Other routing issues
All articles discussed so far assume that the aisles of the warehouse are narrow enough to allow the order
picker to retrieve products from both sides of the aisle without changing position. In Goetschalckx and Ratliff
(1988b) a polynomial-time optimal algorithm is developed that solves the problem of routing order pickers in
wide aisles. Another problem with routing may arise if products are stored at multiple locations in a
warehouse. In this case a choice has to be made from which location the products have to be retrieved. A
model for the problem of simultaneous assignment of products to locations and routing of order pickers is
given in Daniels et al. (1998). Furthermore, heuristics are given to solve the problem. A further routing
problem is that of allowing the order picker to do multiple picks per stop. That is, the order picker travels
through a warehouse with a vehicle. He stops the vehicle and walks back and forth to a number of pick
locations to retrieve products. Then he continues to the next stop location, and so on. The trade-off is between
the time to start and stop the vehicle and the distance walked by the order picker. This problem was analysed
and solved optimally in Goetschalckx and Ratliff (1988a).
Analytical estimation of routing time
Part of the research on routing consists of travel time estimation. Using techniques from statistics and
operations research an attempt is made to give an estimate of how much time (or distance) it takes to collect
an order. Many results are known for systems where the vehicle is confined to a single aisle, see for example
Bozer and White (1984). For travel time estimates of single and dual command cycles in multiple aisle
systems see e.g. Bassan et al. (1980), Francis (1967), Larson et al. (1997) and Pandit en Palekar (1993). A
recent interesting addition is Bozer and Cho (2005) where analytical expressions are presented for an AS/RS
under stochastic demand.
Few researchers have looked for travel time estimates for picking in systems with multiple aisles and multiple
picks per route. Kunder and Gudehus (1975) give travel time estimations for three routing heuristics in a
warehouse consisting of one block. This work is extended in Hall (1993) with more advanced routing
heuristics for one block warehouses. Furthermore, a lower bound on travel time for the optimal algorithm