Before-and-After Study of Freeway Variable Speed Limit Installations Estimated Crash Reductions of 34 Percent in Wyoming and 29 Percent in Georgia.
Multi-State Study Conducted Statistical Analyses of Crash and Traffic Data Before and After Variable Speed Limit Deployment.
Made Public Date

Developing Crash Modification Factors for Variable Speed Limits

Summary Information

Variable Speed Limits (VSLs) that vary based on real-time traffic, roadway, or weather conditions, are often used by transportation agencies as a safety improvement strategy. Researchers evaluated the safety effectiveness of VSL implementation on freeway corridors by state departments of transportation (DOTs) in Virginia, Wyoming, and Georgia. Virginia DOT deployed VSL technology on I-77 in October 2016, with displayed speed limits of 35, 45, 55, or 65 miles per hour (mph) depending on visibility conditions. Wyoming DOT deployed VSLs on 52 miles of I-80 between Laramie and Rawlins in February 2009, with displayed speed limits between 35 and 75 mph, in 5-mph increments. To harmonize traffic flow, Georgia DOT installed VSLs at various locations on I-285 near Atlanta in October 2014. VSL effectiveness was evaluated using before and after data and statistical models. Benefit-Cost (B/C) analysis of VSL deployment on freeways was also performed.


The potential to reduce crashes was assessed by estimating crash modification factors (CMFs) associated with the safety improvement strategy in terms of total, fatal, and injury, and property damage only (PDO) crash frequencies. Safety data from Virginia, Wyoming, and Georgia were collected and used in an interrupted time series study design (with a comparison group for Georgia). Crash data obtained from the Virginia DOT spanned an eight-year period from 2010 to 2017, and data from the Wyoming DOT ranged from 2004-2014. Georgia DOT provided crash data for the 2012-2017 period. The freeways were divided into segments based on the location of the VSLs. Roadway design data such as lane width, shoulder width, the number of lanes, were extracted using online mapping tools. Geolocation data from the crash databases were then used to associate crashes with the corresponding freeway segment and corresponding traffic volume data. Segment regression was used to analyze the compiled datasets for each state.


  • The analysis results from VSL deployment on I-285 in Georgia showed significant evidence of reductions in crashes. In Georgia, the estimated CMFs for total, daytime, and rear-end crashes were 0.708, 0.728, and 0.648, respectively. Overall, crash reductions of 29.2 percent were associated with the VSL installation.
  • In Wyoming, estimated CMFs for total, fatal, injury, PDO, rear-end, and fixed-object crashes were 0.66, 0.49, 0.71, 0.35, and 0.59, respectively. VSL deployment on I-80 was estimated to reduce crashes by 34 percent.
  • Results from Virginia were inconclusive, as the analysis found no statistically significant evidence of a change in overall crashes. Researchers considered the limited amount of active VSL data as a possible factor in the findings. Only ten months of data was available for the post-implementation phase, and since the system modified speed limits only during adverse weather, it was only active approximately five percent of the time.
  • For the locations with statistically significant results, an economic analysis was conducted, yielding an estimated B/C ratio of 40.38 in Georgia and 9.05 in Wyoming.
Goal Areas
Results Type
Deployment Locations