Here is my suggestion;
Operation of model railroads generally utilize a "fast clock" which operates at 8 times normal speed, thus allowing a "days work" to be completed in 3 hours.
Please do not take anything I state as any indication of how much you know or do not already know. I always start at the first floor.
Railroad operating rules, in general, require a 40 second activation of the highway crossing signals, prior to the arrival of the fastest train operated over the crossing. 40 divided by 8 equals 5. Therefore, I would allow for 5 seconds of signal operation, prior to arrival of the fastest train at the crossing island.
Rather than using motion detection to activate, please give some consideration to using approach blocks (isolated track section) to activate your signals. If you are not into electrical wiring, see if you can get someone you know to help. You will be very pleased with the realistic results. There is plenty of information regarding how crossing signals operate. ps.. new systems and upgrades use a carrier system in the track, but it is boring to me. I like relays.
If you have a yard close to the grade crossing, and do not wish to aggravate the motorists with excessive and needless operation of the signals, you can have a short approach on the yard side of the crossing with a CP (control point) signal which displays a red signal upon first being operated by an approaching train, then as the train creeps up to the CP, the grade crossing signals will activate, shortly thereafter the CP will change to green. This system is used in my area where stations are located next to a busy crossing so as not to tie up motor vehicle traffic needlessly, especially during the commutes. It provides another modeling feature.
Just some food for the the "Little Gray Cells"!
Have fun
