The Effect of Expectancy on Visibility in Night Driving

Abstract

The role of expectancy on the perceptions of drivers is an important variable that affects highway safety. But, it is not well understood. An experiment in which the visibility of a pedestrian dummy was measured found that the visibility distance was twice as great when drivers expected the pedestrian than when it was unexpected. The results of that study have been used as though the 2:1 ratio in visibility could be extrapolated to other night driving conditions. Arguments against such generalization are made. They involve analyses of the illumination of headlamp beams, the probability of detection of objects and the effects of speed. Various theoretical approaches are used to evaluate the role of expectancy and other studies are reviewed. No support was found for the 2:1 constant ratio between “expected” and “unexpected” driver visibility in night driving. Further work is needed to study expectancy and to quantify its effect on visibility in a variety of situations.