly enced by the attributes of the transport network in previous supplied to the model but is created by applying an eco- years. Thus, the model is dynamic both with respect to land nomic/land use model. use and transportation. Economic activity models use a modeling technique The economic activity class of models differs from the known as a spatial I-O model. An I-O model distributes four-step commodity class of models in that the former uses household and economic activity across zones, uses links an economic/land use model to forecast zonal employment and nodes of a transportation network to connect the or economic activity prior to the trip generation step and do zones and model the transportation system, and calculates not change those forecasts as a result of the forecast perfor- transportation flows on the network. The spatial I-O mance of the transportation facilities. model uses a land use component to generate and distrib- ute trips and a transport component to generate mode Case Studies split and network assignments. The two sides of the model inform each other, resulting in a dynamic model, as shown Two case studies demonstrate the truck model: the Oregon in Figure 6.6. Economic Activity Model Case Study and the Cross-Cascades Economic activity models typically use an I-O structure to Economic Activity Model Case Study. These are described in simulate economic transactions that generate transportation Sections 8.11 and 8.10, respectively.