A few riders who insist on going fast on M.U.P.'s can spoil it for all of us. While the conditions (usage, sight-lines, etc, do vary) as Bik has noted, we can not always depend on all of our fellow riders to ride at or below a responsible pace, as conditions warrant. This is why, maybe unfortunately, speed limits should sometimes be imposed. Of course, laws and regulations like this are not passed until a 2-year-old gets knocked over and a still-clipped-in rider hits the dirt doing 14mph on a saturday morning at 10:00am.
Let's all keep our speed down and stay alert and considerate, so that we don't need to resort to speed limits.