I just got back from a trip to Yellowstone and surroundings and pretty much every non-Harley rider (about 1/3 to 1/2 of the bikes in the park) were wearing some version of ATGATT. Outside the park the ratio of non-Harley bikes fell by a lot. I will also reiterate the "whole lotta fat" comment for most of the Harley riders.
For the Harley riders the most interesting thing was that the wife/girlfriend on the back never had lesser helmet coverage than the guy in front. So, if the rider was wearing a Harley Yarmulke the lady in back had at least that much helmet or more.