Abstract
Regionwide travel forecasts in small urban areas require good knowledge of through-trip tables for each of several vehicle classes. Survey data are often unavailable or incomplete, so planners would benefit from a means of estimating through-trips from readily available information. A model for a through-trip table from trip-distribution theory and elementary geographical principles is postulated and tested. The model considers the level of through-traffic at external stations, the shape of urban areas, and the necessity that trips between any two external points pass through the region because of barriers that impede travel. Separate model variations are developed for a region of arbitrary shape and for a region of circular shape. To ascertain the validity of the approximations, the model is applied to two cities: Oshkosh, Wisconsin, and Tallahassee, Florida. The model for circular-shaped urban areas has produced excellent results for both cities, each of which previously had prepared external-to-external trip tables by sampling actual traffic. Oshkosh’s geography is particularly interesting because of two large neighboring lakes that constitute barriers to travel. The model explains almost 96 percent of the variation in the trip table in Oshkosh and almost 100 percent of the variation in Tallahassee. These values exceed goodness-of-fit measures that are reported for other models that are described in the literature. Errors in through-trip flows on individual links that potentially serve through-trips are estimated at only about 10 percent in Oshkosh and 14 percent in Tallahassee.
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