Our bike lanes are just a line on the sidewalk and pedestrians routinely walk on them, cross the sidewalk in them without looking, let their toddlers/pets run into them, etc. Also, nobody realizes that a bicycle bell means "someone is coming", so they just ignore it as background noise.

I had to mount an airhorn onto my bike. At least people listen to that, though it's so loud I only use it in emergencies.

I would be worried about a horn like this because if they get startled and move into the path of a car on the actual road, or do any other stupid thing that injures them, you're going to have real problems.