Optimal congestion charges based on private and social marginal costs calculated from traffic microsimulation.

The well known method of determining the optimal congestion charge the difference between private and social marginal cost uses a standard speed-flow function. The applicability of such functions to an urban network is often questioned on the grounds that traffic impedance is due primarily to the intersections. System performance is governed by the functioning of signalled and unsignalled intersections, with mid-block delay being relatively insignificant. A microsimulation of Perth CBD has been used to model the impact of increasing traffic loads on network congestion delays. The system delays resulting from successive small traffic increments increase in an erratic fashion but the trend is easily identified and a BPR function has been fitted. The estimated function is specific to the configuration of the particular network and makes it possible to calculate for this case the private and social marginal cost curves using average values of travel time and fuel. The optimal congestion charge is equal to the difference between the individual motorist's private cost and the social marginal cost he or she imposes on all road users. (a) For the covering entry of this conference, please see IRRD abstract no. E216410.