I think I might die laughing if a park ranger jumped out from behind a tree in Wawayanda and said "Stop, your speeding". Doesn't the ranger have to catch me, too? 15 MPH seems like a pretty low limit when you get a nice stretch of downhill. Not that my average exceeds much more than 6-8 mph in the woods.
I gather these are locations that are seriously over-crowded and filled with selfish people who can't figure out how to share a trail and stop giving hikers a good reason to complain. You would think that would apply to some of the places here in northern NJ and southern NY, just based on the crowding and the # of assholes per square mile. I never feel crowded. But, I never ride someplace like Six Mile on a Saturday at noon either.