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Freeway Management > Lane Management > Variable Speed Limits


Variable speed limit systems use sensors to monitor prevailing traffic and/or weather conditions, posting appropriate enforceable speed limits on dynamic message signs.


Variable Speed Limits (VSL) cut crash rates by more than half during low visibility on I-77 in Virginia.(09/01/2018)

In a recurring congestion scenario, ramp metering, variable speed limits, and hard shoulder running found to improve corridor travel time and network performance by 5 to 16 percent. (09/01/2018)

An implementation of proactive variable speed limits (VSL) was simulated to reduce critical traffic conditions by 39 percent.(February 2018)

Ramp metering and dynamic speed limits shown to reduce congestion and shorten average travel times by about 2.5 minutes on a 7.8 km stretch of the A25 motorway in France.(September 4-6, 2017)

Dynamic Speed Limit Systems in Belgium found to have decreased the number of injury crashes by 18 percent.(July 4, 2017)

When combined, ramp metering and variable speed limits may reduce conflicts by 16.5 percent and crash odds by 6.0 percent in weaving segments.(January 2017)

Variable speed limit pilot found to be effective reducing the number of overall crashes. (08/09/2015)

Variable speed limit systems reduced the number and severity of crashes at three pilot sites in Texas. Benefit-to-Cost ratios ranged from 7:1 to 14:1.(06/01/2015)

Variable speed limit system site selection should be rigorous and incorporate analysis of existing speed profiles and roadway ingress/egress characteristics to assure proper spacing of VSL systems and sensor inputs.(06/01/2015)

Variable speed limit system site selection should be rigorous and incorporate analysis of existing speed profiles and roadway ingress/egress characteristics to assure proper spacing of VSL systems and sensor inputs.(06/01/2015)

Variable speed limit system site selection should be rigorous and incorporate analysis of existing speed profiles and roadway ingress/egress characteristics to assure proper spacing of VSL systems and sensor inputs.(06/01/2015)

Advisory Variable Speed Limit System in Portland, Oregon reduces speed variation and the number of crashes in the area.(May 18, 2015)

In Smart Zone work zones, 71 percent of local resident survey respondents found variable speed limit signs useful.(January 28, 2015)

Variable speed limits have safety benefits and a homogenizing effect, but a German study found no increase in freeway capacity.(January 13, 2013)

Variable Speed Limit model yields up to 42.4 percent reduction on number of vehicle stops and 17.6 percent reduction on the average travel time.(01/01/2013)

Implementing variable mandatory speed limits on four lanes with the optional use of the hard shoulder as a running lane resulted in a 55.7 percent decrease in the number of personal injury accidents on a major motorway in England.(January 2011)

Collisions on I-5 in Washington State have been reduced by 65-75 percent in a 7.5 mile corridor where an active traffic management system was deployed.(November 19, 2010)

A Variable Speed Limit (VSL) system on the I-270/I-255 loop around St. Louis reduced the crash rate by 4.5 to 8 percent, due to more homogenous traffic speed in congested areas and slower traffic speed upstream.(October 2010)

17 percent reduction in NOx on "Ozone Action Days" with Variable Speed Limits.(September, 2006)

On the Køge Bugt Motorway in Copenhagen, Denmark, variable speed limits reduced vehicle speeds by up to 5 km/h and contributed to smoother traffic flow during peak periods.(8 April 2003)

A survey of motorists in Copenhagen, Denmark, found that 80 percent of respondents were satisfied with variable speed limits and the traveler information posted on dynamic message signs.(8 April 2003)

A study of travelers on Snoqualmie Pass, WA found that DMS can decrease mean driving speeds and reduce accident severity.(December 2001)

It was estimated that variable speed limit signs and lane control signals installed on the autobahn in Germany would generate cost savings due to crash reductions that would be equal to the cost of the system within two to three years of deployment. (August 1999)

Advanced traffic management systems in the Netherlands and Germany reduced crash rates by 20 to 23 percent.(August 1999)