An experiment was conducted to evaluate the ability of drivers to judge headway and the closing rate in overtaking situations. The study was conducted on a completed but unopened section of I-95 in Philadelphia. Two cars were used in the tests: a lead car (lc) and an overtaking car (oc). Headway distance (h) is defined as the distance between the front of the overtaking car (c) and the rear of the lead car (lc). The closing rate (cr) is the speed advantage of the oc over the lc. The time headway (th) is h/cr-the time separation between the oc and lc. Ten Philadelphia public school teachers with a minimum of 8 years of driving experience were used as subjects. The only condition that had a significant effect on the variability of judgements was constancy/variability of cr. The variable-cr produced a larger variable error in judgment than did the constant-cr condition. Headway-distance judgment accuracy was good: subject estimated h's to within about 30 ft, independent of the distance, 95 percent of the time. The proportional error decreased with the distance to be judged and at 440 ft 95 percent of the judgments were within 7 percent of the true h. Th estimates varied significantly more when subjects had to judge the cr. On the highway, overtaking drivers must judge and take into account the th with a lead car to make good decisions. Poor th estimates can lead to errors of several kinds. All things being equal, the less the th, the less the time required to complete a flying pass. Since the judgment of cr is a significant factor in error of th judgment, cr must be considered as a candidate for remediation.
Abstract