Effectiveness of reduced speeds on Variable Speed Limit (VSL) equipped corridors

Focus Area: Transportation Systems Management & Operations
Submitting On Behalf Of: TRB Committee on Freeway Operations
Urgency: Important - Medium Priority
Cost: $250,000 - 499,000
Timeframe: 1 - 2 years
Type of Research: Full Research Project

Date Posted: Jun 5, 2025
Status: Not Funded

Research Description

Variable Speed Limits (VSL) are used for a variety of different purposes for speed management to slow traffic across freeway corridors. Applications include responding to environmental conditions (e.g. high winds, ice/snow, fog etc.), supporting on-road events where workers or responders are present (planned or unplanned), slowing traffic on approach to or in congested conditions, and influencing traffic flow to improve stability and efficiency. There have been many studies on the overall effects / benefits of speed limit reductions using VSL for various applications. However, there is less information available on the level of response by drivers, including compliance rate, to reduced speed limits where VSL is deployed.

This research will provide the following learning objectives: • To what extent do drivers comply with “default” (non-reduced) speed limits in VSL-equipped corridors. • How much drivers reduce their speed relative to the increment of speed limit reduction. • How effective is the spacing/regularity of VSL provisions to achieving / maintaining a reduced speed behavior over a distance. • What effect, if any, police presence and/or enforcement have on driver’s responses to speed limit reductions. • What other human factors strongly influence compliance with/response to reduced speed limits. • What effect, if any, does the purpose of the reduction impact reduce speed responses. • What effect, if any, does supporting information (VMS, etc.) have in communicating the purpose for reduced speeds. • Collaboration between States for data sharing

Additional Supporting Information

The project benefits include: • Understanding the response of drivers to VSL changes can assist in understanding a number of questions associated with VSL implementation, including the spacing of VSL assemblies, speed limit reduction increments, placement of VSL signs (side vs overhead mounted). • Assist agencies in understanding the appropriateness of using VSL for specific use cases and local context to inform investment. • Identify potential complementary actions that can improve driver response, and therefore effectiveness of VSL. Through the above, States can establish a consistent strategy for implementing VSL applications.

Submitted By: Jim Katsafanas; Michael Baker International; 412-269-4635

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