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

FHWA Safety Program


August 24, 2012

U.S. Department of Transportation Federal Highway Administration

Safe Roads for a Safer Future. Investment in roadway safety saves lives.

Cover of Guidelines for the Use of Variable Speed Limit Systems in Wet Weather
Cover photos - Source: iStockphoto

Downloadable Version
PDF [2.07 MB]

Table of Contents

Technical Report Documentation Page

Chapter 1. Introduction

1.1 Background
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

Chapter 2. Driver, Vehicle, and Roadway Characteristics Related to Driving in Wet Weather

2.1 The Driving Task
2.2 Sight Distance
2.3 The Driver
2.4 The Vehicle
2.5 The Visual Scene
2.6 The Pavement
2.7 Speed Zoning

Chapter 3. Guidelines for the Design of Wet Weather Variable Speed Limit Systems

3.1 Determining the Appropriate Type of Variable Speed Limit System to Use

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.

3.2 Determining Speed Limits 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.

3.3 Determining Display and Location of Variable Speed Limit Signs

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.

Chapter 4. Guidelines for the Installation, Operation, and Maintenance of Wet Weather Variable Speed Limit Systems

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.

Chapter 5. Guidelines for Enforcement of Wet Weather Variable Speed Limit Systems

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.

Chapter 6. Weather-Related Variable Speed Limit Case Studies

6.1 Alabama
6.2 Delaware
6.3 South Carolina
6.4 Washington
6.5 Wyoming
6.6 Discussion

Appendix A. References

Appendix B. VSL Installations in the United States

List of Tables

Table 1: Alabama DOT Low Visibility Warning System Strategies
Table 2: Washington State DOT Speed Management Control Strategies
Table 3: Wyoming DOT Speed Management Strategies

List of Figures

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

Return to top

Program Contact

Guan Xu


What’s 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