Drivers’ steering reaction to a light stimulus on a dark road

When a car was approaching on a dark road, a small lamp was lit on the road shoulder. The onset of the light was timed by means of infrared beams directed over the road at a constant distance from each other so that for each passing car, a randomly selected beam was used to switch the lamp on. The lateral displacement due to the onset of the light was measured for 815 cars and lorries and these data were plotted as a function of the time available for this avoidance response. The average avoidance response, towards the centre-line, started at a latency of less than 2s, reached its halfway strength at 2-5 s and its maximum at little more than 3 s. It was concluded that the typical driver/vehicle steering response latency, in a fully unexpected situation, is of the order of 2-5 s and the upper, safe limit at little more than 3 s.