DEVELOPMENT AND EVALUATION OF A SYNTHETICALLY SELF-CALIBRATING GRAVITY MODEL
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The development of an alternative to the quick-responses technique of using transferable parameters in trip distribution for small- and medium-sized urban area is discussed. The proposed quick-response procedure involves an origin-zone-specific, self-calibrating gravity model in which the only input data required are the zonal productions and attractions, a zone-to-zone travel time matrix (skim tree), and the origin-zone terminal times. A travel time distribution determined from an origin-destination survey for internal trips is not needed for calibration. Tests conducted on three separate study areas indicated that the proposed model is able to reproduce trip patterns as accurately as the traditionally calibrated gravity model procedure based on origin-destination survey data. The accuracy was achieved by synthetic calibration of the model at the origin-zone level rather than at the aggregate level of the entire study area. Development of the proposed procedure was also based on the consideration that trip distribution is critically dependent on the spatial distribution of land use activities about each of the origin zones. This consideration was incorporated in the proposed procedure through the explicit measurement of the origin-zone-specific opportunity travel time distribution. The opportunity distribution for each origin zone was represented in the model by the origin-zone average travel time, computed from a gravity model trip distribution that has constant friction factors. From this initial key variable the final model was developed and to this the very acceptable results can be credited. (Author)