A Study on Variations of Car-following Behavior at Sag Sections and the Impact of Introducing ACC System

A sag section is a road segment on an expressway in which the vertical slope increases at a small but constant rate. Sag sections are the main traffic bottlenecks on Japanese expressways. Variations in individual car-following behavior are generally recognized as the key cause of traffic breakdown. This research was conducted to develop a quantitative description of variations in individual car-following behavior by obtaining car-following model parameter values from actual trajectories. A simple and reliable model calibration framework considering the effect of sag section is presented. Analysis of the parameter values obtained showed that 48.87% of drivers are affected by the vertical slope change. The parameter values were found to be concentrated in certain ranges rather than being evenly distributed. The dynamic performance of car-following behavior was also analyzed. The results were compared with the results of experiments conducted with a commercial Adaptive Cruise Control (ACC) system. Of the drivers in the analyzed trajectories, 28.83% were found to outperform the commercial ACC system in preventing perturbation in the leading vehicle´s motion from being amplified in the vehicle platoon. The distribution of the parameter values for these drivers was analyzed to identify potential improvements to the ACC system.