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U.S. Department of Transportation U.S. Department of Transportation Icon United States Department of Transportation United States Department of Transportation
FHWA Highway Safety Programs

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

US Department of Transportation

Federal Highway Administration

Subject: INFORMATION: Guidelines for the Use of Variable Speed Limits Systems in Wet Weather

Date: September 26, 2012

From: Tony Furst
Associate Administrators for Safety

In Reply Refer To: HSST

To: Division Administrators
Safety Field
Federal Lands Highway Program Administrator

The purpose of this memorandum is to issue the guidelines on the use of variable speed limit (VSL) systems in wet weather. In response to a motorcoach crash on Interstate 35 near Hewitt, Texas on February 14, 2003, the NTSB recommended that FHWA issue guidance on the use of VSL systems during wet weather conditions at locations where the operating speed exceeds the design speed and the stopping distance exceeds the available sight distance (NTSB recommendation H-05-14).

The guidelines cover the design, installation, operation, maintenance, and enforcement of VSL systems used for wet weather conditions such as snow, rain, and ice. They were developed based on a thorough review of the state-of-the-practice of VSL installations in the United States. 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. The guidelines also provide examples of the challenges and issues that organizations have encountered when implementing VSL systems so that future implementers can develop practices that will increase their likelihood of success.

The successful use of VSL systems during wet weather conditions should take into account factors such as traffic volume, operating speeds, sight distance, and roadway surface condition when posting appropriate speed limits. Some of the information provided in the guidelines may also be adapted for other VSL applications such as for work zones and congested freeways.

Please share these guidelines with your respective State and local transportation agencies and encourage the use of VSL systems for wet weather applications where appropriate. The guidelines are available on FHWA's website at For further information, please contact Ms. Guan Xu at (202) 366-5892, or at

Page last modified on October 15, 2014