USA Banner

Official US Government Icon

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure Site Icon

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

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

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

The following guidelines pertain to coordinating regulatory VSL system installation and operation with law enforcement and the judicial system. This includes guidance on coordinating and collaborating with law enforcement, determining legal authority, and maintaining a system log for evidence of potential violations.

5.1 Guideline 17: Coordinate and collaborate with law enforcement before deploying a variable speed limit system.

The State highway and law enforcement agencies should work together closely to determine how the VSL system will operate and be enforced. An interagency agreement should be developed that specifies the standard operating procedures to be followed and the respective responsibilities of the agencies. The agreement should address items such as:

  • Delegation of authority for setting and changing the VSL;
  • How the VSL will be set, including minimum and maximum speeds to be posted;
  • How often the speed limit can change;
  • How the VSL will be enforced;
  • How best to communicate speed limit changes to patrol officers; and
  • How law enforcement can access VSL sign displays and logs.

For any VSL to be successful in effectively managing speed and reducing crash risk, there must be a clear vision of how the system will operate and be enforced, and all participants must share this vision. Engineers, police, and the courts must know their roles and responsibilities as they relate to VSL. The potential benefits of this coordination and collaboration are as follows: a coordinated approach that supports the integrity of enforcement; an enforcement threat that deters inappropriate speeds and encourages compliance with VSL; speed violations successfully adjudicated; and increased safety for officers writing citations.

A map-based web page, text messages, email, or other methods could be used to notify patrol officers of current speed limits as well as the date and time of last change. A grace period should be allowed after each speed limit change before commencing enforcement of lower speed limits to ensure that the speed limit at time of the violation was seen by passing motorists.

5.2 Guideline 18: Determine the legal authority to post variable speed limits.

The highway agency will need to identify all jurisdictions responsible for speed enforcement in the zone with VSLs. This is important in order to ensure that the agency has legal authority to set and enforce VSLs so that speed violations can be successfully adjudicated. The State highway agency and State police must work with State legislators and council members in local jurisdictions to ensure that the necessary statutes are enacted or revised, as needed, to make VSLs enforceable.

Most, if not all, States have a speed zoning statute that delegates to the DOT the power to establish or change speed limits on the basis of an engineering investigation. The States that have implemented VSLs have done so mainly under the broad authority provided in this speed zoning provision of State Law or through a special provision in its vehicle code to allow for VSL systems.

Speed limits are established by statute or on the basis of an engineering study. Section 11-803 of the Uniform Vehicle Code (17) also includes the following provision:

Such a maximum speed limit may be declared to be effective at all times or at such times as are indicated upon the said signs; and differing limits may be established for different times of day, different types of vehicles, varying weather conditions, and other factors bearing on safe speeds, which shall be effective when posted upon appropriate fixed or variable signs.

This provision specifically supports the setting of VSLs and should be incorporated into the State speed zoning statute. Also, the Judicial Enforcement of Variable Speed Limits report addresses legal considerations for implementing and enforcing VSLs and should be referred to for more detailed information on this topic (18).

5.3 Guideline 19: Maintain system log for evidence of speed limit violations.

A system log should be maintained that provides evidence of the speed limit posted at time of the offense, as well as the prevailing traffic speeds, environmental conditions, or other reasons for any speed limit change. A copy of the engineering study documenting the decision logic and need for VSL at a particular location should be readily available to law enforcement and the courts.

For speed violation to be successfully adjudicated, the State should provide certifiable evidence of the speed limit posted at time of the violation and the reason the speed limit was changed. Speed limits that differ from the general statutory speed limit must be based on an engineering investigation. The potential benefits of maintaining a system log are as follows: successful adjudication of speed violations; fair and reasonable speed management system, as perceived by drivers; greater respect for law enforcement; and enforcement targeted at drivers that disproportionately contribute to crash risk. The system logs could also be used as part of a continuous ongoing evaluation to monitor and determine the effectiveness of the VSL system and changes needed to improve its effectiveness.