Originally Posted by Beachcycle

I got an airzound recently and am very satisfied with it - I would strongly recommend one to anyone who bikes in traffic, even on bike lanes. A horn can alert other road users to prevent accidents - whether the horn is being used by a car, a bike or even a pedestrian (though hopefully pedestrian infrastructure is good enough that this is unnecessary).

A friend was going to get a bike bell, but when he saw the horn in action, he changed his mind. Other cyclists have seen it and expressed interest and fascination.

Some common scenarios for using it:

Drivers are edging into or out of a street parking spot while you are close behind, and you have cars driving on your left (most common situation).

Drivers are turning through your path and don't see you or underestimate your speed.

Driver is making a right turn, and the bike lane has put you on his right - honk if he shows signs of making the turn while you are going through.

Cars blocking bike lanes at length in non emergency situations.