Two studies are described evaluating high-mounted stop lamps using light-emitting diodes (LED's) compared with conventional tungsten units. The first of these studies obtained ratings from subjects who drove one car and followed another car equipped with the test units. The subjects generally liked the LED units.The second study was a laboratory evaluation of the attention-getting capabilities of LED and conventional stop lamps. Under all conditions subjects responded faster to the LED units. The response time advantage for the LED units increased with more difficult viewing conditions, such as high levels of illumination and greater viewing distance.The results of these investigations are discussed in terms of the applicability of the LED technology to high-mounted stop lamps on motor vehicles.