Adaptive Signal Control Systems (ASCS) are typically deployed to improve the operational performance of intersections and traffic corridors. ASCS change the signal timing of intersections in real-time to accommodate for changing traffic demand. The objective of this study was to evaluate the safety benefits of ASCS and determine the type of corridors best suited for ASCS implementation for traffic safety and operational improvements. A literature review and a nation-wide survey involved 28 states were conducted at the beginning of the research to identify ASCS corridor characteristics (e.g., design speed). The research team then evaluated the safety effectiveness of ASCS in terms of reducing the crash frequency at 11 ASCS corridors with a total of 109 signalized intersections in South Carolina. The specific questions that were focused on this study were: (i) the effect of ASCS on the crash frequency and severity, (ii) the effect of ASCS on the likelihood of secondary crashes on freeway sections that have alternate corridors with ASCS, (iii) the effectiveness of ASCS in terms of the travel time and travel time reliability, (iv) the type of corridors best suited for ASCS implementation for traffic safety and operational improvement.
Researchers conducted various statistical methods to analyze crash data before and after ASCS deployment to determine safety and operational effects. To determine the effects of ASCS on crash frequency, the researchers conducted a full Bayesian before-and-after comparison, in which 2011-2014 was the “before period”; and 2015-2017 was the “after period.” The crash prediction models used in the before-and-after comparisons were developed using traffic volumes, geometric roadway features, and statistics on four types of crashes (total, fatal and injury, rear-end, and angle). Geometric features were collected from online mapping tools, while traffic and crash data were obtained from the South Carolina Department of Transportation (SCDOT) databases. Regression models were developed to assess the impact of ASCS on crash severity. To determine the effects of ASCS on crash likelihood, researchers also studied secondary freeway crashes on alternate routes to ASCS-deployed routes. A total of 52 months of crash data from September 2014 to October 2016 (“before” period) and November 2016 to December 2018 (“after”) for the Charleston I-26 and Richland-Lexington I-26 were analyzed. Secondary crashes were considered a possibility if crashes occur within one hour of the primary crash and within two miles upstream of it. The operational impacts of ASCS deployment were evaluated based on travel time reduction and travel time reliability improvements during the day and weekday peak periods, and consistency in these measures in both travel directions.
- ASCS generally reduced the number of crashes for most of the ASCS corridors and intersections. Nine out of 11 ASCS corridors showed a reduction in total crashes and a reduction in angle crashes due to ASCS. Ten out of 11 ASCS corridors showed the fatal and injury crash reduction due to ASCS. However, four corridors showed ASCS increases in rear-end crashes, possibly because of more stop-and-go conditions resulted from high traffic demands at side streets.
- ASCSs generally reduced crash severity, but the reduction was dependent on corridor and intersection geometric features. It also relied on whether the speed limit difference between a major and minor road was less than or greater than 10 mi/h.
- When ASCS was deployed on a corridor parallel to a freeway, the likelihood of secondary crashes were reduced by nearly 47 percent.
- The safety benefits of ASCS were higher at intersections with an annual average daily traffic (AADT) between 20,000 and 50,000 vehicles/day than at intersections with less than 20,000 vehicles/day. Reductions in total, fatal and injury, and rear-end crashes were statistically significant.
- The safety benefits of ASCS deployment were also higher at intersections with a speed limit between 40 and 55 miles per hour (mi/h). Compared to intersections with a speed limit between 30 and 35 mi/hr, reductions in total, rear-end, and angle crashes were statistically significant.
- ASCS deployment reduced travel time about 61 percent of the time during a day and 77 percent of the time during the peak periods (in either direction).
- During the day, ASCS reduced travel time by 6.4 percent on average. Travel time was reduced by about 8.6 percent during peak periods and reduced by about 6.0 percent during off-peak periods.
- ASCS produced higher operational benefits (i.e., reduced travel time) if the average speed of an ASCS corridor is equal to or lower than 35 mi/h, and the number of signals on an ASCS corridor is more than 10.
- Compared to the case when the ASCS was not operational, ASCS deployment improved travel time reliability by 31.4 percent, with an improvement of 35.7 percent during peak periods and 30.0 percent during off-peak.
The effectiveness of ASCS in reducing travel time was found to be consistent in both directions on an hourly basis for 73 percent of the studied ASCS corridors (eight out of 11), whereas the travel time reliability improvement was consistent in both directions on an hourly basis for only 45 percent of the studied ASCS corridors (five out of 11). Figure 1 summarizes the safety and operational impacts of ASCS at the 11 study corridors as found in this study.
Figure 1. Safety and operational impacts of adaptive signal control systems at study corridors.