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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
