We propose a formulation for the pickup and delivery problem with time windows, based on a novel modeling strategy that allows the assignment of vehicles to routes explicitly in two-index flow formulations. It leads to an effective compact formulation that can benefit OR practitioners interested in solving the problem by general-purpose optimization software. Computational experiments indicate that the proposed formulation has interesting features and best overall performance in relation to other compact formulations.
Furtado, M.G.S.; Munari, P.; Morabito, R. Pickup and delivery problem with time windows: a new compact two-index formulation. Operations Research Letters, v. 45 (4), p. 334-341, 2017. (http://dx.doi.org/10.1016/j.orl.2017.04.013)