The concept of circular coloring is introduced and the number of optimized phase of traffic flow signal at an intersection in a city is boiled down to the circular chromatic number of the traffic flow model graphs.The traffic conditions on a few classes of special crossroads are analyzed and, from the cornfliction relations of traffic flow we can get the traffic flow model graphs and their circular chromatic numbers,namely,the optimization phase number of corresponding traffic signal.%通过引入图论中"圆染色"的概念,将城市路口交通信号灯最优相位个数归结为其交通流模型图的圆色数.以几种特殊的五交叉路口交通流状况为例,由车流的冲突关系给出交通流模型图,并求出这些图的圆色数.图的圆色数即为对应交通信号灯的最优相位个数.
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