Abstract
Traffic signals near railroad grade crossings having active warning systems are normally interconnected and receive a signal from the railroad track circuitry when trains are approaching. This train approach signal is used by the traffic signal to interrupt and preempt its normal phasing and enter into higher priority special phasings to clear the track of any vehicles that might be stopped in the pathway of a train. The amount of preemption time needed to clear a vehicle from the pathway of a train is necessary information for appropriate preemption signal settings but there are no definitive guidelines on how to determine this amount of time. It is usually left to the judgment of the signal engineer, and frequently unfounded assumptions are used. This study investigated the time required to clear a vehicle in a queue backed up onto a track at railroad-preempted traffic signals. The two key time components are start-up delay and repositioning time. Queues where all preceding vehicles are short passenger cars cause the longest start-up delays, and heavy trucks at the track involve the longest repositioning times. The model developed is convenient because once it is decided that preemption is needed, only easily determined values are required—the minimum track clearance distance, the clear storage distance, and the types of vehicles that are permitted to use the roadway. The model adopts a high level of confidence to minimize the risk of crashes but avoids the "worst-case" concept to avert an invitation for litigation when an improbable or unforeseen worst case results in a crash.
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