ABSTRACT
Regional travel demand models typically take as their starting point exogenously determined spatial distributions of population and employment (and any required attributes of these people and jobs) as fixed inputs into the demand modeling system. In so doing, they ignore the fact that these population and employment distributions are the outcome of a dynamic process of urban evolution that is partially determined by the nature and performance of the transportation system. That is, as illustrated in Figure 5.1, a two-way transportation-land use interaction exists, in which transportation is a derived demand from the urban activity system, but also in which the transportation system influences land development and location choice through the provision of accessibility to land and activities. From a policy analysis perspective, the need for a consistent, comprehensive analysis of urban systems is generally well understood, if not always put into practice. In particular, note that many transportation problems such as congestion, air pollution, etc., may have their root causes as much in urban form considerations (e.g., excessive urban sprawl) as in transportation system design per se.
