Abstract: Based on the theory of random effectiveness and urban traffic network, a random model used for computing average travel time in a road network is established. According to knowledge about fuzzy mathematics, an algorithm for the model and flow chart is put forward. The method put up in this paper can avoid wasting manpower, finncial resources and material resources which once took place in the past in order to find average travel time through investigation, and can provide basis for accurately computing the capacity of road network .