Not up in the rust belt, about half the cars we've had went from the road, to field car duty, to the wreckers. Usually from terminal rust.That's the thing, I don't believe it's very common. Many people trade in their clunkers instead of junking them.
The other half were sold privately. I guess I've never actually traded a car in, now that I think about it, it always seems its worth just driving it for a couple more years than get a tiny trade in value for it?
I do see lots of "$1000" rust bucket cars that people can't trade in and try to sell privately for more than scrap value, but I think most of those go the wreckers or someone buys them for parts at near scrap value.