Variable Speed Limits for Motorway Off-Ramp Queue Protection

Motorway off-ramp is a main source of traffic congestion and collisions. Heavy diverging traffic and associated lane changing to off-ramp slows down the mainline traffic speed. When the off-ramp queue spills onto the motorway mainline, it may cause a major breakdown of the motorway capacity and a significant threat to the traffic safety. This paper proposes using Variable Speed Limits (VSL) to protect the motorway off-ramp queue to improve safety in congested diverging areas through microsimulation tests. To support timely activation of VSL in advance of queue spill-over, a proactive strategy is proposed based on off-ramp queue estimation and new arrival prediction in the near term. This process determines the estimated queue size on the off-ramp in the near-term, on which the decision to change speed limits is made. VSL can effectively slow down traffic as it is mandated and enforceable in Queensland, Australia. As a possible scenario, reduced speed limits could make drivers more attentive to the surrounding condition, and prepared for a sudden braking of the leading car. This paper analyses and quantifies this possible effect of VSL on traffic safety and efficiency using the microsimulation approach.