Empirical analysis of the causes of stop-and-go waves at sags

Stop-and-go waves are spatially-confined regions of low traffic speed that propagate upstream at a constant velocity. The occurrence of stop-and-go waves on freeways has negative impacts on both travel time and traffic safety. Sags are freeway sections along which gradient changes significantly from downwards to upwards. Stop-and-go waves often emerge on the uphill section of sags, both in uncongested and congested traffic conditions. According to previous studies, the formation of stop-and-go waves at sags can be caused by local changes in car-following behaviour as well as disruptive lane changes. However, it is not clear which of those two causes is more frequent. This paper aims to identify the primary factor triggering stop-and-go waves at sags. To this end, the authors analyse vehicle trajectories collected by means of video cameras on a three-lane sag of the Tomei Expressway (Japan), identifying the causes of formation and growth of stop-and-go waves on the study site. The results show that the primary factor triggering stop-and-go waves is related to car-following behaviour. This finding shows the relevance of developing systems to assist drivers in performing the acceleration task at sags.