A novel approach for modeling order picking paths

We introduce the visibility graph as an alternative way to estimate the length of a route traveled by order pickers in a warehouse. Heretofore it has been assumed that workers travel along a network of travel paths corresponding to centers of aisles, including along the right angles formed where picking aisles join cross aisles. A visibility graph forms travel paths that correspond to more direct and, we believe, more appropriate “travel by sight.” We compare distance estimations of the visibility graph and the aisle-centers method analytically for a common traditional warehouse design. We conduct a range of computational experiments for both traditional and fishbone warehouse layouts to assess the impact of this change in distance metric. Distance estimations using aisle-centers calculates a length of a picking tour on average 10–20% longer compared to distance estimations based on the visibility graph. The visibility graph metric also has implications for warehouse design: when comparing three traditional layouts, the distance model using a visibility graph resulted in choosing a different best layout in 13.3% of the cases.