I put a bell on my bike if I'm going to be riding on a path or trail. Most of my riding is in the Washington DC area on the Washington & Old Dominion trail which gets very busy in nice weather.

Often a bell is the only way to get the attention of someone with headphones on, or (my pet peeve) mothers who insist on walking with strollers 2-and-3-abreast on the trail who don't seem to understand "on your left" even after it is yelled 3 times at them.........