Field data collected over the last two decades show variable speed limit (VSL) systems can reduce crash potential by 8 to 30 percent.
A literature review of VSL systems in the United States and Europe
Made Public Date


United States


United States


Northern Virginia
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United Kingdom



Work Zone Variable Speed Limit Systems: Effectiveness and System Design Issues

Summary Information

In July 2008, a variable speed limit (VSL) system was implemented on a 7.5 mile section of I-495 (the Capital Beltway) in Virginia between the Springfield Interchange and the Woodrow Wilson Bridge. A study was conducted to examine performance and evaluate system impacts, and a literature search was performed to identify measures of effectiveness (MOEs) and gather information on VSL system and algorithm design.


The data below excerpted from Table-1 of the source report highlights findings from the literature search.

Type of Test Location Major Findings
Field Germany, Autobahn 5 30% reduction in injury crashes [1]
United Kingdom, M25 10%-15% reduction in crashes [2]
The Netherlands 16% reduction in crashes and 3%-5% increase in system throughput [1]
Near Munich, Germany, Autobahn 9 Slow flow sustained during times of congestion; similar German VSL sections saw 20%-30% reduction in crash rates [2]; concept of speed-flow-density algorithm uncovered here [3]
Finland, Highway E18 Mean speed and speed variability decreased; projected crash rate decrease of 8%-25% [4]
Utah, I-80 work zone VSLs changed by time of day; some reduction in speed variance at entrance to activity area. [5]
Minnesota I-494 work zone Throughput increased 7% during 1 hour of day; no change in another hour; compliance with speed limits 20%-60%. [6]
Simulation Orlando, I-4 segment Best results involved 5 mi/hr increment change by decreasing upstream limits while increasing downstream limits or just simply increasing downstream limits [9]
I-64 in Covington, Virginia work zone evaluation The two logics concerned with reducing mean speeds produced safer conditions [10]
Northern Virginia VSLs alleviated dangerous drops in speed and reduced queue length but were less effective under heavy congestion [11]

[1] Mirshahi, M., et al. Active Traffic Management: The Next Step in Congestion Management, Report No. FHWA-PL-07-012, U.S. DOT FHWA. 2007.

[2] Robinson, M. "Examples of Variable Speed Limit Applications," Paper presented at the Speed Management Workshop, 79th Annual Meeting of the Transportation Research Board, Washington, DC. 9 January 2000.

[3] Bertini, R.L.; S. Boice; and K. Bogenberger. "Dynamics of a Variable Speed Limit System Surrounding a Bottleneck on a German Autobahn," Journal of the Transportation Research Board, TRR Report No.1978, Washington, DC, 2006, pp.149-159.

[4] Yrjo and Jukka. Southeastern Region, Finnish National Road Administration, December 1995. Research cited in the U.S. DOT ITS Knowledge Resources. (Benefit ID: 2007-00504).

[5] McMurtry, T., et al. "Variable Speed Limit Signs: Effects on Speed and Speed Variation in Work Zones," 88th Annual Meeting of the Transportation Research Board. Washington, DC. 2009.

[6] Kwon, E., et al. "Development and Field Evaluation of Variable Advisory Speed Limit System for Work Zones," Journal of the Transportation Research Board, TRR Report No. 2015, Washington, DC, 2007, pp.12-18.

[9] Abdel-Aty, M. and A Dhindsa. "Coordinated Use of Variable Speed Limits and Ramp Metering for Improving Safety on Congested Freeways," 86th Annual Meeting of the Transportation Research Board. Washington, DC. 2007.

[10] Park, B. and S. Yadlapati. Development and Testing of Variable Speed Limit Logics at Work Zones Using Simulation, University of Virginia. Charlottesville, VA. 2002.

[11] Mazzenga, N.J. ,and M.J. Demetsky. Investigation of Solutions to Recurring Congestion on Freeways, Report No. VTRC 09-R10, Virginia Transportation Research Council. Charlottesville, VA. 2009.