A Model of Car-Following Behavior at Sags

Sags are bottlenecks in freeway networks. The main reason is that the increase in slope has a negative effect on vehicle acceleration, which results in local changes in car-following behavior that reduce traffic flow capacity. Existing car-following models are not able to reproduce the acceleration behavior of drivers at sags and the resulting traffic flow dynamics in a sufficiently realistic way. This paper presents a new car-following model that aims to fill that gap. The model assumes that drivers have a limited ability to compensate for the negative effect that an increase in gradient has on vehicle acceleration. Compensation is assumed to be linear over time; the maximum compensation rate is defined as a parameter. The paper presents the results of a case study using the proposed car-following model. The study site is a particular sag in Japan. Similar traffic flow patterns are observed in simulation and in empirical data from that site. In particular, the model generates a bottleneck caused by the increase in freeway slope, reproducing its location very accurately.