This paper concerns the benefits from vehicle route guidance in urban networks. We suppose that vehicle routes can be altered by En-Route Vehicle Guidance (ERVG) in such a way as to achieve system optimal assignment. Benefits are measured by the savings in total travel time for a given demand 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 some idealized corridors in which a freeway is superimposed over a dense grid of surface streets. Two cases are considered: in the first, the freeway corridor is a distributor and is long compared to average trip length. In the second case the corridor is considered as a link to an end point such as a CBD. In the first case trips of length L are within the corridor and freeway flow along the corridor is constant. In the second, with all trips destined to the end point, the flow on the freeway accumulates as that point is approached. The main role of ERVG in both cases is to divert traffic from the freeway whenever its marginal cost exceeds that of the street system.It is found that travel time savings of the order of 3-4% can be achieved from routeguidance. Benefits are quite sensitive to city street speed. At low speed more users wouldchoose the freeway resulting in congestion, and the potential benefits of route guidanceare relatively high. But as street speed increases and approaches that of the freeway routeguidance would be of less value as more of the motorists would be choosing the citystreet on their own. Route guidance benefits can be enhanced if information is customizedto motorists on the basis of their origins and destinations. Finally, it is shown that routeguidance benefits are reduced when the freeway network is dense. It is recommended thatfuture research should focus on potential opportunities for using ERVG technology inmanaging networks under conditions of non-recurring congestion (accidents/incidents).This paper does not consider important aspects of the evaluation of route guidance, suchas the equity issue stemming from increasing some trip times in order to achieve systemoptimum, or the local impact of diverted traffic.
