1. A detector for detecting light reflected from discontinuous lane dividing lines on a highway surface from a moving motor vehicle, comprising: a photodetector mounted to receive light reflected from the discontinuous lane dividing lines when the vehicle is properly within a lane but not otherwise; circuitry to provide an indication for each lane dividing line sensed; circuitry to provide an indication of a characteristic of the line sensed; logic circuitry to determine if a line is sensed within successive fixed periods of time in which it is expected that a line should be sensed and, if a line is sensed, to determine if the characteristic sensed for that line is consistent with prior lines sensed; and an alarm to, when enabled, provide an alarm indication if the logic circuitry determines that a line consistent with prior lines is not sensed within one of the successive fixed periods of time.