I do tend to rag on California's air quality perhaps too much (It's hard to forget those days when breathing deep meant coughing and chest pain and all because I was doing something natural . .. . .breathing) you are correct in how the smog checks on cars have made vast improvements in the air quality.
An example of how much cleaner the emissions from automobiles have become, Back in the late 60s and early 70s there were just a fraction of the number of vehicles there is today and yet those cars produced as much or more than the cars do today.
In the beginning it was a big hassle to get a smog inspection every other year or in some cases yearly and if you had a "clunker" that didn't run to well and you struggled to keep it going (my first car) it was a bitch meeting the demands of the inspection process.
Its because of the strict emission laws that California's air has got so much better over the years and all the bull shit that everyone has put up with over the years has certainly paid off.