Evaluation of the Impact of Advisory Variable Speed Limits on Motorway Capacity and Level of Service

Variable Speed Limits (VSL) have been introduced to improve the operations of freeway facilities under congested conditions. Experience indicates that the impacts of VSL on traffic performance and safety might be higher if the displayed speed limits were mandatory instead of recommended. This paper focuses on the impact of advisory VSL and proposes a statistical methodology for the comparison of traffic conditions before and after the implementation of VSL using the prevailing flow-density relationships. A case study, with data collected from the E4 motorway in Stockholm, is used to illustrate the methodology and evaluate the impact of advisory VSL. The results indicate that the advisory VSL had no significant impact on traffic conditions, both immediately after the implementation and several months later.