First if you did announce on your left and they didn't hear it that is their problem . Around here once you get a mile from a parking lot the M.U.P.'s get better to ride, you get to the areas that those who can't read the rules at all are not ambitious enough to travel too. That said If your are running 20 mph+ and in a group of more than 2 or 3 there are more appropriate places to ride than M.U.P.
Roy