Traffic diversion due to motorway tolls

The case for a “virtuous circle”, whereby tolls on congested motorways improve vehicle speeds and throughput, is examined. Previous studies, in so far as they are based on monotonic speed-flow curves, exclude the possibility of a virtuous circle. Point data tends to suggest a non-smooth relationship between time-mean speed and flow. However, the case for a virtuous circle rests on the relationship between travel time (or space-mean speed) with respect to flow “bending backward” in a smooth way when congestion sets in. There is little data available, but a priori reasoning suggests that the relationship between space-mean speed and throughput, defined as space-mean speed times density, must bend backwards in a smooth way when congestion arises. A microscopic traffic simulator demonstrates how a toll may improve the throughput of a congested motorway. A macroscopic simulation of motorway traffic was embedded in the path flow estimator, a time-dependent logit path choice tool, to study the effect of network configuration. The results confirm that a virtuous circle is possible where the motorway is congested and where the alternative route is close but not very congested