The Functional Form of the Gravity Model

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Abstract
Gravity models have been used extensively in regional planning and transportation studies in order to examine spatial interaction. These models have been criticized, however, because their precise functional form is difficult to determine empirically. In this paper an application of a maximum likelihood method, developed by Box and Cox, is presented that allows an investigator to choose the most appropriate functional form from a general class of alternatives. This method, which imposes no additional information requirements, is applied to 1970 data on 489 trip flows among approximately 700 different travel nodes in Georgia. The log-linear functional form for the gravity model performs best in this context.