Forecasting Statewide Freight Toolkit (2008) / Chapter Skim
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Pages 27-34
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From page 27... ...
However, the method may be appropriate for developing forecasts for special generators, such as ports, within a more complex model. The facility flow factoring method is used to rapidly apply existing data to determine one or several forecast volumes.
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forecasts of changes in economic variables to estimate the corresponding changes in freight traffic. To simplify the approach for deriving forecasts of future freight traffic from economic forecasts, the demand for transport of a specific commodity is assumed to be directly proportional to an economic indicator variable that measures output or demand for the commodity.
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Case Studies and References Two case studies demonstrate the truck model: the Minnesota Trunk Highway 10 Truck Trip Forecasting Model and the Heavy Truck Freight Model for Florida Ports. These are described in Sections 8.2 and 8.3, respectively.
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Depending on the availability of complete auto and nonfreight truck trip tables within a complete statewide model, the commodity freight trucks can be assigned simultaneously with these other tables to allow the analysis of congestion. Obtaining a Current O-D Freight Table In order to factor an O-D table of freight flows, there must be an existing table of freight flows.
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Kentucky Corridor Model – This model was developed in 1997 by Wilbur Smith Associates.12 The network and base data were updated in 2001 by Wilbur Smith, without changing the model methodology.13 6.3 The Truck Model Description As shown in Figure 6.3, truck models use the trip generation and distribution model components to produce a table of truck trips and uses assignment model components to assign that table of truck trips. As truck models address only the single mode of trucks, they do not require a mode split component.
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Network Assignment Network assignment of the truck trips is based on the multiclass equilibrium highway assignment described in Section 4.5. Multiclass assignment is possible because truck models almost always are used as part of a complete travel demand forecasting process.
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Commodity truck tonnage is converted to daily freight truck trips by applying payload factors. Commodity flow tonnage is converted to vehicles based on commodity-specific factors (tons per truck)
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Case Studies Two case studies demonstrate the truck model: the Oregon Economic Activity Model Case Study and the Cross-Cascades Economic Activity Model Case Study. These are described in Sections 8.11 and 8.10, respectively.
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