Once that bleeding-heart liberal Richard Nixon signed the Clean Air Act in 1970, the writing was on the wall for 11:1 compression hydrocarbon-spewing chrome chariots. Malaise Era, coming up! Still, it's now possible to breathe the air in Los Angeles.
Vehicle-produced hydrocarbons, oxides of nitrogen, and carbon monoxide (not to mention lead, ozone, and sulfur) were certainly getting pretty bad in some parts of the country by the 1960s; outlawing backyard incinerators helped quite a bit in Southern California, but some felt that more could be done. It's possible that the invisible hand would have intervened on its own, with Americans voluntarily paying extra for power- and efficiency-robbing emissions devices on their cars, but we'll never know; the feds and the states stepped in and rammed those devices down the automotive industry's throat. These days, vehicles run pretty damn clean, at least in terms of "traditional" pollutants (we'll leave the carbon-dioxide debate for another day), but it sucks to have The Man sweatin' us car freaks over, say, that 6-71 you want to bolt onto your Roadmaster. What do you think?