The Effect of Car Size on Headways in Freely Flowing Freeway Traffic

In order to determine the effect of car size on freeway capacity, headways were measured and vehicles classified by size for 20,500 vehicles in near-capacity traffic on a Detroit freeway. The data were obtained using a videotape recorder system. Vehicle arrival times were measured from a videotaped image of the oncoming traffic using a photodetector mounted on the monitor screen. Size classifications were made by an observer from the pictures of each vehicle on the monitor. Average headways increased monotonically with lead car size from 1.87 s for “small” cars to 2.02 s for “large” cars. These differences are significantly greater than the differences expected from the greater road space occupied by the larger cars if the front bumper to rear bumper separation were independent of car size. The effect of following car size was nonmonotonic with the shortest average headways, 1.90 s, observed for “medium” following cars, and essentially equal headways of 1.99 s for “small” cars and 2.00 s for “large” cars. The observed car size effects were found for long as well as for short headways. The net result of these effects would be an 8% increase in freeway capacity if the cars observed in the study were all replaced with small (subcompact) cars.