Here's a fun (?) fact: the FAA explicitly refuses to define the term "congested." In one case, they actually claimed that an area with one person was congested. Not sure if that held up on appeal. In any event, they've stated their desire to retain "enforcement flexibility" which, to me, is kinda BS. I've flown over a lot of areas where it wasn't obvious whether it counted as congested or not, and that 500' made a difference. (Next to a mountain, for example.)

This area, though, is definitely congested.