Variable Speed Limits

U.S. Department of Transportation logoU.S. Department of Transportation
Federal Highway Administration

Selecting appropriate speed limits on roadways is important in maintaining a safe and efficient transportation network. Speed limits are established with an engineering study based on inputs like traffic volumes, operating speeds, roadway characteristics, and crash history. However, conditions on the roadway are susceptible to change in a short amount of time (e.g., congestion, crashes, weather). Drivers typically determine their operating speeds under normal weather conditions on a straight roadway section with good pavement quality and adequate sight distances. If ideal conditions do not exist and the roadway does not meet the driver’s expectations, there is a greater chance that a driver error could result in a crash. Providing variable speeds limits (VSLs) capable of adapting to changing circumstances could reduce crash frequency and severity.

Four freeway lanes in the same direction. Over each travel lane is a variable speed limit sign reading SPEED LIMIT 45. To the right of the overhead variable speed limit signs is the left edge of an overhead variable message board sign. Three of the four travel lanes are congested, except for a restricted travel lane on the left.Source: WSDOT


VSLs use pevailing information on the roadway, like traffic speed, volumes, weather, and road surface conditions, to determine appropriate speeds and display them to drivers. This strategy improves safety performance and traffic flow by reducing speed variance (i.e., improving speed harmonization). VSLs may also improve driver expectation by providing information in advance of slowdowns and potential lane closures, which could reduce the probability for secondary crashes. VSLs can mitigate adverse weather conditions or to slow faster-moving traffic as it approaches a queue or bottleneck.

Speed management strategies, including VSLs, are integral to the Safe Speeds element of the Safe System Approach. Because humans are unlikely to survive high-speed crashes, VSLs reduce speeds so that human injury tolerances are accommodated in three ways: improving visibility, providing additional time for drivers to stop, and reducing impact forces.

Agencies can implement VSLs for the following applications:

"Congestion icon" "Incidents" "WORK ZONES" "INCLEMENT WEATHER"


  • Particularly effective on urban and rural freeways and high-speed arterials with posted speed limits greater than 40 mph.
  • Often implemented as part of Active Traffic Management (ATM) plans or incorporated into existing Road Weather Information Systems.
  • When used with ATM, VSLs can mitigate rear-end, sideswipe, and other crashes on high-speed roadways.
  • May be implemented as a regulatory and/or an advisory system.
  • Can be applied to an entire roadway segment or individual lanes.


1. Avelar et al. Developing Crash Modification Factors for Variable Speed Limit. FHWA, (2020).