Detection of the Sign of Relative Motion When following a Vehicle

The ability to detect the sign of relative motion in the driving situation was investigated on a freeway. Subject passengers in a specially instrumented vehicle were given controlled looks, normally of four sec. duration, of a lead car. It was the subjects' task to estimate whether the two cars had come closer or moved further apart during the exposure. The comparative ability of nine stimulus functions to predict the detection of the sign of relative motion was examined. The most consistent results were obtained when it was assumed that the response was to either the average value of relative speed during the exposure divided by spacing or to the spacing change divided by spacing. One of the most consistent effects observed was a response bias in favor of indicating that the cars had come closer when they had not. The results of the experiment indicate considerable sensitivity to the sign of relative motion.