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Rear door boarding on LA MTA buses was initiated in March 2020 in an effort to reduce contact between drivers and riders amidst the COVID-19 pandemic, and remains in effect as of August 2021.
LA Metro has not technically waived fares; officially, the policy is
While you are not expected to use the fare box or TAP validator at the front of the bus, please be in possession of fare during travel.
Fare enforcement is suspended, however, so for all practical purposes, bus rides are currently free.
As with a number of other U.S. public transit agencies, fare-free rides may be permanently instituted for low-income riders after the pandemic; a pilot program is underway in Los Angeles County.
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As Doc notes in a comment, this answer applies only to buses operated by the MTA. Other agencies like BCT, Big Blue Bus, Foothill Transit, or OCTA will have their own policies.