Proof-of-payment

Instead of checking each passenger as they enter a fare control zone, passengers are required to carry a paper ticket, transit pass, transit smartcard — or open payment methods such as contactless credit or debit cards (if applicable) — after swiping or tapping on smart card readers, to prove that they have paid the valid fare.

The honor system can be complemented with a more direct collection approach where this would be feasible—a transit authority using POP will usually post fare inspectors, sometimes armed as a police force, to man entrances to stations on a discretionary basis when a high volume of passengers is expected.

"[1] Disadvantages include higher rates of fare evasion, reduced security on station platforms when no barrier is used, increased potential of racial profiling and other unequal enforcement as "likely fare evaders" are targeted, and regularly exposing passengers to unpleasant confrontational situations when a rider without the proper proof is detained and removed from the vehicle.

Since then, many new light rail, streetcar, and bus rapid transit systems have adopted the procedure, mainly to speed up boarding by avoiding the hassles of crowding at doors to pay fares at a farebox beside the driver as is common practice on traditional buses.

TriMet in Portland, Oregon was the first large transit agency to adopt proof of payment on its bus system, from September 1982 to April 1984.