Traffic congestion has long been a serious problem for urban residents. The development of cutting-edge technologies such as automatic driving, Internet of Vehicles and cloud control not only has a profound impact on the means of transportation for travelling and logistics, but also creates opportunities for fundamental changes in the management and control methods of the transportation system. The highly controllable paths and trajectories of Automated Vehicles (AVs) make it possible to greatly improve the overall operation efficiency, reduce the energy conservation and vehicle emission of transportation system, which will create huge economic and social value. The "Rhythmic Control (RC)" scheme developed by THU-LEAD team is a new traffic control mode for the era of automatic driving. Starting from the basic concept of conflict points, it releases the huge potential of intersections and road networks to handle traffic demands. It can give full play to the advantages of automatic driving and Internet of Vehicles technology, and provide a safer, more convenient and more imaginative blueprint for urban traffic life in the future.
RC is a cooperative outcome of THE-LEAD with Prof. Yin Yafeng (associate dean of the Department of Civil Engineering, University of Michigan) and Prof. Shen Zuojun (vice president of University of Hong Kong). In recent days, two papers of RC have been accepted by Transportation Science. These two papers explain the modeling techniques and application methods of RC scheme in isolated intersections and road networks. By designing repetitive and orderly space-time rhythms in the road network, and embedding vehicle trajectories into these rhythms, the conflict-free and safe operation of the whole network traffic can be achieved without signal lights. Theoretical derivation and a large number of simulation experiments show that RC has excellent performance; in addition to its simple and clear concept, RC does not need complex real-time calculation, and can be implemented on large-scale road network. In low demand scenarios, the average vehicle delay under RC scheme is almost zero. In high demand scenarios, RC can effectively avoid congestion and gridlock. Furthermore, RC scheme can be organically combined with online travel reservation system, which makes it possible to build an integrated platform of future travelling. It is expected that the problem of traffic congestion in big cities, which has plagued mankind for decades, will be eventually eliminated.
A brief introduction of two papers:
• Rhythmic Control of Automated Traffic-Part I: Concept and Properties at Isolated Intersections
Leveraging the accuracy and consistency of vehicle motion control enabled by the connected and automated vehicle technology, we propose the rhythmic control (RC) scheme that allows vehicles to pass through an intersection in a conflict-free manner with a preset rhythm. The rhythm enables vehicles to proceed at a constant speed without any stop. The RC is capable of breaking the limitation that right of way can only be allocated to non-conflicting movements at a time. It significantly improves the performance of intersection control for automated traffic. Moreover, the RC with a predetermined rhythm does not require intensive computational efforts to dynamically control vehicles, which may possibly lead to frequent accelerations or decelerations. Assuming stationary vehicle arrivals, we conduct theoretical investigation to show that RC can considerably increase intersection capacity and reduce vehicle delay. Finally, the performance of RC is tested in the simulations with both stationary and non-stationary vehicle arrivals at both symmetric and asymmetric intersections.
• Rhythmic Control of Automated Traffic Part II: Grid Network Rhythm and Online Routing
Connected and automated vehicle (CAV) technology is providing urban transportation managers tremendous opportunities for better operation of urban mobility systems. However, there are significant challenges in real-time implementation, as the computational time of the corresponding operations optimization model increases exponentially with increasing vehicle numbers. Following the companion paper (Chen et al., 2020) which proposes a novel automated traffic control scheme for isolated intersections, this study proposes a network-level real-time traffic control framework for CAVs on grid networks. The proposed framework integrates a rhythmic control (RC) method with an online routing algorithm to realize collision-free control of all CAVs on a network and achieve superior performance in average vehicle delay, network traffic throughput, and computational scalability. Specifically, we construct a preset network rhythm that all CAVs can follow to move on the network and avoid collisions at all intersections. Based on the network rhythm, we then formulate online routing for the CAVs as a mixed integer linear program, which optimizes the entry times of CAVs at all entrances of the network and their time-space routings in real time. We provide a sufficient condition that the linear-programming relaxation of the online routing model yields an optimal integer solution. Extensive numerical tests are conducted to show the performance of the proposed operations management framework under various scenarios. It is illustrated that the framework is capable of achieving negligible delays and increased network throughput. Furthermore, the computational time results are also promising. The CPU time for solving a collision-free control optimization problem with 2,000 vehicles is only 0.3 s on an ordinary personal computer.