Fare collection is based on a proof-of-payment system. The San Diego Trolley is a barrier-free (no turnstiles), self-service system. Patrons must have proof of fare (ticket or pass) before boarding. Self-serve ticket vending machines are located at each station. Compass Card (see below) users must "tap" at a Compass Card validator (located at various points around each Trolley station, and on MTS buses) or on the Compass Card target on a ticket vending machine before boarding a trolley. Random fare inspections are performed by roving transit enforcement personnel. Based on frequent security inspections, nearly 98% of the 37 million patrons have proper fares.