Research On Ramp Systems’ Characteristics And Ramp Metering Strategies Based On Beijing3^rd Ring Road

With the rapid development of urban economy, private cars increase rapidly and urban road congestion is getting worse. Congestion has brought many negative effects in many aspects. Therefore, how to alleviate urban traffic congestion has become an important issue in urban traffic. And ramp system, which is a typical traffic bottleneck, becomes the main cause of traffic congestion. Thus, ramp control has become one of the most effective methods to alleviate congestion in urban traffic. Based on the empirical traffic data of3rd ring road in Beijing, we have analyzed different mechanisms of traffic congestion by using the traffic flow forecasting and cell transmission models, then proposed different ramp control strategies.First of all, based on the short-term traffic forecasting model proposed by Wang et al., we have analyzed characteristics of the transition probabilities with different time intervals, and then put forward the two-step prediction method and verify its accuracy by using the measured data. The comparison between the predicted state and the critical state can determine the road congestions, the demand could be reduced to alleviate the congestion when the predictive state is in congestion; otherwise the demand can be properly increased to improve the efficiency of the system when the predictive state is uncongested.Secondly, the ramp systems on Beijing3rd ring road are described as double-cell ramp systems with a bottleneck. Then we have analyzed the influence of the initial state on convergence equilibrium under a stationary feasible demand and disclosed the congestion mechanisms of different systems. When the bottleneck in the upstream, the downstream always converges to the uncongested equilibrium, while the upstream bottleneck cell may be in congestion under certain initial states. Therefore the demand should be regulated from the upstream to the downstream. When the bottleneck in the downstream, the congestion starts in the downstream, and then propagates towards the upstream gradually. So the demand should be regulated from the downstream to the upstream.Finally, in previous studies, there is a limitation in the assumption of the off-ramp flow in the original CTM-Cell Transmission Model. In fact, linear relations between the upstream and downstream flows exist in the evening peak rather than morning. Then we proposed a new CTM model based on a flow differenceâ–³between on-and off-ramp flows. There is a critical valueâ–³under which the flow released from the upstream cell can just completely accepted by the downstream cell. If the measured valueâ–³is higher than the critical value, the demand needs to be decreased to relieve the congestion of the system. On the contrary, the demand does not need to be reduced.Simulation and measured results show that the different ramp control methods can effectively reduce the congestion of systems and improve the operation efficiency of the systems effectively.