Spatially Generated Transportation Demands

The spatial distribution of firms and the transportation network are central to the modeling of transportation demands. Firms demand transportation to move products from one location to another. They tend to be located at different points in space and have differential access to the different modes of transportation. In our research, we model the modal decisions of agricultural shippers in the Pacific Northwest. Almost all shipments terminate in Portland, Oregon, and almost all involve a rail or barge shipment. However, only about 12 percent of shippers have direct access to barge, and only about one-half of shippers have direct access to rail. This means that most shipments involve a truck leg. In our model, the truck leg is treated as an access cost in the choice of rail or barge. We find that the access costs (cost of the truck leg) matters significantly as do barge and rail rates and shipper attributes. We then use the results to illustrate how demands can be aggregated to examine transportation market equilibrium defined over the spatial distribution of shippers.

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