The development of public transportation and the promotion of non-motorized methods (e.g. biking and walking modes) are important factors in moving cities towards sustainable transportation. Despite all positive impacts that public transportation has on various dimensions of transportation sustainability, some of its drawbacks are fuel and energy consumption, pollution emission, the initial investment and the maintenance costs. Thus, the need to express the development of public transportation in a quantitative sense, while meeting the travel demand also maximizes the positive effects of this mode of transportation on various aspects of transportation sustainability and minimizes its negative effects. The goal of this research is to estimate the optimum public transit share according to the socio-economic characteristics to improve transportation sustainability indices. For this purpose, cities with the highest transportation sustainability index were identified and then the relationship between the socio-economic characteristics and the share of their public transportation was investigated using the regression analysis. Then, the developed model was implemented to find the optimum public transit share in some Iranian cities based on their socio-economic characteristics. The results indicated that the optimal shares estimated in those cities using the developed model were far away from what was planned in their transportation master plans.