Many transportation providers such as package delivery companies and postal service organizations face the problem of designing a transportation network in order to service their customers. This network must balance the requirement of on-time delivery under tight time window constraints with the goal of low-cost operations of the fleet. Until recently this task was usually performed by planners without sufficient software aid. This paper describes a decision support system (DSS) which has been designed in order to assist planners of the German postal service, the Deutsche Post AG. It helps the planners in designing improved plans which can be generated either manually or with the help of sophisticated intelligent optimization techniques. Both optimization and system design factors which influenced the design of the DSS are discussed.