Bell. People know what a bike bell sound is. They hear the bell, their brain says "Bike. Move over." They know you are about to pass 'em. Ding it a few times and they can sometimes tell how close you are and fast you might be going.

Shout "on your left!" Their brain says, "Left. Huh? What? Turn and walk left i guess."

Hear a whistle and my brain says, "ball is down. play is over." or maybe "Who is blowin' that whistle? Turn and look."

For me, the bell is A #1 solution to riding on the MUP.