Research On Ramp Optimization Metering Strategy And Method In Urban Expressway

Traffic congestion has become an outstanding character of urban traffic problem. It is an emergent task to alleviate and prevent traffic congestion, because it relates to the continuous development of our country. As the artery of urban traffic, urban expressway system is one of the key decisive factors to reflect traffic efficiency, especially in Beijing. Therefore, the research on urban expressway congestion is pressing. In this dissertation, the control strategy and method with the measure of ramp metering are sought to deal with traffic congestion on the basis of the studies on traffic flow characteristics. It is expected that the findings will provide some references for traffic management and control in urban expressway system.At first it is introduced supporting projects and research backgrounds. The construction and planning status on Beijing urban expressway are analyzed, and the main current problems are presented. Research methods and achievements undertaken by overseas and domestic researchers are generalized. In light of above work, the research contents, significance and frameworks of the dissertation are proposed, which aims at the construction characteristics of Beijing urban expressway.In second part of the dissertation, the focus is on the analysis to ramp metering strategies in urban expressway system. Firstly, the control strategy on freeway system oversea is summarized simply. Secondly, three kinds of types, the on-ramp disturbance type, the off-ramp restriction type and the conflict type between the on-ramp and off-ramp, resulted in traffic bottleneck of Beijing urban expressway are described. In light of the bottleneck characteristic analysis above, the ramp metering strategies, i.e. the local ramp optimization control and the ramp coordination control on urban expressway system in our country have been put forward. For the local on-ramp and off-ramp, the various control modes are set according to the traffic state in mainline of urban expressway. For a pair of the on-ramp and off-ramp, the mainline capacity will reduce because of the weaving behavior between the traffic streams of entering and exiting resulted from the distance from one entrance ramp to the next exit ramp is too short. In order to alleviate the conflicts, the coordinated control strategy between the close on-ramp and off-ramp is researched. It is well known that solving a local congestion only, may have as consequence that the vehicles run faster into another (downstream), whereas still the same amount of vehicles have to pass the bottleneck, and so the average travel time on the network level will still be the same. During peak hours the density on the mainline can be so high that the queue on an on-ramp spills back to the surface streets of the city, whereas metering of upstream on-ramps could reduce the density of the mainstream flow and prevent spill back of the on-ramp queue. So, it is necessary to realize the coordinated control between the on-ramp and the signal intersection upstream surface street, because it can to prevent the queue on an on-ramp spill back to the surface streets. Another source of degrade network performance is that congestion may block traffic flows on routes that do not pass the bottleneck, such as an urban expressway with a congested off-ramp where the vehicles that want to leave the urban expressway block the mainstream traffic. Therefore, it is necessary to adjust the signal timing of intersection downstream at off-ramp. The strategy follows such a principle that the control to mainline surpasses over the one of the surface street, but the latter excels the ordinary urban street in control concept. With the exception of that, it is necessary to combine dynamic routing guide with control to put out real-time traffic information by Variable Message Signs. Finally, the construction of ramp control system is designed and the factors of partitioning control area are analysed.In the third part of the dissertation, research is made on the local ramp metering methods in urban expressway system. The characteristics of traffic flow parameters, i.e., the characteristics indicated by flow, speed and time occupancy during the bottleneck forming are analyzed. According to the analysis of time dependent characteristics of three traffic flow parameters, the control methods are discussed. Here, the local on-ramp metering method adopts both parameters of time occupancy and speed as control variable to realize feedback control. The on-ramp metering rate is determined by the comprehensive control effect from two parameters. In the model, the critical threshold is calibrated using linear decide function in pattern recognition and the method is simulated by VISSIM. The result of the simulation denotes that the method can improve average speed in mainline upstream at on-ramp about 8% and reduces the travel time in mainline section by 7.2% compared with un-signal control mode. The off-ramp control method doesn't regulate the off-ramp, but holds properly up vehicles of the surface street to ensure the vehicles'exitting from off-ramp by setting signal at the surface street. The method is simulated by VISSIM and the result shows that it can increase average speed in mainline about 13.58% and decreases the travel time in mainline section by 11.3% compared with un-signal control mode. Furthermore, the research finds further that the control effection is obvious during traffic rush hour, but isn't in smooth state. On the basis of the research above, the signal control method coordinated entrance ramp and its near exit ramp is proposed in order to reduce the conflicts and ensure the optimal traffic state on the mainline. The method is also simulated by VISSIM and the result shows that it can improve average speed in mainline and reduce the conflict at weaving area effectively.The forth part of the dissertation is the research on coordination ramp control method of urban expressway. At first, there is a simple discussion on the necessity brought coordination ramp control on the network level into effect. Then, the equations of the traffic dynamics on the mainline and the traffic dynamics on the surface street with model predictive control framework are described. The coordination control framework requires an objective function that expresses the whole performance of the traffic network. The objective function we use is the total time spent by the vehicles in the network to realize the systemic optimization. At the same time, the phase adjustment method between the off-ramp and the signal intersection downstream at surface street, and the coordination method between the on-ramp and the signal intersection upstream at surface street, are sought and explained by the examples of four-phase control schemes. Furthermore, a control method of dissipating congestion is discussed. The method with the castigatory factor of the queue at surface streets adopts the change rate of inbound car numbers in control area as an object to alleviate traffic congestion, and an example is presented to illustrate its feasibility.In the end, the research works are summarized. The findings and achievements are analyzed, and the innovations are concluded. Finally, future research works are brought forward.