have there been any studies to determine if loud horns are safer than the stock horns on bikes? Seems like they would help prevent accidents. I've seen a lot of air horns on motorcycles. Any ideas?  

If you have the time to use the horn and wait for someone to figure out where the noise is coming from, then figure out if the noise is aimed at them, then figure out if they should do anything about it...  You had time to take evasive action.

If you're paying attention to what is going on around you, you have no need for a horn. If you're not paying attention, the horn won't save you. Outside of that, it's purely a matter of personal preference.

There are people out there who are not going to notice you, until you come through their windshield. You need to be ready to dodge them.

regards,
Joe  

My wife and I both have 128 decibel air horns on are bikes and they have saved our lives more than once. As the first response to this post had mentioned there is no substitute for diligence and total concentration while riding, however when you are trying to get someones attention who is riding in a cage with the windows up, talking on their cell or just not paying attention an air horn will definitely get their attention.  

Anything you do to make your bike stand out will help with safety. Loud exhaust, air horn, l.e.d. lights, blow up doll etc.

pay attention because most cagers are NOT  an air horn is a cheap effective way to let them know your there
(stebel air horns are about 50 bucks)  

Go into most any auto parts store and pick up a PIAMM "Freeway Blaster" for about $15.  Well over 100 decibels and it's a plug 'n play deal on most bikes.  A very inexpensive safety mod.  

To post purely anecdotal data on the other side of making noise. I'm alive today because I DON'T have loud pipes.
I was riding on a back road, and coming up on a tight left, when I heard the squeal of truck brakes. I grabbed a couple of handfuls, and about 2 sec later, a truck came thru the turn, mostly in my lane, doing about 25mph. I was doing 30.

I'm a firm believer in making noise when it's important, and not when noise can get you killed.
A quick toot on a horn can stop someone from coming out of the intersection  

Loud pipes, or loud horn, . . . ?

My pipes are quiet but my horn is LOUD!  Mac-truck LOUD!

I ride defensively and rarely get into a situation where
the loudness of my pipes would make a difference at all.
Since most of the sound from the pipes goes to the rear,
the loud pipe issue is a matter of choice, and the law.
My horn will let a cager notice me more than the exhaust
sound because I try not to position myself where I can't
be seen.  If they forget, I remind them FORCEFULLY!!!

Case in point, a loud piped cruiser was in the left lane
and in the blind spot of a cager in the right lane ahead
of me.  The cager started to move into the left lane w/o
signals or a mirror/blind-spot check over-the-shoulder.
I BLASTED my Bad-Boy horn and the cager jumped back
into the right lane avoiding a brush with the cruiser.
I could hear the pipes from the rear but the cager did
not!  After the cruiser passed the cager moved into the
left lane.  I attempted to pass on the right and AGAIN
the cager tried to move into the lane without looking.
Another BLAST and there were brake lights and a look of
'WTF was that?'  I 'beeped' my stock horn as I past and
gave a stern stare.  Cager was on a blue-tooth cell.

BTW - I ride a Maxi-scoot so I NEED the LOUD HORN!
Saving for a V-Max to restore!

Hope this helps?