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32 Common Model Components 4.5. Multiclass assignment is possible because truck models almost always are used as part of a complete travel demand Trip Generation forecasting process. Trip generation components will produce daily truck pro- ductions and attractions using equations whose coefficients were developed based on local surveys or using parameters Case Studies borrowed from other sources such as the Quick Response Two case studies demonstrate the truck model: the New Freight Manual. Trip distribution is accomplished using a Jersey Truck Model Case Study and the SCAG Heavy Duty gravity model that recognizes that the friction factors from Truck Model Case Study. These are described in Sections 8.6 internal-internal and external-internal/external-internal trips and 8.7, respectively. will vary, reflecting the difference in average trip length between these types of trips. External-external trips are established based on surveys and factored independently. 6.4 The Four-Step Commodity Model Trip Distribution Description In truck models, the trip distribution component follows As shown in Figure 6.4, the four-step commodity model the process described in Section 4.3. The geographic scope of most closely resembles the four-step urban travel demand the model area typically requires that external trips be dis- model for passengers; both use the trip generation, trip dis- tributed differently than internal-internal trips. Light, tribution, mode split, and assignment model components. medium, and heavy trucks are distributed from origins to The economic forecasts that serve as the basis for the four- destinations using the gravity model technique. This is the step commodity model are not modified in response to the same distribution method used in any typical auto passenger results of the model. model. The friction factors in the gravity model can be devel- Four-step commodity models and the more familiar four- oped from surveys or borrowed from other sources such as step passenger models both require the development of a the Quick Response Freight Manual. statewide network and zone structure. If a statewide passenger model exists, it is often used to provide the zone and network structure within the state. Since trip distribution and mode Network Assignment split for freight typically involves average distances of hundreds Network assignment of the truck trips is based on the mul- of miles, a skeletal freight network is typically appended to that ticlass equilibrium highway assignment described in Section statewide highway network. While commodity models can Figure 6.4. The four step commodity model.