I have this bell and I'm quite impressed (bought in Germany but also available on UK Amazon ... don't know about availability in the US).

It sounds like a bike bell and it's very loud. I never have problems with people not moving to the right. They don't even turn around.

http://www.amazon.co.uk/Bbb-Loud-Cle.../dp/B000VSY8NG

A whistle would be silly at best and distracting/disruptive at worst.