Through the experience of urban planning in the first half of the twentieth century, transportation planning predominantly involved developing highways and facilitating the transport of private vehicles without addressing the improvement of public transportation (i.e transit) systems. Accordingly, the promoted trend towards private vehicles led to quite low levels of traveling demand for public transport modes. It was soon noticed by urban authorities that the use of private vehicles renders the traffic congestion out of control, and in light of this, public transport was raised as an essential option in moving towards urban sustainable development. As an integral part of sustainable urban development, transit planning deals with a broad spectrum of strategic, tactical and operational problems, involving long-term decisions to short-term ones. Transit Routes Network Design (TRND) problem, as a basic strategic decision making problem, aims at finding the best configuration of public transportation routes, while keeping the budget constraint into consideration. This problem is categorized as the first step and the most important one in the process of urban transit planning and affects all subsequent planning problems e.g. frequency setting, designing exclusive lines, vehicle scheduling, etc. As a result, it is of great importance to study TRND problem to establish the best configuration of network routes. To define the “best” configuration in TRND problem, various mathematical objective functions have been defined and taken into account, including minimization of users’ average travel-time, number of transit routes, number of transfers between the routes, the amount of unsatisfied demand, and also maximization of routes directness, number of passengers using transit system, etc. In an ideal condition, it is best to address all the objectives simultaneously. However, the contribution of several objectives in a real-world problem brings about an intractable complexity.
