This study examined the implementation of a traffic signal preemption system used to give trucks priority at a high-speed intersection (US-83 and FM-886) in Sullivan City, Texas. The Texas Department of Transportation installed a pair of magnetic loop detectors under the pavement 550 feet upstream from the intersection in order to identify trucks and determine their individual speeds. The loop detectors were connected to a Traffic Counter/Classifier (TCC) and a personal computer capable of communicating with and activating intersection signal control devices. Based on the signal phasing plan and truck speed data input, the computer placed holds on signal timing as necessary to extend green time for approaching trucks. The duration of each hold was based on truck speed. If a truck was identified during a red light, no hold call was placed. A timer relay was also used to limit hold times to 2 minutes in order to reduce the excessive wait times resulting from consecutive truck actuations.
The data was collected and analyzed to evaluate the impact of using the preemptive signal control system to minimize truck stops. The research team collected data three weeks before implementation, and then three weeks after. The "before" and "after" data were collected during typical traffic volume conditions.
The signal control system benefited about four percent of the trucks on the roadway since holds were placed only when trucks needed extended green time. For a weekly volume of 2500 trucks, 100 less stops were made.
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