Boulder is like California (North and South) in this regard...  I don't
bother waving in "urban" areas.  I'd be waving at a couple people per day.
Not that that would be "bad" just it gets old.

I do always wave when I'm road tripping and out away from the "urban" areas
and of course always when in national/state park, campground area, etc...  I
also tend to wave anytime I havn't seen another van for a week or more, but
that invaribly only happens when I'm road tripping, so I guess I already
covered that.

Locally, I do still take a momment to wave or geture to anyone with a
particualrly nice, spiffed up van...  like the the gold Syncro Westy wtih
Gary Lee Rack and 15" SA Rims I happened to pull up next to on the 405
Freeway in LA.   He also happened to be my near twin, excepting the Rack and
style of 15" wheels...  or when I notice someone with an out of state
license plate...