https://arstechnica.com/cars/2018/12/california-transit-agencies-have-21-years-to-build-zero-emissions-bus-fleets/

On Friday, California's Air Resources Board (CARB) unanimously approved a regulation that would compel the state's public transit agencies to build zero-emissions fleets by 2040. According to the San Francisco Chronicle, the regulation would also prohibit transit agencies from investing in diesel- or gas-powered buses after 2029. Buses usually last about 12 years before they need to be replaced, the Chronicle noted.