The fare paid is a contribution to the operational costs of the transport system involved, either partial (as is frequently the case with publicly supported systems) or total. Many bus and rail systems in the United States recover only around one-third of their operational costs from fares (the farebox recovery ratio).The rules regarding how and when fares are to be paid and for how long they remain valid are many and varied. Rail and bus systems usually require the payment of fares on or before boarding. In the case of taxis and other vehicles for hire, payment is normally made at the end of the ride.Some systems allow transfers: that is to say that a single payment permits travel within a particular geographical zone or time period. Such an arrangement is helpful for people who need to transfer from one route to another in order to reach their destination. Sometimes transfers are valid in one direction only, requiring a new fare to be paid for the return trip.
