Safety and Operational Benefits of Variable Speed Limits under Different Traffic Conditions and Driver Compliance Levels

Control of traffic by a variable speed limit (VSL) has been researched for a long time, but there is a lack of consensus on its safety and operational benefits and on the impact of driver compliance. This study provides quantitative evaluations of these benefits and shows the importance of driver compliance with VSLs. VISSIM and the technique of simulated vehicle conflicts (Surrogate Safety Assessment Model) were used in the study. To validate the safety analysis, an attempt was made to correlate the simulated vehicle conflicts with historic crashes. Three scenarios of traffic conditions were considered: heavily congested, lightly congested, and uncongested. Each scenario was further examined under four driver compliance levels: low, medium, high, and very high. The results con-firmed that the VSL had safety and operational benefits for motorway traffic. The system had the highest safety benefits during highly congested traffic conditions, followed by lightly congested conditions, and the least during uncongested conditions. Moreover, the system had the highest operational benefits during lightly congested traffic conditions, little benefit during uncongested conditions, and no benefit during heavily congested conditions. It was also found that the safety benefits of VSL were not at the expense of an increase in travel time. However, these benefits showed significant variations that depended on drivers' level of compliance with the system. The study concludes that the success of the VSL is highly dependent on the level of driver compliance.