Even when the warehouse had wide aisles, it is evident from Table 5 that the cost
savings were substantial with WITP compared to the sequential ITP+WP. However, these
savings reduced compared to the narrow aisle case. This is because although the ITP
solution remained unchanged in both wide and narrow aisle cases (e.g., column
“Shipments” in Tables 3.4 and 3.5 are identical), blocking in wide aisles was
substantially lower. So when the WP was optimized, the number of workers required in
wide aisles was lower compared to narrow aisles for the same ITP solution; e.g., for DS
#3 in Tables 3.4 and 3.5, the total pickers was 27 (22 permanent and 5 temporary) for
narrow aisles and 15 (12 permanent and 3 temporary) for wide aisles. Consequently, the
total cost of plans with ITP+WP for the wide aisle case was lower than for the narrow
aisle case ($42,855 vs. $49,015). The corresponding WITP plans for both narrow and
wide also exhibited a reduction in the number of pickers; i.e., 13 (narrow) and 9 (wide). Given that warehouse workforce cost now has a relatively lower contribution in the objective function of WITP for wide aisles (compared to narrow aisles), the corresponding optimal distribution plans were largely driven by transportation and inventory cost reductions. The resulting effect was that the optimal plans for the warehouse with wide aisles may sometimes have a higher workload variation compared to narrow aisles, but still much lower than the ITP+WP plans (see Figure 3.6).