On-ramp Selection Methodology for Coordinated Ramp Metering Schemes

Coordinated ramp metering is an effective means to reduce freeway bottleneck delay by postponing the moment of a flow breakdown and the associated capacity drop. To this aim, vehicles are stored at upstream located ramps to reduce the flow towards the bottleneck, causing delays at the ramps to both vehicles that move towards the bottleneck and to those that leave the network elsewhere. There are several different network characteristics that determine the resulting total ramp and bottleneck delays, and hence, whether it is beneficial to include a ramp into the set of coordinated ramps. In this contribution an approach is put forward to make an adequate decision on which ramps should be coordinated given the situation at hand. To quantify the resulting ramp and bottleneck delays, cumulative inflow and outflow curves are developed as a function of the involved situation-specific variables. This enables us to determine the optimal set of coordinated ramps beforehand and to gain insight into the effects of the system variables that determine the delays. By means of a test case the effects are illustrated and a worked example is given of finding such optimal set of coordinated ramps. Results show that the duration of the peak period and the size of the capacity drop strongly determine the delay at the bottleneck, and hence the coordination benefits of postponing the moment of the capacity drop. These variables also have a large influence on the minimum required share of ramp traffic to the bottleneck (i.e. break even share), that determines whether it is beneficial to include the ramp into the coordination.