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10 Table 4.1. Freight model classes by component. Model Component Direct Trip Trip Mode Traffic Economic/Land Model Class Factoring Generation Distribution Split Assignment Use Modeling Direct Facility Of facility Flow Factoring flows Method O-D Factoring Of O-D tables Included Included Method Truck Model Based on exo- Included Not Included genously Applicable supplied zonal activity Four-Step Based on exo- Included Included Included Commodity genously Model supplied zonal activity Economic Based on out- Included Included Included Included Activity Model puts of eco- nomic model flow matrix or (in the absence of a matrix) incorporated trip generation, a production and attraction file for all geo- into a gravity model of shipment distribution. graphic zones, customarily serves as input to other model components used in freight forecasting. However, the pro- duction and attraction file can be useful on its own, showing 4.2 Trip Generation freight trips that end in zones. As shown in Figure 4.2, the trip generation model compo- The trip generation models used in statewide freight fore- nent forecasts the productions and attractions of freight casting include a set of annual or daily trip generation rates movements that begin or end in a geographic zone based on or equations by commodity, providing annual or daily flows the characteristics of that zone. The most common charac- originating or terminating in geographic zones as functions teristic used in trip generation is the employment by industry of TAZ or county population and disaggregated employment that produces and consumes various goods. The output of data. Production and consumption tonnages for special gen- erators like seaports, airports, and other intermodal transfer terminals are directly obtained from the port or terminal for 1 the base year. The commodity flow tonnages for external Data zones are obtained from the commodity flow database and are disaggregated at the TAZ or county level based on the dis- tribution of employment within each TAZ or county. Direct Facility For the truck model class of freight models, trip generation Flowing Factoring is usually calculated separately for internal trips between zones (I-I) and external trips between internal and external zones (E-I, I-E, and E-E). Trip rates are derived from national Link sources such as the Quick Response Freight Manual and/or Volumes regional sources, if available. These are applied to households 5 and employment data to obtain truck trips internal to the state. Different trip rates by truck type are used for truck trip Figure 4.1. Direct productions and attractions. The socioeconomic data used in factoring. a typical truck model are consistent with those data used in