Variable Speed Limits

Variable speed limits are speed limits that change based on road, traffic, and weather conditions. Variable speed limits offer considerable promise in restoring the credibility of speed limits and improving safety by restricting speeds during adverse conditions.

Field Operational Tests On Variable Speed Limits In Work Zones

Field tests are being carried out in cooperation with Maryland and Michigan to implement and analyze the effectiveness of variable speed limits (VSL) in work zones. These projects are a joint effort between Safety and ITS Joint Project Office. The VSL are displayed on portable trailers and rely on input of vehicle speeds, presence of road work activity, and other information to post an appropriate speed limit, allowing motorists to maintain the most efficient and safe speeds, without endangering themselves, other drivers, or workers.

Results of several deployments in an 18-mi work zone in Michigan are now available [PDF - 159 KB]. Field trials of variable speed limits in a Maryland work zone are planned to begin in December 2005. A synthesis report of the results and lessons learned from the tests will be distributed to all states, so best practices for safe and efficient travel through works zones can be applied.

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Program Contact

Guan Xu


What’s New

Speed Management Program Plan NEW

Guidelines for the Use of Variable Speed Limit Systems in Wet Weather


USLIMITS2 a web based tool for recommending speed limits

Speed Management Webinars

Methods and Practices for Setting Speed Limits: An Informational Report

Procedures for Setting Advisory Speeds on Curves

State Practices to Reduce Wet Weather Skidding Crashes

Speed Concepts: Informational Guide

Speed Management Information Resources

Analysis of Speeding-Related Crashes (PDF 594 KB)

Engineering Countermeasures for Reducing Speeds: A Desktop Reference of Potential Effectiveness

Traffic Safety Facts: Speeding – 2011