Short explanation on routing strategies

The routing strategy only includes double sided picking strategies. Single sided picking is in general outperformed by double sided (Goetschalckx and Ratliff, 1998a). The routing strategy determines the sequence in which the pick locations are visited. The efficiency of a routing strategy depends on  the order profile, the storage strategy and the lay-out of the warehouse. It is possible to select more than one routing strategy. The routing strategies, used here are:

• the S-Shape heuristic, in which every aisle containing pick item is traversed entirely in the shape of an S. Compared to the other heuristics, the S-Shape heuristic (also called Traversal strategy) will lead to short routes if the number of items per aisle is high. This heuristic is easy to use; when an aisle contains a pick item, the aisle is traversed completely, otherwise this aisle is skipped. This probably is the reason why this heuristic is used so frequently in practice.
• the Largest Gap heuristic, in which a picker enters an aisle as far as the largest gap. A gap is defined as the distance between two adjacent picks or between the first pick and the front aisle, or between the last pick and the back aisle. Unlike the S-Shape heuristic, the Largest Gap heuristic will lead to a short travel time if the number of items per aisle is low. The largest gap is the part of the aisle that is not visited by the order picker.
• The combined  heuristic is a combination of the Largest Gap and the S-Shape heuristic, which means that an aisle either is traversed totally or entered and left from the same side of the aisle. The best option (= the shortest route) from these two opportunities is chosen, after which the next aisle is entered. These procedures are repeated until the last item is picked and the best route from the two options is chosen.
• The optimal method can calculate the shortest route, regardless of layout or location of the items. Optimal routes will typically look like a mixture of S-Shape and Largest Gap. On this site it will only be possible to calculate optimal routes for warehouses with 0 or 1 cross aisles. This method is based on the algorithm of Ratliff and Rosenthal (1983). Also Roodbergen en De Koster (1998).