Ensure proper placement of variable speed limit (VSL) signs in a work zone and operate the VSL system consistently on a long term basis.
Virginia’s experience using variable speed limit signs in a work zone
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.
Author: Fudala, Nicholas J. and Michael D. Fontaine
Published By: Virginia DOT
Source Date: 03/01/2010
Other Reference Number: Report No. FHWA/VTRC 10-R20URL: http://www.virginiadot.org/vtrc/main/online_reports/pdf/10-r20.pdf
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