As a result of a Portable Variable Speed Limit System installed along a construction zone in Alaska, the percentage of vehicles speeding decreased by 57 percent.

Study sets out to determine the effectiveness of a Variable Speed Limit (VSL) System in reducing speed through construction zones.

Date Posted

Effectiveness of VSL Signs in Reducing Crash Rates on Roadway Construction Work Zones in Alaska

Summary Information

Due to Alaska’s severe winter weather, most construction within the state is restricted to summer months. Summer also comes with the state’s peak annual traffic volumes due to tourism and more people taking part in outdoor activities at that time. The larger traffic volume and road construction coinciding creates potential for conflict, worsened by drivers speeding through construction zones. To combat this, the Alaska Department of Transportation and Public Facilities (DOT&PF) has implemented portable variable speed limit (VSL) feedback systems statewide, consisting of a radar speed detector and an electronic LED display that provides real time feedback to the drivers. The programmed speed limit may be adjusted to suit the construction site’s needs, and flashes to indicate to when a driver is speeding in order to alert the driver.

A typical Before-and-After study of speed data at a particular construction zone was conducted by a research team from the Civil Engineering Department of University of Alaska- Anchorage. For the system evaluation, speed data was collected from a construction project along an 8-mile segment of Parks Highway (from Church Road to Big Lake Road).

Vehicle speeds for the "before" condition (without the use of the speed display system) were collected using a Doppler radar speed gun, and 112 samples were recorded for this condition. A day and time with similar road conditions and the same posted speed was chosen to collect data for the "after" condition (with a speed display system) for comparison. The "after" condition recorded 378 samples using a portable radar speed display trailer.

Alaska Department of Transportation and Public Facilities (DOT&PF) provided data of violation and crashes over the work zone arena from June 2017 to use as the "before" data for the comparative analysis.


  • The results indicated an overall positive change towards addition of VSL along the roadway. The mean speed observed a reduction of 3.5 mi/h and the 85th percentile speed received a 1.0 mi/h reduction.
  • The overall percentage of vehicles speeding decreased from 21.8 percent to 9.4 percent (a 57 percent decrease). However, the overall percentage of vehicles within the pace also decreased, indicating a greater variation in speed for the "after" condition.
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