Americans had become acutely aware of environmental damage post-war, primarily through the smog hanging over many major cities, and so it became a key issue in the 1968 US presidential election.
That was won by Republican Richard Nixon, who said in his 1970 state of the union address: "The automobile is our worst polluter of the air. Adequate control requires further advances in engine design and fuel composition.
We shall intensify our research, set increasingly strict standards and strengthen enforcement procedures, and we shall do it now." Two months later, an astonishing 20 million Americans came out in protest on the inaugural Earth Day.
That December, Nixon signed into federal law a Democrat's bill, the Clean Air Act, and created the Environmental Protection Agency (EPA) to enforce its requirements.
Already makers were required to cut emissions by 70% from 1967 (these limits set by California) to 1970, and yet the new bill required a 90% reduction in hydrocarbons to 0.41g/mi and in carbon monoxide to 3.4g/mi by 1975, and to 0.41g/mi of nitrogen oxides (NOx) by 1976.
The required technology didn't yet exist, but that was the point: regulation to spark innovation.
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LAND ROVER DEFENDER
Our new 130-all 5358mm and £96,745 of it - has its first taste of life as a daily wagon
RENAULT CLIO
Trip down to Spa reveals its talents aren't diminished with another two men aboard
Make mine a slightly smaller one
The Little Car Company has hit new heights of scaled-down kerb appeal with its recreated
Loyal to the end
Old-school vehicle breakers' yards are rare these days, but a handful of marque specialists are soldiering on to provide a vital service. John Evans pays one a visit
THAT'S WHY WE GO TO ICELAND
Smooth, empty roads snaking through epic scenery are one reason. Experiencing erupting volcanos is another. Matt Prior goes in search of both in Kia's latest Sorento and finds 50mph ways to leave your lava
TESTED 27.4.24, DEVON ON SALE NOW - PORSCHE 718 SPYDER RS
Dream-like on German asphalt, does this curious Rennsport also excel in the UK?
TESTED 20.5.24, FRANKFURT, GERMANY ON SALE NOW - FORD TOURNEO CUSTOM
New Transit sprouts seats and windows, gets a PHEV system that works differently
TESTED 24.5.24, BERKSHIRE ON SALE NOW - MINI COUNTRYMAN SE
Flagship version of the larger and pricier third-generation SUV arrives in the UK
TESTED 29.5.24, BEDFORDSHIRE ON SALE NOW - PORSCHE TAYCAN
Refreshed saloon targets class supremacy with electric powertrain updates
TESTED 23.5.24, BEDFORDSHIRE ON SALE NOW PRICE £446,742 - LAMBORGHINI REVUELTO
We've sampled Sant'Agata's new-era, 1001bhp supercar on track. Now it's time to see how it fares on a decent B-road