Advance preemption holds several benefits over simultaneous preemption, one of which is how advance preemption can provide a lower activation time for the railroad warning system at locations where a large APT is expected to adequately clear the crossing of any vehicles. As a result, advance preemption has garnered popularity over simultaneous preemption. However, this has led to highway authorities requesting increasingly higher preemption times, resulting in increased delays for motorists queued upstream of a grade crossing during preemption events. Long activation times can be linked to a lack of compliance with the warning system and can lead to undesirable motorist behavior.

Prior to 2018, as highway authorities requested longer preemption times at interconnected grade crossings, railroad warning times also began to fluctuate. This led to some concern regarding the reliability of railroad warning time. Grade crossing warning equipment manufacturers reported that as approach times increases, the railroad equipment’s ability to accurately detect a train’s approach became less reliable.

As a result, AREMA adopted the 50 second rule to encourage industry best practice to avoid unnecessarily increasing vehicular delays and provide reliable preemption timing at interconnected grade crossings.

Per the AREMA C&S Manual in Part 3.1.10, Section C, “For grade crossing warning systems, interconnected with highway traffic signals, System Design Time minus Equipment Response Time shall not exceed 50 seconds.”

In this case, the term “System Design Time” may be used interchangeably with “Total Approach Time”. Railroad authorities utilize various types of signal equipment, which can lead to varying ERT. The designer shall verify the railroad specific ERT and BT to confirm that the requested TAT will not exceed the AREMA 50 Second Rule. Long railroad approaches create complexities in the railroad warning system and can impact the reliability of warning times. See Figure 4-1 for a graphical representation of how System Design Time relates to the overall railroad approach.