A study was conducted to test the primary hypothesis that driving speed is a factor in discrimination of relative motion in low illumination. Thirty subjects ranging in age from 19 to 54 years were used, each making 84 separate observations for a total of 2,520 judgments on: (1) the difference in speed of a moving vehicle ahead and his own speed, (2) the difficulty of making a judgment, and (3) distance of the vehicle ahead. Data were analysed on the basis of the following variables as aspects of measurements made by the scotometer: (1) perception time, (2) estimation of speed differential, (3) estimation of distance, (4) judgment of difficulty, and (5) errors made in direction of movement. A correlation of 0.45 was obtained between visual acuity and perception time, although no one had less than about 67% vision. The difficulty of perception and judgment was greatest for all lower speeds. It is concluded that differential speed is a factor in judgment of relative distance. Increasing visibility of a moving target or vehicle at mesoptic levels of vision will significantly decrease the time for accuracy determination of direction and rate of movement, increase the accuracy of estimating or judging actual difference in speeds, increase the safety factor for stopping distance by reducing the apparent distance of the brighter targets, and decrease the errors in judgment of direction of movement. Reflectorization of tailgates greatly increases perceptability of relative motion over conventional nonreflectorized treatment of tailgates.
Abstract