We study the potential benefit assessment and identification of the best alternative of an urban mass transportation project. The vehicle miles produced by each of the individual alternatives are given as constants. The basic input data are the capital costs and the user savings. These numbers are often impossible to be estimated with enough precision. Also there is uncertainty in the estimation of the future benefits. Fuzzy set theory is a convenient mathematical device with which we are able to solve problems that involve uncertainty, ambiguity and indetermination. We represent these input data as fuzzy numbers. Triangular Fuzzy Number (TFN) is an easy way of expressing the input data. On the basis of the economic criteria we identify the best alternative from the examined ones as that which yield the maximum difference between the total benefits and the costs; or alternatively, the system for which the marginal benefits and the marginal costs are equal. By varying the width of the triangular fuzzy numbers that represent the user savings and the capital costs, both individually and simultaneously, we obtain the best alternatives for every system of user savings and capital costs.