The VSL signs were activated in late July 2008, but initial evaluations of the system showed inconclusive effects. Changing site conditions in a work zone made a direct before-and-after evaluation of the system deployed in the field problematic, and some problems with the control algorithm were also noted. Given the difficulties in evaluating the system deployed in the field, a calibrated simulation of the site was constructed to assess the effects of the VSL system on traffic operations and safety surrogate measures.
Work zone lane closures were simulated by removing lanes that were closed because of construction. VSL signs were simulated at the same locations as in the field deployment. As such, the sign locations may not have been optimal for the specific lane closure simulated, but they were chosen to allow for consistency between the field and simulation environments.
The results indicated that the VSL could create substantial improvements in traffic operations provided the demand did not exceed capacity by too large a margin. Further, a cost/benefit analysis indicated that VSL signs may be most appropriate for long-term applications
Variable speed limit (VSL) systems improve traffic flow and increase safety. A VSL system uses detectors to collect data on current traffic and/or weather conditions. Posted speed limits on VLS signs are then dynamically updated to reflect the conditions that motorists are actually experiencing. Presenting drivers with speed limits that are appropriate for current conditions may reduce speed variance, a concept sometimes called speed harmonization. If properly designed, VSL systems have been shown to reduce crash occurrence and can also reduce system travel time through increased uniformity in traffic speeds.
Lessons learned from a Virginia Department of Transportation (VDOT) study on the effectiveness of VSL signs on an I-495 construction work zone in the vicinity of Woodrow Wilson Memorial Bridge (WWB) are presented below.
- Ensure proper placement of VSL signs in a work zone. VSL signs are to be located in such a way that they facilitate driver understanding and smooth operations. Signs should be placed so that they are not at risk of being obstructed and are not generally difficult to see under normal circumstances. The field visits showed that VSL signs on high volume roads should be placed on both sides of the road or on overhead sign structures to decrease the likelihood of a motorist missing a posted speed attributable to an obstructed sign. Further, the simulation results indicated that speed limits should also be posted past a bottleneck to “pull” vehicles through the capacity reduction. Given that work zone operations and traffic control can change rapidly, this suggests that VSL signs should be portable so that they can be moved to appropriate locations as construction activities change. In permanent applications, VSL signs should be placed just past any bottlenecks that are constraining flow.
- Operate the VSL system consistently on a long term basis. A concept of operations for future VSL systems should be developed and followed to ensure consistent application of VSL. In the field test, the WWB VSL system was not operated in a consistent manner initially, which may have had an impact on its effectiveness. The WWB VSL system was not consistently operated in a traffic-responsive mode, even when work zone lane closures were present. This could have worked against the effectiveness of the system. It is possible that motorists did not perceive the VSLs to be dynamic since the signs did not change the vast majority of the time. This could have caused the signs to blend into the background visual noise of the work zone.
- Design VSL control algorithm to facilitate rapid response to changing traffic patterns in a work zone. Agency operations staff has to ensure that the VSL control algorithm is designed to facilitate rapid response to changing traffic in a work zone. The simulation results show that algorithm design can make a significant difference in performance. Likewise, it appears that allowing speed limits to be set sign by sign rather than by zone can improve operations. It is important the agency staff have a clear working knowledge of how the system works, even if this requires signing non-disclosure agreements.
- Consider operational and safety tradeoffs prior to installing VSL systems on roads where demand far exceeds capacity. Agencies should carefully consider operational and safety tradeoffs prior to installing VSL systems on roads where demand far exceeds capacity. VSLs do not appear to provide significant operational benefits where there is a sudden onset of severe congestion. Indications from the VDOT study, as well as several other studies, suggest that VSLs offer no substantial operational benefit in these cases. The VSLs do, however, appear to offer operational benefits when demand is at or just above the capacity of the road. This implies that the primary operational benefits would be obtained during the shoulders of the peak hours. The simulation results show a reduction in the standard deviation of speed with VSLs during the onset of congestion, so the VSLs could offer some safety benefit by alerting drivers approaching the end of the queue to the slow or stopped traffic ahead.
Virginia’s experience suggests that a well-configured VSL system can provide operational benefits and improvements in safety surrogate measures provided that demand does not exceed capacity by too large a margin. A volume-to-capacity threshold where VSLs would provide an operational benefit was not defined in the VDOT evaluation because site-specific characteristics, such as ramp geometry and spacing, will likely have a significant effect on operations. Prior to deploying future VSL systems, it is suggested that departments of transportation perform site specific simulations to determine likely operational impacts.