MULTI-OBJECTIVE TRANSIT FREQUENCY OPTIMIZATION: SOLUTION METHOD AND ITS APPLICATION TO A MEDIUM- SIZED CITY

Frequency setting takes place at the strategic and tactical planning stages of public transportation systems. The problem consists in determining the time interval between subsequent vehicles for a given set of lines, taking into account interests of users and operators. The result of this stage is considered as input at the operational level. In general, the problem faced by planners is how to distribute a given fleet of buses among a set of given lines. The corresponding decisions determine the frequency of each line, which impacts directly on the waiting time of the users and operator costs. In this work, the authors consider frequency setting as the problem of minimizing simultaneously users’ total travel time and fleet size, which represents the interest of operators. There is a trade-off between these two measures; therefore the authors face a multi-objective problem. The authors extend an existing single-objective formulation to account explicitly for this trade-off, and propose a Tabu Search solving method to handle efficiently this multi-objective variant of the problem. The proposed methodology is then applied to a real medium-sized problem instance, using data of Puerto-Montt, Chile. The authors consider two data sets corresponding to morning-peak and off-peak periods. The results obtained show that the proposed methodology is able to improve the current solution in terms of total travel time and fleet size. In addition, the proposed method is able to efficiently suggest (in computational terms) different trade-off solutions regarding the conflicting objectives of users and operators.