Evaluation of the Safety Effectiveness of “Vehicle Entering When Flashing” Signs and Actuated Flashers at 74 Stop-Controlled Intersections in North Carolina
North Carolina Department of Transportation (NCDOT) has utilized "Vehicle Entering When Flashing" (VEWF) systems to provide an active, real-time warning that delivers motorists with more information about intersection conditions. Depending on the assembly placement, they may be used to warn drivers approaching an intersection if a vehicle is entering the intersection from the minor road, or they may be used to provide guidance on gap selection for stopped drivers. The assemblies include vehicle-actuated warning signs for stopped vehicles, vehicle-actuated warning signs for through vehicles, or a combination of both.
The purpose of this project was to determine if the installation of VEWF intersection warning systems reduce the number and severity of crashes at various types of two-way stop controlled intersections. Because a variety of sign configurations are used, this study determines if a particular sign placement and usage provides more safety benefit. This study will compare the crash data of stop controlled intersections before and after the assembly installation.
74 intersections covering four categories of VEWF intersection warning systems were evaluated.
- Category 1 – Overhead Signs and Flashers at the Intersection on Major, Loop on Minor (24 sites)
- Category 2 – Overhead Signs and Flashers at the Intersection on Minor, Loop on Major (19 sites)
- Category 3 – Post Mounted Signs and Flashers in Advance of Intersection on Major, Loop on Minor (23 sites)
- Category 4 – Locations with Combination of Category 1 through Category 3 (8 intersections)
A before and after crash analysis was performed at each intersection utilizing the Traffic Engineering Accident Analysis (TEAAS) software that accessed the North Carolina Traffic Records Database. The time periods analyzed for each location varied depending on when the treatment was installed. In most cases, the ending dates for the analyses were determined by the available crash data at the time the crash analysis was completed, which was through October 31, 2011. Some of the analysis periods were over ten years in duration, and the average before period was approximately five years.
Empirical Bayes before and after techniques were utilized to account for selection bias and to overcome the threat of regression to the mean, along with other potential deficiencies in a naïve before and after analysis. The Empirical Bayes approach requires the use of reference sites as well as before period data from the treatment site to estimate the expected safety of the treatment site had no improvements been made. Approximately five reference sites per treatment site were chosen.
- For all 67 treatment locations, the results of the Empirical Bayes analysis with the traffic volume adjustment yielded
- 3 percent (+/- 5 percent) reduction in target crashes
- 7 percent (+/- 4 percent) reduction in total crashes
- 6 percent (+/- 6 percent) reduction in injury crashes
- 16 percent (+/- 16 percent) reduction in severe injury crashes
- There was a noticeably greater reduction in total crashes for Categories 3 and 4 (19.3 percent, 25.1 percent) than Categories 1 and 2 (-9.1 percent, 3.5 percent). Category 1 actually showed an increase in total crashes.
- Crash analysis results for four-lane divided at two-lane intersections (11 of the intersections studied) showed no apparent reduction in crashes. Overall, the locations with this intersection geometry did not experience a reduction in total, target, injury or severe injury crashes. The use of VEWF systems at four-lane divided at two-lane intersections may be more a “band-aid” treatment that does not address the root cause of crashes in this situation and a geometric change (i.e. directional crossover, offsetting minor road legs, or crossover closure) may be a more appropriate solution to address crash patterns likely related to gap-acceptance.