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



This report provides guidelines for the design, installation, operation, maintenance, and enforcement of wet weather variable speed limit (VSL) systems. The use of VSL systems during inclement weather or other less than ideal conditions can improve safety by decreasing the risks associated with traveling at speeds that are higher than appropriate for the conditions. By using VSLs, agencies can take into account traffic volume, operating speeds, weather information, sight distance, and roadway surface condition when posting speed limits. The guidelines were developed from a thorough investigation of the state-of-the-practice of VSL installations in the U.S. and a complete review of the practices and procedures that states use when implementing and operating VSL systems. The guidelines include:

  • Identification of driver, vehicle, and roadway characteristics affected by wet weather driving conditions
  • How to set appropriate speed limits, develop appropriate algorithms, and select signage
  • VSL system installation, operation, and maintenance
  • Coordination of VSL system installation with law enforcement and the judicial system
  • Case studies of VSL systems in Alabama, Delaware, South Carolina, Washington, and Wyoming
  • A table of known VSL installations in the U.S.

The guidelines are intended for a broad range of audiences, from the transportation policy professionals who are considering whether or not their agency should use VSL systems to the engineers who are actually designing VSL systems for their jurisdictions.

Source Organization Location

Washington, DC

Operations Area of Practice

  • Freeway Management
  • Roadway Geometric Design
  • Road Weather Management

Organizational Capability Element

  • Freeway Operations
  • Road Weather Management
  • Roadway Geometric Design

Content Type

  • Best Practice

Role in Organization

  • Associate Engineer
  • Engineer
  • Principal Engineer
  • Researcher/Academic
  • Senior Engineer

Publishing Organization

  • FHWA

Document Downloads

Project Website

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

Prime Contractor
SAIC, Texas A&M Transportation Institute, and Brudis and Associates
Bryan Katz, Cara O'Donnell, Melisa Finley, Davey Warren et al.
Issue Date
August 24th, 2012
Publication Number
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