Although it is possible to construct optimal routing algorithms efficiently, simple heuristics such as the traversal and return policies are widely used in practice because they are easy to understand and the resulting routes are more consistent. For the traversal policy, the picker will cross through the whole aisle that contains at least one pick, and therefore always enters at one end of the aisle and exits at the other end. For the return policy, the picker always enters and exits at the same end of the aisle, and the same aisle may be entered twice from its two ends. Detaileddescription of these routing policies and their variations can be found in Hall (1993) and Caron et al. (1998). Performance evaluation of these different routing algorithms can be found in Hall (1993) and Caron et al. (1998, 2000) based
on analytical models, and Petersen (1997, 1999),Petersen and Schmenner (1999), and Petersen and Aase (2004) based on simulations.