Existing bike horns do work a fair amount of the time, but you wouldn't want to rely on them in any kind of emergency situation.
See these horns use extremely high frequencies which are harsh on the ears and people often can’t even tell where the sound comes from. Zoe Williams from the Guardian wrote that pedestrians "get the 140 decibels, but [they think] ‘What's a tumble dryer doing in the sky?” Auditory illusions like these are well studied [source] and are caused by the narrow range of frequencies that cheap piezoelectric sound generators and cheap air horns can make. In an emergency, horn volume is much less important than getting a driver to recognize and react appropriately to the sound.
This is how you might look listening to a traditional bike horn. photo by littleprincessdiaries / Creative Commons
Some say that drivers could eventually learn to associate a particular screech with a speeding bicycle, but "abstract-sound warnings were found to be learnt and retained with far greater difficulty" than relevant warnings including car horns [source]. To make matters worse, the creators of all the different screechy bike horns have patents on their unique sound, effectively destroying any hope for a standard recognizable "bicycle-warning."
Car horns, however, are universally recognized by drivers everywhere. The LoudBicycle car horn for bikes is the best solution for protecting cyclists in emergencies.
Read about why the best bicycle horn sounds like a car horn; order a Loud Bicycle horn at www.LoudBicycle.com