The first thing I noticed when I went to South Africa was the older cars on the road. Some cars that I remember from my childhood but had completely forgotten about until that point. I never realised the turnover in the UK was so high until I came back.
As an example my sister lives in a county in Colorado that doesn't do emissions testing. I bought a car - 2001 Subaru Legacy - from her several years ago and had to pass emissions as I do live in a county that requires it. It failed spectacularly. The car was not 'seriously ailing' either. The forward O2 sensor needed to be replaced and then it passed just fine.
Granted from what I hear California has pretty strict emissions requirements, but it's not necessarily a cake walk outside of California either, although sometimes it is. It's highly variable.
Cops will pull you over if your brake/head lights are out.