California is taking the lead in the US in cutting greenhouse gas emissions by mandating public transit agencies to transition to 100 percent zero-emission bus fleets by 2040.

The California Air Resources Board (CARB), the state’s clean air agency, on Friday announced a phased-in schedule from 2023 to 2029, by which year agencies will no longer be allowed to buy buses that run on diesel or gasoline.

The transition will mean a reduction in greenhouse gas emissions by 19 million metric tons from 2020 to 2050 — the equivalent of taking 4 million cars off the road, according to the CARB.

Despite opposition from the natural gas industry and concerns of the performance of electric buses, Stanley Young, CARB spokesman, told China Daily that they didn’t anticipate challenges.

“The regulation was adopted by the California Air Resources Board, after extensive consultation by staff with the transit agencies of the state,” he said on Monday.

California has 200 public transit agencies which operate about 12,000 buses statewide. CARB expects rapid deployment of electric buses in the coming years — from the current 153 electric buses to 1,000 by 2020.

Eight of the 10 largest transit agencies in the state are already operating zero-emission buses, including battery electric and hydrogen fuel cell vehicles, according to the agency.

Some transit agencies have made their own schedules, even ahead of the state’s requirement.

Last year, the Los Angeles Metropolitan Transportation Authority voted to shift its entire 2,200-bus fleet to zero-emission by 2030. San Francisco Municipal Transportation Agency announced in May to only purchase all-electric buses starting in 2025 in order to have an all-electric bus fleet by 2035.

“The transit agencies themselves will make the decision about which company’s buses to purchase or lease,” said Young.

Several incentive programs are available to help them manage the cost. “The upfront incremental cost to transition the state’s fleet to battery electric buses is estimated to be about $1 billion, but we also estimate that, as a result of savings in fuel and maintenance, there will be $1.5 billion in savings over the lives of the buses,” said Young.

California also is home to some key players in the electric bus industry, such as Chinese automaker BYD and Proterra, a startup that recently secured a $155 million investment.

BYD, which has a factory in Lancaster, California, has been aggressively marketing its electric buses. In California, the company has delivered 79 electric buses to transit authorities, with another 122 on the way. It currently has 19 public transit customers in the state.

“Our Lancaster facility has the capacity to produce 1,500 buses and we have partnered with Generate Capital to make a leasing program available to accelerate this transition,” said Stella Li, president of BYD Motors, in a statement.

But experts said the technology still need to evolve, as some BYD buses were reported to have broken down on the road in Los Angeles. San Francisco’s authority also said it needs more proof from manufacturers that their electric buses can withstand heavy ridership and steep hills.

“The ability of a bus to climb hills is a motor issue and a measure of its torque and horsepower,” said BYD spokesperson Sheila Given. She declined further comment, saying BYD was in the bidding process.