Effectiveness of combined advance detection and advance warning flashers

Advance detection (AD) is a common method of providing dilemma zone protection at high-speed signalized intersections. AD systems enable approaching vehicles to extend the green and prevent the onset of yellow while they are in the dilemma zone. However, AD systems could be counterproductive as they tend to increase the likelihood that the green will be extended until it reaches the maximum allowable at which point it transitions immediately to yellow (max-out) such that any dilemma zone protection is lost. Consequently, the Nebraska department of roads (NDOR) developed a new system that combines AD and advance warning (AW) systems to reduce the frequency of loss of dilemma zone protection due to max-out, thereby improving efficiency without unnecessarily compromising intersection safety. A number of states have adopted similar systems; however, the operating algorithm is unique to Nebraska. Currently NDOR has implemented thirty eight combined AD-AW systems around the state. While these have received many favourable comments, especially from truck drivers, there has been no comprehensive analysis of their safety effectiveness. This paper examines the actuated advance warning system in Nebraska with respect to safety. Crash records from before and after the implementation of the system are compared. In addition control intersections are used to compare crash rates over time using a Bayesian technique to ensure that no exogenous variables affect the results. Results of the analysis are encouraging (a crash reduction rate of 0.384) and suggest that the use of AD systems in conjunction with AWS systems should be encouraged as effective safety treatments for high-speed signalized intersections.