It was 1995 when the Los Angeles County Metropolitan Transportation Authority (MTA) bought its first bus fueled by natural gas. Fifteen years later, MTA has “2,221 buses powered by compressed natural gas (CNG), as well as one electric bus and six gasoline-electric hybrids.” Today, announced the Los Angeles Times, MTA “will retire its last diesel bus” to “become the only major transit agency in the nation with a fleet that is totally equipped with alternative-fuel technologies.”

The alternative buses in the city have traveled about a billion miles collectively. Transit officials estimate the switch “has reduced the release of cancer-causing particulates from the bus fleet by 80% and greenhouse gases by about 300,000 pounds a day in one of the smoggiest areas of the country,” the LA Times says.

Compressed natural gas buses cost more to buy and maintain compared to diesel buses, but officials say the increased expenses are offset by lower fuel costs. (The buses that run on natural gas cost about $450,000.)  According to the Los Angeles Times, MTA will ceremoniously retire the last diesel bus, a 40-foot New Flyer purchased in 1998 running in MTA’s Venice division. The bus logged 383,180 miles and will be towed away during an event in downtown Los Angeles.

A few smaller agencies in the U.S. boast fleets that run only on alternative fuels, including a few in California, but Los Angeles is the only major U.S. city to entirely switch to alternative fuel, showing the agency’s commitment to clean fuel technology. And important to note: “Though the MTA has converted its fleet, the agency still contracts with private bus lines that rely on diesel fuel. Of the 187 private buses, 82 have diesel engines, but transit officials say those should be phased out in the next several years.”