here's something
Scroll down a bit to "Pollution" and you'll find
Other things being equal, going slower produces less pollution ... just 10% of the cars generate half the polution from mobile sources. The \ufffdgross emitters\ufffd are improperly tuned or lack catalytic converters.
If that 10% generating 50% is true then it seems to me that emisions testing would do a lot towards cutting down on the pollution. A few years back we had emissions testing in Houston, but it was stopped because it "penalized the poor" - even though they'd let a car pass if the owner spent at least $100 in an attempt to make it complient (ie: give the car a tune up).