I saw a guy with one of these duct-taped to his handlebar:
I get near-miss side-swipes by SUV's and delivery vans almost all the time here in downtown SF. My last encounter riled me up so bad that I chased-down the offending vehicle and punched the passenger-side door as I went zooming past. Probably shouldn't have done that, but in a life-or-death instance, you really don't think clearly.
Those safety horns are often brought as party favors to Critical Mass...and they're loud...really loud...but a bell should be sufficient for warning pedestrians.