Is there anyone on here who can explain exactly how traffic signal preemption at a grade crossing works? As is commonly known, when traffic signals are located very near a grade crossing, they are preempted whenever a train is approaching, meaning they provide a special sequence which allows vehicles queued up on the tracks to clear the tracks, prior to the activation of the crossing warning devices. What I would like to know is the process by which the traffic signals are notified that a train is approaching. The traffic signal controller must somehow be able to detect the presence of an approaching train, and then provide the preemption sequence. This is a question to which I have trying to find the answer to for quite some time now, and hopefully, someone on here will be able to provide it, perhaps someone with a background in traffic engineering and/or railroad systems engineering.
I had a pre-emp on 1 crossing and I'd need to show you on a set of plans/prints how it worked. Basically the best I can explain it was when a train got on my approach circuit it tripped a relay in the the traffic light control case which set the traffic lights red at the same time my xing activated. I hated pre-emps because if a xing ever got in trouble from bad weather or a battery
charger failure street traffic would get snarled up and by the time I got there the cops were primed and ready to jump in my ass even though my xing had no gates, just flashers.