Those days of “Test Pipes” and carburetors with open idle mixture screws are LONG past.
Not necessarily. Take a close look at this thread http://community.cartalk.com/posts/list/2135156.page where the OP has stated that he has removed the catalytic converter from his car. Where I live (Florida) our cars aren’t inspected or tested, and I know people who have removed the catalytic converters from their pick-up trucks. Emissions testing deters and identifies this kind of deliberate polluting.
If states like CT, CA, and TX didn’t do emissions testing, emissions from motor vehicles wound not have “greatly reduced over the years.” The car companies didn’t reduce emissions out of the goodness of their hearts. They did it so they would meet state standards. If the state standards aren’t enforced, you may as well not have them.