Any new public transit bus purchased in California by 2029 must be a 100 percent electric vehicle, according to a new unanimous vote by the California Air Resources Board, the state’s clean air agency. It is the first statewide policy in the United States to require an entire vehicle class go electric, the Union of Concerned Scientists wrote.

Within five years, 25 percent of transit agencies’ new bus purchases must be electric vehicles. That jumps to 50 percent by 2026, and 100 percent by 2029. This equates to about 14,000 zero-emissions buses on California roads by 2040 — up from 132 today —and will reduce the state’s carbon emissions by 1 million metric tons per year in just two decades. School buses and privately owned buses are not covered under the policy.

In addition to the policy’s climate benefits, another major motivator was that the shift to electric vehicles will help further combat California’s air pollution; the state is home to eight of the top 10 most polluted cities in the U.S., including Los Angeles and Bakersfield.

“This is the biggest public transportation breakthrough since we switched from trolleys to diesel buses a century ago,” Jimmy O’Dea, a senior vehicles analyst at the Union of Concerned Scientists, said in a statement. “Bus riders, bus drivers, and anyone who has gulped the exhaust from a passing truck or bus knows we must do something about these vehicles. Electrifying them is a one-two punch: we reduce carbon emissions that worsen climate change and we clean up the air we breathe.”

Green energy experts said the impact of California’s new policy will ripple across the transportation industry. The state is the largest U.S. bus market in the U.S. In addition, four of the five major U.S. bus manufacturers are headquartered in the state.

The new ruling “sends a clear market signal that the fifth largest economy in the world is serious about zero-emissions transit buses,” Adrian Martinez, a staff attorney at the nonprofit Earthjustice, told Fast Company. “Even though you’ve had a lot of transit agencies step up to the plate to commit to 100 percent, getting the stamp of approval from a state as big and as powerful as California is important to send a signal nationally.”