A Combined Trip Generation and Distribution Model

We develop a model useful for analyzing and predicting trip generation, trip distribution, or both at the aggregate level of analysis. The generation component of the model is structured around the fundamental notion of a diminishing marginal effect of accessibility on tripmaking; the distribution component is a variant of the familiar gravity model. This new approach is seen to overcome many of the shortcomings of conventional models. For example, it is shown that (a) under certain conditions the combined model has structural properties that are fully consistent with substitution principles of microeconomics and (b) supply-demand feedbacks are explicitly taken into consideration. Organizationally, the modeling postulates first are set forth. Then, the combined model is formally developed and its structural (behavioral) properties are enumerated. Finally, an application is provided.