Abstract: In order to guide traffic flow efectively，this paper firstly divided the process of identifying traffic signs and operating vehicle into seven po ints and six phases from the viewpoint of driving psychology．And by analyzing the existing setting conditions of traffic sign pointed，the basis of setting traffic sign is got．Then the process of changing lanes from the outer second one to the outer first one on urban expressway is an alyzed in detail．On the basis of it and by means of probability analysis，the theoretical model of operating distance when vehicles merging from the second lane to the first one is obtained，and then theoretical distance model of traliqc sign to the beginning po int of off-ramp is also acquired Lastly the distance oftraffic sign to the beginning po int of off-ramp belonging to diferent design ing velocity was calculated out．

Key words: urban expressway, trafi c sign, gap, headway, Erlang distribution, M3 distribution

摘要： 为了达到有效引导交通流的目的，首先从驾驶心理学的角度将驾驶员对交通标志的识别和对所驾车辆的操作过程分为7个点6个阶段进行研究，在对已有交通标志设置条件进行分析的基础上，得到了交通标志位置设置的依据，然后对驾驶员由城市快速路外侧2车道向外侧1车道的车道转变过程进行了微观分析，在此基础上利用概率论方法，得到了车辆由外侧2车道向外侧1车道汇合时操作行动距离 的理论模型，进而得到了城市快速路出口标志位置设置前置距离D的理论模型，最后通过实例计算得到了城市快速路不同设计车速下出口标志设置的前置距离．

关键词: 城市快速路, 交通标志, 间隙, 车头时距, Erlang分布, M3分布