Search

Although time constraints on travel behavior have been widely recognized, little effort has been made to incorporate such constraints into the traditional Stochastic User Equilibrium (SUE) framework. The major objective of this research is to fill this gap by incorporating travel time constraints into the SUE model by means of a non-linear perceived travel time function. This modified model, designated as the Travel Time Budget (TTB) model, focuses primarily on discretionary travel behavior (such as shopping trips), and hence also allows the possibility of deferring travel decisions by incorporating an additional choice alternative designated as the Shop Less Frequently (SLF) alternative. This model is compared to the traditional SUE model by means of simulated travel scenarios on a test network designed to reflect a range of practical planning situations. These simulations show that when attractiveness levels are increased by the introduction of new shopping opportunities, the presence travel time constraints can lead to significantly smaller predicted travel volumes than the traditional SUE model. More importantly, they show that the overall pattern of travel can be quite different. In particular, travel to those shopping destinations with enhanced attractiveness can actually decrease for some origin locations. These findings suggest that when attempting to evaluate the impact of planning alternatives on future traffic patterns, it is vital to consider not only the cost of time itself but also the time tradeoffs between travel and other human activities.