Abstract: A system for alerting drivers of vehicles which have an overall height too great to clear an overhead obstruction in their path. Respective pairs of cooperating light sources and light sensors are spaced at appropriate distances from each other and in advance of the overhead structure, with the light beam from each light source being directed to the corresponding light sensor with which such light source is paired. The respective light beams are momentarily interrupted or broken as a vehicle having an excessive overall height passes the successive pairs of light sources and light sensors. When the light beams have been broken in sequence and within a preset, given time period, a signal is sent to the control station which in turn activates a visible, flashing, electric sign indicating that the approaching vehicle is too high to clear the obstruction and warning the driver of the vehicle to stop or exit from the thoroughfare.