An advance train detector for each direction of travel is added to a conventional two-direction, overlap highway crossing protection system. Detection of a train, prior to its occupancy of the primary approach warning section, by an advance detector actuates a test logic means to drive the conventional crossing protection control logic through an operational sequence simulating the passage of a train. The test logic monitors the warning signal outputs from the conventional protection logic during this simulated train passage to assure that a proper, safe warning operation is possible for the approaching train. Failure to detect proper warning signal outputs activates a failure warning indication and inhibits response by the crossing logic to occupancy of the approach warning sections by the train.