The California Air Resources Board (CARB) has unanimously approved the Innovative Clean Transit measure, setting a firm timetable for all public transit agencies in the state to convert bus fleets to zero-emission vehicles by 2040.

While many cities across the state have been implementing zero-emission policies already, the ICT measure will give municipalities still struggling along a clear timeframe by which to replace their fleets.

Cities such as Los Angeles and San Francisco have come out in support of the transition, with many calling it uniquely California idea.

“Transitioning to 100 percent zero-emission buses across California is good for our cities, good for our state, and good for the planet,” San Francisco Mayor London Breed said earlier this year. “California has always been at the forefront of environmental innovation.”

Don’t expect to see new buses overnight, though. According to the ICT documentation, transitions will be slow and steady across the state, and build on pre-existing plans. Gov. Jerry Brown in 2016 mandated 1.5 million zero-emission vehicles by 2025, and an executive order signed in January calls for 5 million zero-emission vehicles in California by 2030.

This article has been updated.