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Every delivery operation lives or dies by one ratio: demand against fleet capacity. Queueing theory says the relationship is brutally nonlinear — at 70% utilization customers barely wait, yet as demand approaches capacity the backlog and waiting times blow up, not gradually but like a wall. Logistics planners call the safety margin between the two 'slack', and this model shows why they pay for it.
Houses scattered around a central depot raise orders at random; idle couriers claim the nearest open order, drive out, hand it over, and return for the next. Watch the backlog monitor: with a quiet order stream the fleet idles green at the depot, near capacity the waiting-order count wanders upward in long excursions, and past capacity it grows without bound while average wait climbs a straight line. Adding a few couriers — or shaving demand slightly — snaps the system back.