Mail sorting optimization: a case study of the French postal company La Poste

This paper addresses a mail sorting optimization problem encountered by a major French mail delivery provider La Poste. To simplify the operators’ handling during the sorting process, it is necessary to balance the mail flow between the outputs of each sorting machine. This problem can be considered as a variant of the well-known assembly line balancing problem (ALBP) with a particular structure of precedence constraints. At first, to handle efficiently small- and medium-size academic instances of this problem, we propose a mixed-integer linear programming (MILP) model aiming to minimize the difference between the most and least loaded outputs. Finally, to deal successfully with real industrial large-size instances, we develop an efficient heuristic, inspired by simulated annealing, which focuses on minimizing the nonlinear standard deviation between the output loads. Both approaches have demonstrated a very good overall performance for the respective instance categories. Computational results are reported as well.