^ This.

MUP user myself, most of the time, where they occur.

Plenty of MUPs near where I live. And most people don't seem to readily "get" the language used. In my own experience, they tend to be surprised by the warning call (whatever it is they imagine was said), then all too frequently jet to one side in fear of being struck.

By far, the safest technique I've been able to employ is: allowing a good 5-7+ seconds prior to ever coming near a person on the MUP, if passing them where they're currently unaware I'm approaching, and not actually passing until they've had sufficient time to register and "settle" into a straight and sane path along the MUP. Rushing upon a person who's looking away then shouting something (even "On your left" or "I'm passing on your left") often doesn't allow a person sufficient time to do anything but react badly to an oncoming collision they fear.

The occasional MUP near me has serious, brightly-painted lane lines dividing the flow of traffic on the MUP, and these seem to be somewhat safer with far fewer people playing the part of the dolt with the head in the clouds. But the narrow, unmarked MUPs where I'm hoping the person won't do the unconscionable seem the worst. It's generally safer on the right hand side of the street, with those "bad" (narrow, unmarked) MUPs.

Of course, then there are the frequent people who demand to walk in the narrow street area instead of the MUP twelve feet away that's designed to keep them safe. As if that makes them safer. (Had a handful of those a few weeks back, sad to say. And their utter indignation at being asked to use the MUP and not clog traffic wasn't taken well by a couple of them.)

Takes all kinds.