One thing I've long wondered, along these lines, is why cars only have an aggressive sounding horn, and don't have a less aggressive "toot toot, I'm here, mind your back" type sound. Metrolink trams have this feature (the pleasant "toooooot" and the almighty scream), and when I'm cycling I have the same option, the bell being a fairly pleasant "mind out, there's a bike behind you, do you mind moving over a little please?" kind of noise, but I retain the option to shout loudly if someone steps (or more commonly allows a dog on a long lead to stray) directly into my path in a manner I would have no way to avoid hitting them unless they moved.