Exploring the Influence of the Congested Downstream on Traffic Flow at its Upstream Bottlenecks

With the rapid increase of the private cars, urban road congestion is getting worse. As known, when the downstream section is free flow, the congestion for every bottleneck section starts from the bottleneck cell, and then gradually propagates towards its upstream. Many studies about bottlenecks are based on the assumption that the downstream of each bottleneck is free flow. In this paper, we have investigated ramp systems with the congested downstream. Based on the cell transmission model, a tri-cell ramp system with two bottlenecks is used as an example to explore what the convergence state of the bottleneck section is under the congested downstream section. Three cases have been studied: free-flow downstream, middle-state downstream, and the most congested downstream, and the different convergence states. The results show that 1) if the downstream of the bottleneck is the most congested, the upstream bottleneck section converges to the most congested state no matter what the initial state is; 2) if the downstream is congested, the upstream section converges to congested states, and the degree of congestion in the upstream section increases with; 3) the larger the density in the congested downstream is, the more congested the final state of the ramp system is.