I don’t recall exact statistics—US states vary widely in their requirements from no inspections to annual safety and emissions testing.
Virginia has been reported. The safety inspection provides a checklist of about 30 items. If the vehicle has sufficient support for OBDII, the tailpipe emissions test can be replaced by consulting the vehicle’s computer. And now northern Virginia (at least) has monitoring stations that will record your vehicle automatically, making the emissions inspection automatic.
The safety inspection primarily checks tire tread depth, parking brakes, service brakes, lights, steering, interior safety equipment, and exhaust system. I have on occasion been advised on mechanical issues not part of the inspection—“there’s a big chunk missing from your CV joint boot, Rick”—and have failed the test in most of the above categories at one time it another. That’s the price of driving cars to extinction as I often do. (Three of the vehicles in Fleet Denney have earned antique vehicle plates, but those are kept and used consistently with the usage restrictions placed on such vehicles.)
When I lived in Texas, safety inspections included (in big cities) emissions testing and were conducted annually. As far as I know they still are.
Judging by what I see on the roads, there is a lot of corruption in the system, but the penalties for such are quite high in Virginia, for both the vehicle owner and the service facility licensed to conduct the inspection. Safety inspections in the US are usually conducted by private certified inspection stations; emissions less so.
I believe only California certifies aftermarket equipment for emissions (California Air Resources Board—CARB) and then requires modifications to use only certified products.
Rick “vehicles are generally bound by the laws in effect when new” Denney