First, I like the idea and I believe it is useful. The main reason for posting it is because it makes a lot of sense to me. However, without sounding defensive, I read a write up on the LOUD horn that seemed to see the horn as a misguided way of handling the rider/driver relationship. I'm not sure why, I guess because to me a bike horn makes perfect sense.
First off the bike horn isn't really necessary for every bike. If you ride on trails and leisurely around the suburbs where roads are more open, you might not need a bike horn. On the other hand if you commute to work by bike in a city like Manhattan where there isn't much space, this might be exactly what you need.
Yes, the driving community should be more aware of the riding community. And yes, cities should be more accommodating when planning on how roads will be used. But ideals aside, there is a reason why cars have horns. It allows for a means of communication between drivers. And if we expect everyone to share the road, we should all be speaking the same language.
As the designer, Jonathan Lansey stated, drivers react to the sound of a horn before knowing where it's coming from. As a driver I know this is true and as a rider I feel like it will help to prevent accidents.
You can support the project through it's Kickstarter page.