Contrary to my fellow Texan's above posting, most of the more populated counties in Texas have mandatory emission and safety testing. The safety inspection has been around forever, making sure that all the lights work, etc. On the whole, no real complaint with that. If you've ever lived in a state where they don't have at least the safety inspection you see a lot of really ratty cars that you'd rather not be following or have behind you.
The emission testing is another thing entirely. If you fail, there are very limited choices, and most dealers flatly refuse to work on anything pre-ODB2. Theoretically, if you do everything you can and it still fails, the state will buy your car for something like $500, but you will have spent $1,500 or more in parts trying to get it working before that happens. As it is something of an unofficial law that every Texan must own a really ratty pickup truck, we seem to have a contradiction in cultural goals.
Given that any air pollution blows downwind to Louisiana, I can't see why we bother. Texas toxic waste would still improve the average in that misbegotten place.
I moved to Nevada. Reno has emission testing, but Carson City 20 miles south (where I live) does not. As we've got nothing but desert and Mormons downwind of us, I assume the emission testing is to prevent California cars that don't pass from being dumped into their market. Wish they could find a way to keep Californians from being dumped into Nevada as well.