The effect of contrast on perceived speed in a road-crossing scenario

It is well known in the psychophysical literature that low visual contrasts can lead to amisestimation of an object’s speed. This effect can have important consequences for trafficsafety when navigating under low visibility due to adverse weather conditions (e.g., fog) orvisual impairments. So far, road traffic research has primarily focused on the perception ofself-motion during driving when evaluating the effects of contrast. Drivers underestimatetheir own driving speed when contrast is reduced uniformly, but overestimate speed whencontrast is reduced in a distance-dependent manner, simulating visibility in fog. It remainsunclear whether pedestrians are subject to similar biases when estimating the speed ofapproaching traffic. In two experiments, we used a two-interval forced choice discriminationtask to investigate the effects of a uniform and distance-dependent reduction of contrast onthe perceived vehicle speed in a simulated road-crossing scene. The results point towardssimilar  effects: Speed  was  underestimated  when contrast  was  reduced uniformly  andoverestimated in simulated fog. The overestimation of speed in fog occurred in bare andvisually more complex road environments including roadside trees or road markings. Whilethe effect of fog has previously been explained by drivers relying on peripheral optic flowwhen the visibility of the central optic flow is reduced, this cannot account for the effectsobserved in pedestrians. We discuss the findings considering the overestimation of vehicledistance in fog and the integration of motion signals across time.