Impact of Driver Compliance on the Safety and Operational Impacts of Freeway Variable Speed Limit Systems

Variable speed limit (VSL) systems enable freeway system managers to change the posted speed limit on a section of roadway in response to varying conditions. VSL system goals may include homogenizing traffic flow, improving safety, and/or reducing driver stress. Although it is understood that the effectiveness of VSL systems is impacted by the level of driver compliance, which itself is influenced by the extent of speed limit enforcement, very little is known about the strength of these impacts. This paper uses a simulation model to evaluate the sensitivity of the safety and operational impacts of VSL to driver compliance. Several scenarios for driver compliance were modeled using the PARAMICS microscopic traffic simulator. Findings indicated that VSL impacts are very sensitive to the level of driver compliance. Safety was shown to be positively correlated with the level of compliance, and travel time was shown to be negatively correlated. However, it was also found that the magnitude of the impact is strongly influenced by the VSL control strategy (i.e., set of rules for incrementing and decrementing the speed limits) being used. Therefore, selection of the VSL control strategy cannot be done independently of the decision regarding speed limit enforcement.