Using freight models
Thursday, May 28th, 2009Freight models in states that are geographically small and densely populated with adjoining urban areas, such as Connecticut and New Jersey, tend to take the form of urban truck models. Freight models in larger states, particularly those with larger rural areas and/or large percentages of pass-through traffic, such as Indiana, Florida, and Wisconsin, forecast freight in “four-step” commodity models, are a principal focus of this section. Still other states, such as Virginia, Tennessee, and Georgia, follow the general form of commodity model, but use acquired commodity freight tables in lieu of forecasting those tables in the trip generation and trip distribution.
State “four-step” commodity models are truly multimodal. The modes considered in these models typically include truck, rail, water, and air, even though the assignment step may only address trucks, and sometimes rail. As multimodal commodity models, the flow unit is common to all modes, and is typically tons. These models tend to be calibrated from annual commodity flow tables and the forecasts in the first forecasting steps will be annual tons.
Freight forecasting models, as all models, should have boundaries such that they internalize most of the trips that will be subject to forecasting. In the case of passenger modeling, these boundaries can be set at the jurisdictional boundaries of the state. Internal freight traffic within a state is typically no more than 25 percent of the flow total, and the flow to, from, and through the state due to national traffic comprise the majority of the freight flows. In order to properly forecast this traffic, the geographical area covered by state freight models typically is most of the continental United States, if not all of North America. The inclusion of modes that primarily travel distances of over 500 miles, such as rail, water, and air also suggests that the freight modal boundary should be much greater than just the state boundary. States that have developed “four-step” commodity freight models typically already have developed detailed travel-demand model zones and networks within the state boundary. These models and zone systems have been extended by inclusion of national highway and rail networks.