Advance Warning Signals Long-Term Monitoring Results

A driver approaching a signalized intersection at which the light has turned yellow must make a decision whether to stop or proceed. A signal that is properly timed will generally provide an opportunity for a safe and legal maneuver. As approach speeds increase, however, executing a safe maneuver becomes more difficult. One countermeasure used to provide advance warning of an approaching intersection or an impending signal change at such an intersection is an advance warning signal (AWS). Several AWS systems have been installed throughout the United States, including four in Utah. The effectiveness of the Utah systems has been evaluated as a function of intersection safety by using crash history, speed distribution, and red-light running (RLR) metrics. The results of the research suggest that the contribution of the AWS systems to the safety and operation of the intersections can be interpreted as both positive and negative. The AWS systems have helped to maintain higher operating speeds and may also have improved capacity when vehicles do not need to stop. Meanwhile, speeds have been reduced in the time just before the onset of the yellow interval. However, the additional information provided to drivers may have encouraged some drivers to attempt to beat the light, as evidenced by an increase in RLR even with shortening of the lead flash time. The increase in RLR, however, has not led to an increase in the overall crash frequency or crash rate at these intersections.