ABSTRACT
The previous chapters have so far discussed a variety of planning problems arising in the design of a freight distribution network, separately at strategic, tactical and operational levels of planning. Each problem was modelled and solved in isolation within the planning level in which it arises, with no integration considerations with problems arising in other levels. However, there is an inherent interdependency between the decisions made at different levels of planning. To address this interdependency, planners sometimes take a hierarchical approach, whereby strategic problems are solved initially, the outputs of which are then used as inputs to the tactical and then to operational problems. For example, the vehicle routing problem (VRP) introduced in Chapter 4, which is at an operational level of planning, assumed that the strategic decision concerning the location of the depot(s) was already made. Such an approach, however, is likely to result in suboptimal solutions for the system as a whole, as the decisions made at one level will be highly dependent on the quality of the decisions made at other levels. It is for this reason that some integration between problems of different levels would be necessary.
