Time headway and platooning characteristics of vehicles on interstate highways

Time headway and platooning characteristics of vehicles on interstate highways in low to high traffic volume conditions (136 to 1704 vph) were studied. The lognormal model with a shift of 0.36 second was most appropriate headway distribution. Relationships between traffic volume and mean and variance of variable of lognormal distribution were found to be logarithmic and linear, respectively. One can generate time headway from the shifted lognormal distribution by knowing only traffic volume. A modified geometric model most appropriate function for platoon size distributions. A power function was found between traffic volume and probability of having platoons. A power function was also found between traffic volume and number of platoons. Thus, one can generate platoon size from modified geometric distribution by knowing only traffic volume.; Mean time headway for platooning vehicles decreased as traffic volume increased. About 18% to 31% of platooning vehicles had a space gap that corresponds to a reaction time of 0.5 second or less, with more than half of these in large platoon of 5 vehicles or more. About 15% to 85% of platooning vehicles were in large platoons. For left and right lanes, platoons led by heavy vehicles were moving at slower speeds than platoons led by light vehicles. Proportion of vehicles in platoon on right lane was significantly higher than that on left lanes. Proportions of heavy vehicles leading platoons in right lanes were significantly higher than that for left lanes. Mean headways of platooning vehicles in right lanes were significantly higher than that in left lanes. Mean platoon sizes in left lanes were significantly higher than that in right lanes. In two out of three data sets for left lanes, light vehicles following heavy vehicles, and heavy vehicles following light vehicles kept longer headways than light vehicles following light vehicles. For right lanes, for two out of three data sets light vehicles following heavy vehicles maintained longer headways than heavy or light vehicles following light vehicles.