Oh, I remember now!
I use a whistle, but I've never been ticketed for it, and it has saved me a few times from motorists coming in from side streets who didn't appear to see me. It works even when windows are rolled up, and never runs out of power. But the downside is knowing when to have it ready in your mouth, if it's not already in there, forget it--trying to put it in is a waste of time. Probably better to go with an airhorn, but I'm used to my whistle.
Now I'm thinking, maybe it's time to install the Airzound.