I seriously doubt that the horn has an output of 115db. This reference shows the levels for various sounds. http://www.gcaudio.com/resources/howtos/loudness.html As you can see, 115db is equivalent to sand blasting or a loud rock concert; it's also 5db higher than a power saw at 3 feet. My impression is that the horn is not quite that loud. N.B. if it is that loud, please note the cumulative exposure to 15 minutes of such sound in any day will lead to permanent hearing loss. That's the standard on which the OSHA regulations are based.

The basic problem is that sound insulation in the standard car, with windows sealed, air conditioner and radio on will make even a 115db horn inaudible. The NYC Fire Department kept making sirens louder because motorists were not getting out of the way. By the late 1960's the sirens had become so loud that firemen were going deaf and cars would still not move out of the way. I was taking an engineering course in sound and acoustics at the time. The professor arranged for us to measure sound levels for different sirens on the fire trucks, on the street and in cars. We concluded that the motorists in their cocoons could not hear the sirens and that the firemen had plenty of cause for going deaf. I believe that Detroit's penchant for removing the driver from road sounds stemmed from a Rolls-Royce advertisement at that time. The ad proclaimed that at 60 mph the only sound a Rolls-Royce driver heard was the sound of the [mechanical] clock.