Well, right now, if you want to get anywhere in Dallas on a bicycle, the trails are the worst ways to do it. Adding a low speed limit will just enforce that. It'll be a good way to keep newbie riders from riding, period, but won't accomplish much otherwise. It does reinforce the idea that bikes belong on a street, and is at just exactly the wrong time for the city to discover that. It would also be a way to shift the blame for virtually any bike accident on the trails to the cyclists' fault, regardless of the pedestrian actions.

If I remember right, though, the city does have a law requiring cyclists to give an audible warning when overtaking cyclists or pedestrians. Unfortunately, they don't have any law requiring people to listen to those audible warnings. That would probably be a better solution.