Interaction Between System Design and Operations of Variable Speed Limit Systems in Work Zones

Variable speed limits (VSLs) have been shown to improve safety and operations on European freeways, but work zone applications of VSLs have been limited. In July 2008, a VSL system was installed on a congested portion of the Washington, D.C., Beltway (I-95—I-495) in a major work zone. A limited field evaluation of that system revealed some deficiencies related to the system’s configuration, resulting in inconclusive operational effects. Those results raised questions about the relative importance of different VSL system parameters on the ultimate effectiveness of work zone VSLs. This paper describes an evaluation that used a calibrated simulation model to evaluate the influence of several parameters. The results showed that VSLs can delay the onset of congestion and help produce more rapid recovery from congestion, provided that demand volumes are not too far above the work zone capacity. When demand volumes are extremely high, VSLs offer no appreciable benefit over static speed limits. The simulation also showed that VSL sign location is extremely important, and signs must be positioned so that drivers will accelerate back to a reasonable speed once they pass through a bottleneck. Algorithm design also proved to play an important role in system effectiveness.