The MTA is asking bus drivers not to open the rear doors when passengers are boarding.

The new protocol, first reported by the New York Post, is part of a recent effort to crack down on what the MTA believes is widespread fare evasion on city buses. In April, MTA chair Janno Lieber announced the creation of a blue ribbon panel to examine ways to reduce fare and toll evasion, which he said is now causing a $500 million shortfall.

“The policy is intended to continue to offer customers easy exits while deterring fare evasion by reducing the time that rear doors are open while no one is exiting on local bus routes,” MTA spokesperson Aaron Donovan wrote in a statement on the rear-door orders. “All-door boarding on regular bus service is being implemented in conjunction with OMNY, where fares will be accepted at all doors.”

In a memo sent to bus operators, the MTA notes the center and rear doors on buses should no longer automatically open when the bus stops, unless there’s an emergency. And the rear door should only open when riders need to exit.

The one exception is on Select Bus Service buses, where the MTA says it encourages rear door boarding because riders pay at a machine at the bus stop, not exclusively at the front of the bus. Those buses also have OMNY installed at all doors, which allows riders to pay at any bus door entrance as well. The MTA has installed electronic OMNY fare readers at the back of all local buses, but it hasn’t activated them. An agency spokesperson said the MTA hasn’t decided when to begin using the rear door devices, but it is available at the front of all buses now.

The MTA said fare evasion across its system, including subways, buses, bridges, tunnels, as well as the commuter railroad, costs the agency $500 million a year. The agency notes that its current estimates are that it loses $245 million annually to fare evasion on subways, and $205 million on buses. It estimates that there are $25 million in fares not collected on the commuter railroad and $50 million lost to fare evasion on the bridges and tunnels.

Lieber said last month that fare evasion is about more than just money.

“Pervasive fare evasion is a threat to the spirit that makes New York not just a great city, but a great community,” he said, referring to buses and trains as a “sacred space” for New Yorkers.

This story has been updated with additional information.