U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
FHWA Safety Program
August 24, 2012
Cover photos - Source: iStockphoto
PDF [2.07 MB]
1.2 Roadmap to the Guideline Document
1.3 Design Speed, Operating Speed, and Maximum Safe Speed
1.4 Sight Distance and Factors Affecting Stopping Distance
1.5 When to Consider Variable Speed Limit Systems
1.6 Other Countermeasures to Consider
Guideline 1: Conduct an analysis to make sure that a wet weather variable speed limit system is justified.
Guideline 2: A regulatory variable speed limit system is preferable over an advisory variable speed limit system.
Guideline 3: Consider a semi-automated or automated approach for variable speed limit systems.
Guideline 4: Incorporate a weather responsive decision support into existing variable speed limit algorithms to determine the displayed speed limit.
Guideline 5: All speed limit algorithms and manual display determinations must be approved by a traffic engineer.
Guideline 6: For freeways, set a minimum regulatory speed limit of no less than 30 mph or 15 mph for advisory speed limits.
Guideline 7: Use speed limits in 5 mph increments.
Guideline 8: Display variable speed limit changes for at least 1 minute.
Guideline 9: Do not display reduced speed limits more than 1 mile upstream from the section of roadway where the reduced speed is desired.
Guideline 10: Where variable speed limit signs are closely spaced, do not allow speed differentials of more than 15 mph without advance warning.
Guideline 11: Use Changeable Message Signs (CMS) to communicate reasons for speed reduction.
Guideline 12: Similar to static signs, variable speed limit signs should be placed at all entrances to the roadway.
4.1 Guideline 13: Develop a comprehensive concept of operations for the variable speed limit system.
4.2 Guideline 14: Install appropriate weather sensors or use accurate weather data at problem locations.
4.3 Guideline 15: Ensure that equipment required in the design is incorporated into the installation requirements.
4.4 Guideline 16: Develop an Operations and Maintenance Plan.
5.1 Guideline 17: Coordinate and collaborate with law enforcement before deploying a variable speed limit system.
5.2 Guideline 18: Determine the legal authority to post variable speed limits.
5.3 Guideline 19: Maintain system log for evidence of speed limit violations.
Figure 1: VSL Sign Installation in Pennsylvania
Figure 2: Wet Weather Variable Speed Limit Flowchart
Figure 3: Regulatory Versus Advisory VSL Flowchart
Figure 4: Example Regulatory and Advisory Variable Speed Limit Signs
Figure 5: Example VSL Algorithm for Wet Weather
Figure 6: Variable Speed Limit System on the New Jersey Turnpike
Figure 7: Changeable Message Sign in Wyoming Used to Communicate Speed Reduction Warning