Previously, the circuits used for activating a crossing's warning devices were very simple, and activated them whenever a train came within a fixed distance (hundreds or thousands of feet) of the crossing. This method required that the crossing be designed to accommodate a train approaching at the track speed limit. Unfortunately, this made for longer warning times for all trains approaching the crossing at lower speeds. Very slow (5 mph, for example) trains could receive many minutes of warning time, thus holding up highway traffic unnecessarily.
This inductance can be measured by connecting a constant current a.c. (alternating current) source to the rails, and measuring the voltage which results. Ohm's Law tells us that with a constant current source, the voltage measured will be proportional to the impedance. The absolute magnitude of this voltage and the time-rate-of-change of that magnitude can then be used to compute the amount of time remaining before the train arrives at the crossing. (For a constant-speed train only.)
The crossing's warning devices are activated whenever the computed amount of time remaining before the train's arrival at the crossing is less than or equal to the desired (programmed) amount of warning time. However, each equipment manufacturer has their own special algorithm for determining this. The earliest grade crossing predictors used analog computers to perform this calculation, but modern equipment uses digital microprocessors to do this.