We've heavily promoted KEEP RIGHT EXCEPT TO PASS on our MUPs and it's worked well. The longer bit says it's great to walk or ride three or four abreast but QUICKLY move right if someone is coming from front or behind. Always keep dogs off to the walkers RIGHT. If wearing EarPods then stay hard right at all times.

More MUPs also helps as the number of people using them for recreation will often not grow as much so they get thinned out a bit. We've now got 8-10' wide MUPs on just about all county roads near us and some have them on both sides (which helps even more since people have learned to use the side traveling with traffic). Recently we've been pushing for physcially separating some of the more heavily used ones, usually with a 1' strip of grass, in to separate 8-10' bikeways and 6' walkways.