U-turn movement is a common form of traffic organization in urban streets.A traffic flow model for U-turn movement based on the NaSch cellular automaton model is proposed to simulate and analyze the dynamics of U-turn movement.The results indicated that the U-turn movement has significant effect on the traffic flow, and it is a typical bottleneck.The U-turn movement can cause phase transition at small probability of vehicle generation and the critical flow also decreases obviously.The system with two U-turn movement is different from the system with one U-turn movement in traffic features such as phase transition and critical flux.The one U-turn system has a good performance.It is suggested that the system with a couple of U-turns may help mitigate the negative effect of U-turn movement in traffic planning and management.%以NaSch模型为基础建立设置U形转向的路段交通流模型,对其特性进行模拟分析.结果表明,U形转向路口对交通流影响显著,会导致在较小进车概率时发生自由流状态到拥挤状态的相变,相应的临界流量也随之降低.设置双U形转向系统的相变、临界流量等交通流特性与单转向系统具有明显差异,其系统效率较高.在实际交通规划与管理时应避免设置单转向系统,以减小U形转向的负面影响.
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