Eventually the program was scrapped, due to fewer cars failing (8%, down from 14%, which is still too high IMHO), and likely as a way of gaining political capital with the public in advance of an election.
So, for example, in California (one of the strictest) you must pass a visual inspection, a tailpipe sniffer, and an engine computer scan. There are also many rules about acceptable engine modifications. In other states you may only get an engine computer scan.
As for standards, cars are (rather reasonably) held to the standard they were made to, not the standard of today, and modern cars really are dramatically less polluting than cars from several decades earlier.
...which seems shortsighted. Except for trailers, all of those can have mechanical problems that cause it to pollute far more than they should. My daily driver for a long time was from 1974 and I was never sure why older cars were exempt. If it's a hobby car with historic plates, sure, but if you're driving it every day you should have to follow the same rules as everybody else. In my defense, my old car got 68 mpg and was well fussed-over.
Part of the rationale with the 1975 exemption is that vehicles 1975 and older represent a very small portion of the number of cars on the road, and are driven far fewer miles than average. That beautiful 1966 Corvette didn't last fifty years by being driven every day.
The major loophole is that if your car fails, you only have to spend $450 to fix the problem. Spending $450 to improve emissions gives you a pass whether or not it actually fixed the problem or not.
Ontario uses a lot of salt in the winter, so there aren't a lot of old cars on the road.
In places where there's no extended winter weather I've noticed the cars are often older, with 40+ year old junkers from the 1980s looking, by local standards, no more than ten years old.
Any surprise that emissions tests are required in Fairfax and Arlington, but not in Bland County? Largest city in Bland County has a population of 75; it is a different world from the DC suburbs.
Testing is generally done via the OBD-II port, so the vehicle self-reports it's condition. Vehicles without OBD-II and "other" vehicles use a tailpipe probe to report on exhaust gases.
Across the water in Vancouver you do have to pass an emissions test.
People still find a way to drive, buy and sell pieces of scrap.
http://www.ontario.ca/environment-and-energy/drive-clean-tes...
Saying that, go to many less developed country and the cars are far worse condition.
My friend always used to claim that the environmental cost of producing a new car took 9 years to counter with the efficiencies in fuel consumption. No idea how true that is, anyone know?