The objective of routing policies is to sequence the items on the pick list to ensure a good route through the warehouse. The problem of routing order pickers in a warehouse is actually a special case of the Travelling Salesman Problem, see also Lawler et al. (1985). The travelling salesman problem owes its name to the problem described by the following situation. A salesman, starting in his home city, has to visit a number of cities exactly once and return home. He knows the distance between each pair of cities and wants to determine the order in which he has to visit the cities such that the total travelled distance is as small as possible. Clearly, the situation of the travelling salesman has many similarities with that of an order picker in a warehouse. The order picker starts at the depot (home city), where he receives a pick list, has to visit all pick locations (cities) and finally has to return to the depot. An example layout of a warehouse with pick and a corresponding graph representation is given in Figure 7.
Depot
Figure 7 Illustration of an order picking situation (left) and its graph representation (right)