genejockey 

I no longer ride on MUPs, partly because there aren't any nearby that go anywhere I care to go, but mostly because of the (admittedly) small percentage of pedestrians (and rollerbladers) who can't or won't respond to a bell or 'On your left'.

My worst experiences:

Old couple walking on MUP, old man on left. He responded to "On your left" by pivoting TO THE LEFT into my path to find my front wheel almost in his crotch.

Young couple who thought it would be a great idea to try out their BRAND NEW rollerblades while walking their dog on a retractoleash. He was all the way across the path in the bushes on the other side, while they were almost completely unable to control their own speed and direction. Fortunately, we were approaching face to face, so they could see just how much I went out of the way not to hit their dog. It wasn't his fault, after all. He was just being a dog.

Rollerblading girl with a walkman (yes, it WAS that long ago) bopping along the MUP, who did an impromptu pirouette right into my path, after it SEEMED LIKE she had responded to my bell.

Idiot runner who kept going smack down the middle of the path as I bore down on him, pinging like mad on the bell. "Say left or right!" he shouted at me. Why in hell would I pass on the RIGHT?? This is America, after all. Who passes on the right?

To be fair, those folks were only a few out of hundreds. Most let me pass without incident, without panic, politely. But those few put me off of MUPS for good.