Spacing Communal Letter Boxes Along a Road: The Interval That Balances Walking Against Collection Cost

Where door-to-door delivery is unaffordable, households collect mail from communal box clusters along the road, and someone must choose how far apart the clusters stand. This paper balances the two costs that spacing controls: each cluster carries a stated annual cost of USD 180 (bank, shelter and collection stop), while households walk an average of a quarter of the spacing and value that walking at USD 6 per household-kilometre per year in aggregate. At a density of 80 households per kilometre the balance falls at a spacing of 1.225 km, a mean walk of 0.306 km, and a combined cost of USD 293.94 per kilometre of road per year. Halving or doubling the spacing each raises cost by about a quarter (25.0 and 25.0 percent). The optimum follows a square-root rule in density, so a road four times as dense needs clusters only twice as close. SIMULATED, NOT MEASURED.