The purpose of spare capacity

A bus company may keep a few vehicles out of its daily timetable. At first glance, these unused buses appear wasteful: they cost money without carrying passengers. Their value becomes clearer when a scheduled vehicle develops a fault or an unexpected event creates additional demand.

[A] A reserve vehicle allows the company to respond without immediately cancelling a service. [B] Maintaining the reserve still has costs, including storage, inspections, and the possibility that a vehicle becomes outdated before it is used much. [C] The company must compare those costs with the consequences of being unable to respond to disruption. [D] Keeping more spare vehicles is therefore not automatically the best decision.

This reasoning applies to other organizations. A school may leave some timetable space available for rescheduled activities, and a warehouse may retain empty floor space for temporary deliveries. In each case, judging efficiency only by how completely a resource is used can miss its protective function. The appropriate amount of spare capacity depends on how likely disruptions are, how serious their consequences would be, and how quickly an alternative could be arranged. A resource can contribute value even when it is not in constant use.