Because transit agencies all run in the red and need revenue. You want a system that brings people onto the bus who would not have come on at full price — ideally enough of them to make up for the household pairs who already were riding together. (Though not quite since you have goals of reducing cars on the road here too.)

They use proof of payment here on the streetcars, but some evidence suggests there is massive cheating. Sometimes I’m the only one who pays it seems. Does everybody have a monthly pass?