Best Practice

BEST PRACTICE:

J1-5: Variable Speed Limits

DESCRIPTION:

Variable Speed Limit (VSL) systems in work zones typically consists of sensors, Portable Changeable Message Signs (PCMS), and a processing system that calculates prevailing speeds for traffic in and approaching the work zone. As congestion begins to form, measured downstream speeds are used to determine desirable posted speed limits upstream, resulting in smoother transition from “free flow” speeds further upstream to lower speeds in the vicinity of the work zone. Supplementary notification of the reduced speed limits can be provided further upstream via PCMS units. Variable speed limits may be advisory or regulatory depending on agency preference, enabling legislation, and the availability of a mechanism for enforcement.

REASON(S) FOR ADOPTING:

VSL systems can help reduce speed variation through work zones. They can also help reduce speeds when workers are present, work activity near travel lanes has increased, or weather conditions are unfavorable. Visible enforcement of the speed limit may also be necessary in order for drivers to realize that speed limit signs are credible and are to be obeyed.

PRIMARY BENEFIT(S):

When a VSL system is deployed far enough ahead of the work zone, it can reduce the number of crashes that could occur as drivers approach the back of the queue. Another benefit of VSL systems is to reduce congestion and ease traffic through a work zone depending on current conditions.

MOST APPLICABLE LOCATION(S)/PROJECT(S):

VSL systems should be considered for deployment in situations where frequent lane closures are expected, where frequent work activities are occurring and lower, temporary speed limits would be beneficial, and where traffic speeds vary widely throughout the length of the project. In general, these systems are most suitable for longer length, long-term work zones.

STATE(S) WHERE USED:

Michigan, New Hampshire, Ohio, Utah, Virginia , Washington

SOURCE/CONTACT(S):

Chris Brookes, Work Zone Delivery Engineer, Michigan DOT
Phone: (517) 242-6486
E−mail: Brookesc@michigan.gov