Speed Estimation from a Moving Automobile

An experiment is described in which the role played by the senses of sight and hearing in the estimation of speed from a moving automobile was investigated. Subjects were driven in the front seat of a car whose speedometer was hidden from their view. The subjects' task was to estimate the speed of the car under the following four conditions of sensory awareness: (A) normal passenger; (B) unable to see— the subject wore a blindfold; (C) diminished hearing— the subject wore a sound excluder; (D) the subject wore both a blindfold and a sound excluder. Under all four conditions slow speeds (25 mph or less) were, on average, underestimated. For the two conditions which permitted hearing the mean estimates of normal driving speeds were without systematic error. However, for the two conditions involving diminished hearing the mean estimates were always lower than the set speeds, indicating that the sense of hearing is of great importance in the task of speed estimation. A subjective speed scale, which is not of the common power-law form, is derived for the normal passenger condition. This is used to predict how drivers would perform certain tasks which have previously been investigated experimentally, and the predictions agree satisfactorily with the observations.