A manager planning a distribution network faces a multiple-vehicle routing problem. The first step is to assign customers to vehicles. One way to do this is to partition a given territory into zones assigned to each vehicle. This paper presents a decision support system that enables the distribution manager to interactively design the zones at the screen using his own judgement as well as suggestions made by the system. The latter are based on analytic formulae that determine the optimal shape of a zone. This helps the planner design the zones while taking into account the speed of the vehicle, the density of the demand, the location of the zone, and a maximal duration or a maximum number of stops in a route, as well as his own knowledge of the territory.