A yellow trap, or lagging yellow, is a special scenario that the designer shall account for to mitigate collisions from occurring at intersections during preemption. A yellow trap occurs when the track clearance green phase and the opposing traffic’s through and permissive left turn phases are being served prior to a preemption call. When preemption is activated, the traffic signal controller will hold the track clearance green phase, while the opposing signal heads will go to yellow. Opposing left-turn vehicles will wait in the intersection for an adequate gap. Depending on the traffic, motorists may need to wait until the traffic signal transitions from green to yellow or even red. Motorists typically assume the opposing traffic is transitioning as well. When the opposing motorists on the track clearance phase remain green, to transition to track clearance green sooner, a collision can occur since both drivers are used to seeing simultaneous yellows on both approaches when their phase ends. Figure 3-8 illustrates an example of a yellow trap occurring during preemption with a grade crossing located north of the displayed intersection.

The designer may apply either of the following methods to prevent a yellow trap during preemption:

The traffic signal should have protected-only left turn for the track clearance green phase and opposing traffic, or
If the design proposes permissive left turns for opposing traffic, transition the traffic signal to all-red when preemption is activated. Once the traffic signal is all-red then transition to track clearance green. The TxDOT Form 2304 should account for the amount of all-red time that will be provided during preemption to prevent the yellow trap.