Abstract: An apparatus for alerting the driver of a vehicle to the presence of another vehicle in an adjacent lane while changing lanes, in which an ultrasonic transmitter emits acoustic energy which, if reflected from an adjacent vehicle within a zone of coverage, is received on an ultrasonic receiver and processed to produce an audible warning to the driver indicative of the relative speed and position of the adjacent vehicle. One of three distinct audible warning tones alerts the driver to an approaching vehicle, a receding vehicle, or a vehicle whose relative speed is the same.