There have been several studies conducted by various agencies on the effectiveness of implementing variable speed limits on interstate (or similar) roadways.
Northwestern University Transportation Center
The Northwestern University Transportation Center conducted a review of previously implemented variable speed limit systems. According to their website, these are their general findings:
- VSL systems increase safety by reducing speed variations
- Speeding violations are reduced
- Using VSL in congested areas can smooth traffic flows and reduce the occurrence of stop and go driving
Link to the study webpage
Michigan - VSLs in Work Zones
Michigan established a variable speed limit system within workzones on Interstate 96. Before VSLs were used, the work zone speed limit was 50 mph. After implementing the system, posted speed limits ranged from 40 mph to 60 mph (with some areas restricted to a maximum of 50 mph).
- Average speeds generally increased while the VSL system was operating (speeds were increasing because congestion was decreasing)
- Travel times generally decreased when the VSL system was operating
- The percentages of motorists traveling 60 mph decreased while the VSL system was operating
- Summary: "The VSL system can present far more credible information to the motorist, responding to both day-to-day changes in congestion as well as significant changes in congestion and geometry as motorists go through a given zone."
More information on the Michigan work zone study
Delaware -- VSLs on Interstate 495
Delaware recently started using VSLs on I-495 to control traffics due to crashes or weather conditions. The signs have been in use for a short time, so haven't had any studies completed on them, yet.
Europe
The United Kingdom has installed a VSL system on M 25 in London. Speeds range from 50 mph to 70 mph based on volumes. Crashes on M 25 have reduced by 10 to 15 percent, and the initial results show a decrease in travel times and smoother flowing traffic.
Germany installed VSLs on rural autobahns. Speeds there range from 60 km/h to 130 km/h. Using VSL reduced crashes by 20 to 30 percent.
More information on VSLs in Europe
European tour by FHWA and AASHTO on Speed Harmonization/Active Traffic Management
