At exit, to driver or conductor

In this practice, passengers either board at the rear of the vehicle and pay [usually] the driver on alighting at the front, or otherwise they enter at the front of the vehicle and pay the conductor on alighting at the rear. In both examples, the objective is to discipline the flow of passengers through the vehicle as well as establishing a consistent approach to the sequence and location of payment.

The advantage is that dwell time is improved to the extent that entrance and exit movements are segregated, although delays may occur at busy times where many passengers are alighting at the same stop and this can extend the dwell  time to process the payment of the fares.

However this payment practice is only effective where there is a flat fare system, as there is no evidence of the point of boarding and little visibility of that action by the fare collector.