Do horns and bells really prevent accidents?

In order for e.g. a horn to work you need enough time that the driver processes the situation and decides the horn will communicate something AND enough time for the pedestrian or whatever to process that and react to it. Generally it's a lot easier just to press the brake, and more importantly be travelling at a speed and in a manner where the brake is sufficient.

Structurally, we'd be much better off reducing conflicts between the different tiers of users. I.e. properly segregated infrastructure for each class of vehicle.