Traffic signal timing techniques designed to achieve efficiency often have an adverse effect on safety by stranding drivers in the "dilemma zone," defined as the length of roadway on a signalized intersection approach where drivers are uncertain about whether to proceed or stop at the onset of a yellow signal. This uncertainty results in hesitation that can lead to rear-end, left-turn opposed, or sideswipe collisions.
As a solution to this problem, a dilemma zone detection and control system was developed that overcomes the limitations of traditional multiple advance detector systems. The new Detector-Control System (D-CS) intelligently forecasts the best time to end the signal phase based on consideration of vehicle presence in the dilemma zone, vehicle type (i.e., truck or car), and the presence of vehicles waiting for a conflicting phase. To evaluate the D-CS, a field study was conducted by Texas Transportation Institute between 2008 and 2012 at a total of eight sites in five U.S cities:
- Fort Lauderdale, Florida (three sites)
- Peoria, Illinois (two sites)
- New Orleans, Louisiana (one site)
- Waco, Texas (one site)
- San Antonio, Texas (one site)
A before and after study of crash data was used to evaluate the safety of the D-CS algorithm. The results were based on data for a 5-year "before" period and a 2-year "after" period, depending on cooperation of the local agency and availability of data. At each participating site, two-loop detector traps in each approach lane were used to obtain the necessary information about vehicles approaching the intersection. The traps were located 700 to 1,000 ft upstream of the intersection.
A real-time data collection system was used to collect performance data, including:
- Number of red light runners.
- Number of vehicles caught in the dilemma zone at the onset of yellow on main street phases.
- Analysis results suggested that the after study periods experienced 82 percent fewer red-light violations and 73 percent fewer vehicles in the dilemma zone than the before study periods.
- State crash data indicate that by combining angle plus rear-end crashes (because of small sample sizes), the D-CS reduced crashes by 9 percent.
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