A sign reading "Fare Required" has been appearing on buses. This message is being displayed to address an issue with fare evasion. The sign is quite straightforward, as public transit is not free.

Fare evasion is a problem that the San Francisco Municipal Transportation Agency is dealing with. The agency is taking steps to remind passengers that they need to pay to ride.

Further details about the sign and the issue of fare evasion are available from the source, which provides more information about the situation and the actions being taken by the transportation agency.