We introduce the visibility graph as a new 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 realistic \travel by sight.” We compare distance estimations of the visibility graph and the aisle-centers method for traditional and fishbone warehouse layouts. Our results suggest that the aisle-centers method overestimates the length of a picking tour by 10-20%, depending on the number of lines picked and the size of the warehouse. The visibility graph method also has implications for warehouse design: in an experiment comparing three traditional layouts, the distance model using a visibility graph chose a different layout in 12.5% of the cases.