This issue here is context.

Twice in the ten years I've been cycling I've had cyclists come up behind me on public sidewalks. One said "on your left." The other rang a bell. Both times I completely blanked out, and had no idea what was happening behind me. Neither time resulted in a collision, but they easily could have. On the other hand, many times I've had a cyclist come up behind be on a MUP when I'm walking or riding, and understood instantly what was being communicated to me.

The difference was that when I was walking on a sidewalk, where cyclists are not entitled, I had no subconsicous expectation of a cyclist coming up behind me. So I couldn't process in the information. On the path I was expecting it, so it was easy to understand.

What we all experience on a MUP or trail while cycling are people who are in that mindset...not expecting a cylist to come up behind them, when of course they should. No horn, bell, or shout out will shake them out of their lack of awareness at that moment.