A simple model for route guidance benefits

This paper concerns the benefits from vehicle route guidance in urban networks. We suppose that routes can be altered in such a way as to achieve system optimal assignment. Benefits are measured by the savings in total travel time when comparing this assignment with the user equilibrium, which is assumed to occur in the absence of route guidance. A continuum approach is used to analyze an idealized corridor in which a freeway is superimposed over a dense grid of surface streets. The main role of route guidance is to divert traffic from the freeway whenever its marginal cost exceeds that of the street system. It is found that saving in total system travel time of the order of 3-4% can be achieved from route guidance. Benefits are quite sensitive to city street speed. At low speed more users would choose the freeway resulting in congestion, and the potential benefits of route guidance are relatively high. However, as street speed increases and approaches that of the freeway, route guidance would be of less value as more of the motorists would be choosing the city street on their own. Benefits can be enhanced if information is customized to motorists on the basis of their origins and destinations. Finally, it is shown that benefits are reduced when the freeway network is dense. This paper does not consider important aspects of the evaluation of route guidance, such as the equity issue stemming from increasing some trip times in order to achieve system optimum, or the local impact of diverted traffic.