Variable advisory speed limit (VASL) systems encourage drivers to slow down gradually as they approach rural work zones.
An investigation of work zone traffic control in Missouri
Date Posted

Evaluation of Variable Advisory Speed Limits in Work Zones

Summary Information

This study evaluated the mobility and safety impacts of using variable advisory speed limit (VASL) systems on interstate highways in Missouri. Case studies were conducted and simulation modeling performed to evaluate performance across a wide range of treatment scenarios. The first set of case studies focused on VASL in congested and uncongested urban work zones on I-270 near St. Louis. The second set examined VASL in rural work zones on US-54 and US-63. Simulation modeling was also used to expand the range of treatment scenarios evaluated.

Urban Study

In 2010, permanent VASL dynamic message signs (DMS) were installed on I-270 to control traffic and adjust speed limits to accommodate prevailing traffic conditions. In June 2012, researchers used traffic data collected at or downstream of static speed limit signs over a three day period to assess system impacts in areas with and without congestion. At sites with congestion, the system did not operate continuously, but operated periodically to encourage drivers to reduce speeds gradually as they approached work zone bottlenecks. At sites without congestion, the VASL system operated continuously to detect average speeds and remind motorists of safe travel speeds.

Rural Study

To determine the impacts of VASL on rural work zones data were collected at two locations in central Missouri. The first site on US-54 located south of Jefferson City was monitored on September 21, 2011. The second site on US-63 located south of Columbia was monitored on March 13, 2012. Since traffic conditions did not warrant lower advisory speed limits in these areas, researchers evaluated the effectiveness of the system to display static speed limits and reinforce posted speed limits in control areas. These work zones generally experienced less average daily traffic, fewer lanes, less traffic, and more trucks compared to the urban work zones.


Two traffic simulation models were constructed to represent congested work zone conditions with and without VASL.

Lessons Learned

The following recommendations were based on the case studies investigated.

  • Use VASL systems to lower speeds and achieve better compliance with posted speed limits in areas without congestion in urban work zones.
  • Use VASL systems to complement static speed limit signs in areas without congestion in rural work zones. This can lead to safer traffic conditions by encouraging drivers to slow down gradually as they approach work areas.
  • Use simulation modeling to optimize VASL system algorithms. A well designed VASL algorithm, like the 5-minute (P5) algorithm developed in this study can improve mobility and safety in congested work zones.