Three Dilemma Zone Strategies for High-Speed Rural Intersections: Comparison of Field Results

High-speed, rural intersections with both passenger car and truck flows present dilemma zone challenges. Three signal-timing strategies were tested in the field at three intersections of varying geometries with the goal of minimizing the occurrence of dilemma zones without sacrificing efficient operation. The strategies focused on hardware changes impacting vehicle sensing as well as mainline green phase extension and termination. The base strategy was volume-density control. The second one (NQ4) added advance detection of high-speed trucks triggering a fixed green extension. The final strategy replaced the second system with a sophisticated advance detection and control system (D-CS) logic based on the work of Bonneson et al. (2002). Results indicate simple volume-density control is surpassed in safety by both other strategies; moreover, the detection-control system dramatically reduced the number of vehicles trapped in dilemma zones at the onset of amber. The second strategy works fairly well in the field. The drawbacks to the NQ4 system are most noticeable at high-volume intersections — it does not actually find times when no vehicles are in dilemma zones, vehicle speeds are not directly used for computing main street hold times (which is accomplished in the D-CS control strategy resulting in improved efficiency), and it is a bit cumbersome and expensive to install. Based on our findings, the addition of the DC-S algorithm into a controller is a worthwhile investment for mixed-traffic, high-speed, rural intersections with dilemma zone issues.