In many states it's a largely solved problem, with annual or biannual inspections needed before you can renew your car registration. (along with the ability for police to site you if there's a visible or audible problem)
There are loopholes, though. There are fair number of trucks in my state that are allowed to run dirty diesel trucks with basically no exhaust protections. Their industries (construction? mining?) get some exemptions under the law, I assume. It's utterly stupid, as those trucks probably account for more particulate emissions than all the other vehicles on the road put together. But God forbid someone increase their "cost of doing business" by requiring some improvements.
There are loopholes, though. There are fair number of trucks in my state that are allowed to run dirty diesel trucks with basically no exhaust protections. Their industries (construction? mining?) get some exemptions under the law, I assume. It's utterly stupid, as those trucks probably account for more particulate emissions than all the other vehicles on the road put together. But God forbid someone increase their "cost of doing business" by requiring some improvements.