The opportunity model is theoretically based on the behavior of individual trips. In practice it is necessary to deal with trips grouped together into zone, with all trips in a zone assumed to originate at the zone centroid. This causes errors in the estimation of trip interchange between zones. The magnitude of these errors is dependent upon zone size as well as trip density and propensity of people and activities to interact with each other. This study develops formulas which exhibit the dependence of zonal interchange calculations upon the variables mentioned, with emphasis on the variable of zone size, and presents tables showing the relative error as a function of zone size, trip density, and the trip interaction constant used in the opportunity model. These tables will suggest appropriate zone sizes to use in order to hold a constant level of bias for the entire study region.
Abstract