I ride a MUP on part of my daily loop. In the evenings it's packed with peds, bladers, runners and bikers. Sometimes the peds make it difficult by walking 3-4 abreast. Signs say bikes must yield to everyone.
I recently wired my bike with a 12V light system and included a small 110dcb car horn. Last night on a crowded part of the trail there was a group of teens blocking the whole lane and part of the other. Just rapping, drifting along with their hands in their pockets, holding up their pants. ;-)
As I approached from the rear I decided this was the ideal time to try my new toy as there was just enough room to pass before oncoming traffic if only I had a little squeeze in the middle.
Two short taps and the seas parted. They literally jumped to the side. One said he thought it was a car or motorcycle on the trail as I breezed past.
I don't plan on using it much, it is obnoxious, but seems to work. A couple of cars pulling out of driveways and hooking got the horn too, and they both stopped.