I'm not sure how useful a loud horn is with traffic, at least in my part of the world. In some places, like Italian cities, honking a car/scooter horn seems to mean Watch out, I'm near to you or Ciao. However, in my town, drivers only honk if there is a problem or maybe if they see a friend. But, when I'm biking, I'm busy evading if there is a problem, and I can just wave or say Hi if I see a friend.

I believe that my safety should not depend on someone hearing my horn; I should be riding in a way that allows me to avoid any threat. Besides, in an emergency I will automatically focus on maneuvering to save myself. I might not remember the horn's existence until after the danger has receded.

Still, there might come a time when the horn will save me from my own careless behavior, lack of vigilance, or bad luck.

I doubt that honking to express displeasure with a moronic driver will do any good. Chances are good that they will either be confused, or get defensive and react negatively to the honking.

You can buy small portable air horns at boat supply stores and some sporting goods stores. They are shaped like a small aerosol can with a horn on top, and they should fit into a water bottle cage. Some might be refillable, but I'm not sure how you do that.

Another option is Nightsun's Alert horn. I think it's powered by one of those standard boxy-shaped 9 volt batteries. It's compact and mounts on the handlebar, so it appears to be an excellent choice for getting people's attention.

But I have not tried either of those solutions. I instead bought an air horn marketed to cyclists, the Air Zounds horn.

Bicycle air horn accessory

My air horn is rated to emit up to 115dB, though it has a volume control. I hope I'll have the horn ready for the next time a dog chases my bike. I'm guessing that this will be a great deterrent, second only to a weapon. The horn is also useful if someone fails to hear the bicycle bell that I prefer to use.

The air is stored in a bottle which can be slipped into a water bottle cage, as shown below, or attached to the frame or handlebar using zip ties. For pumping up the bottle with air, the horn has the same Schrader valve used on all car tires and many bike tires.

Bottle of air for horn accessory

Are you curious about what other horn owners think about this device? Check reviews on mtbr.com.

Last time I checked, several places had these horns at a good price. I ordered mine from the source but normally this will cost more.