Control Strategy of Variable Speed Limits for Improving Traffic Efficiency at Merge Bottleneck on Freeway

Queue forms at merge bottleneck area on freeways if the mainline and on-ramp traffic demand exceeds the capacity of bottleneck. Then bottleneck discharge flow further drops below the capacity of bottleneck which is termed as the capacity drop phenomenon. This study aims to develop control strategies of Variable Speed Limits (VSL) for improving traffic efficiency at freeway merge bottleneck area via preventing the capacity drop. The Cell Transmission Model (CTM) was developed and calibrated with field traffic data to evaluate the effectiveness of VSL. The results showed that by reducing the speed limit in VSL at upstream area, the capacity drop at the merge bottleneck was prevented and the discharge flow rate was kept near the bottleneck capacity. The control strategy of VSL developed in this study significantly reduced the travel delay by 56.10% and was effective in improving the traffic efficiency at the merge bottleneck area.