A federal grant program aimed at speeding America’s transition to zero-emissions transit is being undermined by a requirement that one-quarter of the spending be allocated to low-polluting vehicles — leading to shortages in grants to agencies seeking to bolster their no-pollution fleet.

After passage of the 2021 infrastructure bill, the Federal Transit Administration awarded grants to support clean or cleaner bus fleets and facilities in rural, suburban, and hyper-dense areas across the country. By law, 25 percent of the five-year $5.5-billion Low or No Emission Vehicle (a.k.a. “Low No”) program must go to low-emission vehicles such as diesel hybrids or buses powered by compressed natural gas — and every agency that sought grant money for the polluting vehicles obtained it, while only 33 percent of agencies that sought zero-emission grants earned them.