Urban storage space selection method for integrated control on a freeway bottleneck

Ramp metering is an effective means to reduce freeway bottleneck delay that results from the capacity drop phenomenon when congestion sets in. The bottleneck flow is kept at free-flow capacity by temporarily storing vehicles traveling toward the bottleneck at the ramp. The metering duration is normally limited because of the finite amount of ramp storage space available to prevent undesired spill back to the urban network. A beneficial extension of the metering duration might be achieved by strategically choosing upstream intersection arms that reduce their inflow to the ramp. For that purpose, coordination needs to be realized between the ramp meter and its intersection controllers located upstream, which will also hinder vehicles not traveling toward the ramp. In this contribution, an evaluation approach is put forward to decide objectively which intersection buffers (arms) should be included in the coordination. To quantify the resulting delays in the system, cumulative inflow and outflow curves are developed as a function of the involved situation-specific variables. This approach enables one to determine the optimal set of coordinated buffers beforehand and to gain insight into the effect of the various system variables on the delays. By means of worked examples, these effects are illustrated, and the way to determine the optimal set of coordinated buffers is shown. Results show that the length of the peak period and the size of the capacity drop strongly determine the coordination benefits, and, hence, a buffer's minimum required fraction of traffic traveling past the bottleneck needed to result in beneficial coordination.