Phased variable speed limit for off-ramp on highway

Variable Speed Limit (VSL) is an Intelligent Transportation System that can determine appropriate speed limits at which drivers should be travelling, given current roadway and traffic conditions. This study aims to develop the phased VSL for dispersing the shock wave and reducing congestion and queue length at a recurrent bottleneck based on traffic flow, theoretically, using the fundamental traffic model and shock wave theory. In this algorithm, the proper control speed is determined based on the difference of traffic demand and the capacity of the bottleneck, and we apply the algorithm not only to the mainline highway, but also the segment with the off-ramp. The performance of the VSL algorithm is evaluated with the case studies of the Sungsan off-ramp which is located at the northern part of the Seoul Naebu Expressway. As the result, it is found that the proposed VSL algorithm is able to improve the upstream congestion of the bottleneck without increasing the total travel time. In particular, the effects of reducing the queue length and diffusing the bottleneck congestion to the upstream are significant. The proper VSL control is also able to harmonize the traffic speed and help to delay the onset of flow breakdown and recover the traffic flow from congested conditions.