https://phys.org/news/2018-12-california-mandates-percent-zero-emission-bus.html

While clean buses cost more than the diesel and natural gas vehicles they'll replace, advocates of the mandate say they have lower maintenance and fuel costs. Supporters hope creating demand for thousands of clean buses will bring down the price of those buses and eventually other heavy-duty vehicles like trucks.

California has 153 zero-emission buses on the road today with hundreds more on order. Most of them are electric, though technology also exists for buses powered by hydrogen fuel cells.