This project bulletin discusses the preempt trap and how to deal with it. The preempt trap is a potentially hazardous condition that can occur under advance preemption and is worsened by variation in warning time. The preempt trap occurs when the track clearance phase - designed to move stationary vehicles off the tracks before the train arrives at the crossing - ends before the gates and the warning devices are activated by the grade crossing predictor. Because the warning lights have not been activated and the gates are still vertical, vehicles can continue to cross the tracks and possible stop on the tracks. However, the track clearance phase has already expired and there will be no further opportunity to clear! The effect is that vehicles may be trapped between the intersection stop line and the moving train or, even worse, trapped on the tracks in the path of the oncoming train. (Author/publisher)
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