My husband has the Zound Air horn. He uses it on his commute and mostly just for dogs. I don't think that he's had to alert any cars with it yet, but I don't doubt for a minute that they would have any trouble hearing it...even with the volume turned down as it is extremely loud. I found myself wanting one today on my commute...twice!

Oh...do NOT accidentally set it off in the garage with the door shut...eek!