Signs alert drivers: Slow down; there’s congestion ahead

By Bill Holloway
Drivers headed for traffic jams largely disregard advisory variable speed limit (VSL) signs intended to slow traffic in advance of downstream congestion. A recent report from the ITS Institute at the University of Minnesota found that drivers are not adjusting their speeds in response to Minnesota DOT’s VSL signs, which cover portions of I-35 W and more recently I-94 in the Minneapolis area. Because the speed limits posted on the signs are advisory, drivers are not legally required to abide by them. However, while drivers are not adjusting their speeds to comply with the limits, the signs do appear to be having a positive effect on congestion. Researchers observed that both the frequency and spread of severe congestion waves were reduced substantially by the signs. Morning peak period congestion was reduced by more than 17 percent while the most severe congestion—speeds below 10 mph—was reduced by roughly 40 percent.
The authors speculate that the beneficial impacts of the VSL signs may be due to drivers using them to gauge the severity of downstream congestion and being more prepared to react to slower traffic ahead. By limiting the spread of extreme congestion waves, which contribute to rear-end collisions, the VSL signs appear to be having a beneficial impact on both safety and congestion despite the unwillingness of most drivers to abide by the limits displayed.
VSL signs have been deployed in a number of states and European countries. This report from the University of Missouri provides a case study of VSL sign implementation on I-270 in Missouri, where the limits displayed were enforceable. The Missouri DOT has since changed the system to make the speed limits advisory instead of enforceable. With before and after data from this system, future research may be able to better identify the impacts of advisory versus regulatory (enforceable) VSL systems.
Bill Holloway is a Transportation Policy Analyst at SSTI.