Variable Speed Limit System shows promise; crashes reduced to lowest level in a decade.

Results of Wyoming Department of Transportation's VSL implementation in the Elk Mountain Corridor

October 2010
Wyoming,United States

Summary Information

The Wyoming Department of Transportation (WYDOT) implemented a Variable Speed Limit (VSL) system along the Elk Mountain corridor during February 2009 with the goal of improving safety and reducing closure frequency and durations. The Elk Mountain corridor is located in southeastern Wyoming on Interstate 80 between Laramie and Rawlins. The corridor carries approximately 11,000 vehicles per day. On average, approximately half or more of those vehicles are trucks carrying freight.

The VSL system included 20 variable speed limit signs at ten locations (5 in the eastbound and 5 in the westbound directions) and 10 speed sensors. The VSL system was expanded in the 2009-2010 winter season to include 8 additional variable speed limit signs in four new locations (2 in the eastbound and 2 in the westbound directions). Another major data source was the information collected from the Road Weather Information Systems (RWIS) station located approximately in the middle of the corridor.

The overall goal of this project is to improve safety along the corridor as measured by the number of crashes that occur. Crash records for the first full year of VSL system operation were analyzed along with records for the years prior to the VSL system to set the baseline crash history.

The year after the VSL system was implemented in February 18, 2009 was the period when Elk Mountain Corridor had the fewest crashes of any of the 10 years prior. During this time the total number of incidents and the number of injury crashes fell to 0.999 and 0.208 per Million Vehicle Miles Traveled (MVMT) respectively. These are the lowest crash rates in the last decade. However, The number of fatal crashes remained consistent in the last ten years and was equal to three fatal crashes per year on average.

It is too early to make final conclusions about whether the VSL system is improving safety on the Elk Mountain corridor on I-80 due to the nature of safety data. There are other baseline factors to consider, such as exceptionally harsh winter conditions and an overall decrease in number of vehicles on the road in 2009. Crash records must be analyzed for a minimum of three years with the system in operation in order to determine with statistical confidence if the safety along the corridor has improved. In the future, crash records will be analyzed to determine the effectiveness of the VSL system on improving safety.

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Variable Speed Limits System for Elk Mountain Corridor

Author: Buddemeyer, J., et. al.

Published By: Wyoming Department of Transportation

Source Date: October 2010



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Benefit ID: 2011-00733