Proof-of-payment or POP is an honor-based fare collection approach used on many public transportation systems. Instead of checking each passenger as they enter a fare control zone, proof-of-payment requires that each passenger carry a ticket or pass proving that they have paid the fare. Ticket inspectors such as conductors or fare enforcement officers periodically (and often randomly) check to ensure that passengers have paid their fare(s) and are not committing fare evasion. On many systems, a passenger can purchase a single-use ticket or multi-use pass at any time in advance, but must insert the ticket or pass into a validation machine before use. Validation machines in stations or on board vehicles time stamp the ticket. The ticket is then valid for some period of time after the stamped time.
The proof-of-payment method is implemented when the transit authority believes it will lose less money to the resultant fare evasion than it would cost to install and maintain a more direct collection method. It may be used in systems whose passenger volume and density are not very high most of the time—as passenger volumes increase, more-direct collection methods become more profitable. However, in some countries it is common even on systems with very high passenger volume. Proof-of-payment is usually applied on one-man operated rail and road vehicles as well as on automatically operated rail lines.
POP can be complemented with a more direct collection approach where this would be feasible—a transit authority utilizing 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. For example, transit users leaving a stadium immediately following a major concert or sporting event will likely have to buy a ticket from an attendant (or show proof of payment) to gain access to the station(s) servicing the stadium. Direct fare collection methods may also be used at major hubs in systems that otherwise use POP. An example of this is the Tower City–Public Square station on Cleveland's RTA Rapid Transit Red Line, which uses faregates.
Travel without a valid ticket is not usually considered a crime, but a penalty fare is charged.
Advantages and disadvantages
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Advantages of proof-of-payment include lower labor costs for fare collection, simpler station design, easier access for mobility-impaired passengers, easier access for those carrying packages or in case of an emergency, and a more open feel for passengers. On buses, proof-of-payment saves drivers the time needed to collect fares, and makes it possible for all doors to be used for boarding. Validated tickets can double as transfers between lines.
Disadvantages include potentially higher rate 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. Visitors unfamiliar with a system's validation requirements who innocently misunderstand the rules are especially likely to get into trouble.
Proof-of-payment is popular in Germany, where it was widely introduced during the labor shortages resulting from the Economic Miracle of the 1960s. It has also been adopted in Eastern Europe and Canada and has made some inroads in newer systems in the United States. The first use of the term "POP" or "Proof of Payment" on a rail line in North America is believed to have been in Edmonton in 1980. Since then, most new light rail, streetcar, and bus rapid transit systems have adopted the procedure, mainly to avoid the hassles of crowding at doors to pay fares at a farebox beside the driver as is common practice on traditional buses - Toronto being a prominent example of this.
Systems using proof-of-payment
|TFL||London, England, Britain||Bus and Tram||Open boarding provided on routes that are served by the New Routemaster and Routes 507 and 521 which allows passengers to board by the middle doors. Oyster or Contactless payment card users must touch in as they board.|
|Ruter||Norway, Oslo||T-banen Metro, Rail, Tram, Bus, Ferry||some underground stations have inactive turnstiles for possible future use or removal|
|Muni||United States, California, San Francisco||1993–2000 light rail phased introduction, 2005 bus ||Light Rail, Bus||Muni Metro has faregates in the Market Street Subway but otherwise operates on a POP basis for all light rail, historic streetcar, and bus lines. The San Francisco cable car system uses manual fare collection.|
|ZTM||Poland, Warsaw||Rail, Tram, and Bus||Warsaw Metro has faregates since 1996 |
|Dubai Metro||Dubai, UAE||Metro live since 9-9-2009||Metro train, Bus||Travel Card in Dubai is called NOL card|
- All German S-Bahn, U-Bahn and Stadtbahn (light rail) systems, as well as most German tramway systems
- Majority of public transport systems in Central Europe and Eastern Europe
- Altamont Commuter Express
- Baltimore Light Rail
- Buffalo Metro Rail
- Calgary C-Train
- Charlotte, North Carolina LYNX
- Greater Cleveland Regional Transit Authority Red Line Rapid Transit
- Copenhagen Metro, Copenhagen S-Train, The Coastal Railway
- Regionstog, Lokalbanen and Arriva
- Dallas Area Rapid Transit's Light Rail system and Trinity Railway Express
- Denver, Colorado Regional Transportation District
- Edmonton Transit System
- GO Transit train lines
- Helsinki commuter rail
- Helsinki Public Transit
- MTR Light Rail in Hong Kong
- METRORail of the Metropolitan Transit Authority of Harris County, Texas in Houston, Texas
- Line 1 (ISAP) of the Attiko Metro in Athens, Greece
- Los Angeles County Metro Rail
- Docklands Light Railway, London, United Kingdom
- Luas, Dublin, Ireland
- NJ Transit light rail lines: Newark Light Rail, Hudson-Bergen Light Rail, and River Line
- Metrolink in Southern California
- Minneapolis-Saint Paul's METRO Light Rail and BRT
- Montreal's Agence métropolitaine de transport commuter trains
- Mumbai Suburban Railway
- Norfolk, Virginia's Tide Light Rail of Hampton Roads Transit.
- Ottawa, Ontario O-Train
- Phoenix, Arizona Metro Light Rail
- Portland, Oregon TriMet
- Sacramento, California RT Light Rail
- San Diego Trolley
- San Francisco Bay Area Caltrain
- San Francisco Municipal Railway
- Santa Clara Valley Transportation Authority (VTA)
- Seattle, Washington Sounder Commuter Rail and Central Link light rail (Sound Transit)
- St. Louis MetroLink
- Toronto Transit Commission: 501 Queen, 504 King, 509 Harbourfront and 510 Spadina streetcars (and will be extended to all streetcar lines as more new vehicles arrive)
- Translink (Vancouver) - According to Translink, turnstiles will begin to operate on late 2013.
- Tyne and Wear Metro
- Utah Transit Authority TRAX
- Vienna U-Bahn and Trams
- Virginia Railway Express
- Zagreb ZET
- Transport for London in London, England
- sbX in San Bernardino, California.
- Emerald Express bus rapid transit in Eugene, Oregon
- Greater Cleveland Regional Transit Authority HealthLine
- Coast Mountain Bus Company (Vancouver) and Translink (Vancouver)
- Copenhagen buses
- Innsbruck Public Transport
- LACMTA Orange Line
- MTA New York City Bus Select Bus Service
- VIVA - York Region's Bus Rapid Transit in the Regional Municipality of York in Ontario, Canada
- Ottawa, Ontario articulated buses
- Paris Bus (RATP)
- Pioneer Valley Transit Authority Five Colleges (UMass Transit) fixed routes
- San Francisco Municipal Railway (all bus routes) 
- Tallinn Public Transport
- RTA buses, Dubai
- SFMTA website , accessed July 6, 2011
- SFMTA website , accessed July 6, 2011
- SFMTA website , accessed July 6, 2011