Traffic signal phasing during normal and preemption operations shall be clearly defined on the design plans. Figure 6-14 presents a generic railroad preemption sequence. Each piece of this sequence should be represented on the plans.

The right-of-way transfer time (RWTT) is used to transition the traffic signal from any conflicting pedestrian and vehicular phases to the Preemption Clearance Interval. This includes the minimum green, pedestrian clearance, yellow change, and red clearance time intervals.

The Preemption Clearance Interval is then started to begin clearing any motorists queued on or approaching the tracks. The Preemption Clearance Interval must be long enough to allow a motorist to perceive and react to the green indication and travel safely through the MTCD. If the CSD is not long enough to allow a design vehicle to safely queue in, then the Preemption Clearance Interval should also allow vehicles to travel through the CSD.

Once the Preemption Clearance Interval is completed, the normal yellow change and red clearance intervals will cycle through their respective durations prior to the traffic signal entering limited service, or dwell/hold intervals. Once preemption is complete, the traffic signal transitions back to normal operations. Typically, this transition consists of returning to green indications for vehicular movements that are crossing the tracks as they have been delayed the longest. However, various factors such as traffic volumes, the time of day, and special events may dictate which phase the traffic signal will prioritize serving.

If another preemption call is placed while the traffic signal is still transitioning back to normal operations from the previous preemption event, then it is imperative that both the grade crossing warning equipment and traffic signal are capable of re-entering preemption operations efficiently and continue to provide Preemption Clearance Interval.