Instructables user frenzy put it well in this “Putting A Car Horn On Your Bicycle” Instructable:

I’ve found in my biking adventures, drivers are aloof, distracted and just plain blind to us little pieces of metal on the road. I’ve also found giving a little shout before a driver makes a right hand turn with out signaling is a great way to save your life, but i like my voice to not be raspy after running around town… SO the car horn project was BORN!

Here’s my take on the same idea:

Video (with noise) here; another Maker having a blast with his bike car horn in action here. Any other loud pedal-powered projects you’d like to share, please comment below!