I don’t know, I can’t decide between having every swear word I’m uttering at the offending driver being broadcast loudly and in quadrophonic sound within their vehicle, or whether I’d rather have a psionic ray which could spin their car off the road, at which point Mario Andretti would slap them and say “Give me your licence!”
A Wibble to anyone who gets the reference! 
Ooooh, I like this one. When I get stuck behind a painfully slow grandad-driver, I tend to occupy myself with an expletive-filled monologue/rant directed at them, most often concluding with one particular line when I overtake or they turn off. Spoiler for the easily-offended:Yes, of course, 40 is perfectly fucking fast enough. No bloody point in wearing out that fifth gear, is there, you shit. Oh, yes, there’s a curve up ahead, better slow to fucking 20. Yes, please do stay there until well after the fucking bend, I’m enjoying gazing at the fucking scenery. Fuck off, you cunt.
I only do this when I’m alone, of course. Or when I forget that I’m not alone.
In Saint Maarten, everyone toots their horn in a friendly way to thank others. It is kinda cool. You enter the roundabout, need to move over, and someone chirps their horn to say go ahead. You move over and chirp back a thanks. All very friendly. My first day driving there, I was freaked out because I assumed people were pissed off at me.
Semi-horn? meh.
A 120 decibel recording of an 18-wheeler locking it’s brakes at 60 mph, now that would be impressive.
(Yes, I stole that from the Fabulous Furry Freak Brothers, but I’ve wanted one since I read it.)
Haven’t read the thread, but…
Last week I took the MG out for a longish trip. A girl in an SUV got on the freeway, crossed the rightmost lane, and then started moving into the left lane without hesitating or (apparently) looking where she was going. (Humans should come with some sort of swivel for their heads as standard equipment. I have one; it’s called a ‘neck’. Very useful thing for looking where one is going.) I had nowhere to go, so I leaned on the horn while I edged toward the median grass. A three-second honk stopped her from hitting me or forcing me off the road.
When driving down an alley and approaching the end, where a sidewalk may be crossing at the alley, I always give a tiny beep before stopping, so as to warn walkers not to blithely step out into the alley. It’s just a little heads-up for folks who may be preoccupied with conversation or something. And I try to be quiet so that the folks in the apartment buildings, for example, don’t have to get jolted out of their chairs when it happens. However, the other day, I did my little tap on the horn, saw no one peeking around the corner of the building at me and began to move out into the street when a dick-wad jogger with earphones came running out in front of me. I did play my horn then, and for good purpose. Yes, it allowed me to say, “You stupid git! You scared the shit out of me and I almost killed you!” but I’m hoping it also put a bit of a scare into him and he’ll be a bit more careful now.
You guys forgot one other purpose of the horn. It is a short hand way of saying
“Hey I noticed you and your friends are really smoking, and I would like to have sex with you, so If you guys will jump into my car while I pass by at 45MPH that would be acceptable”
Women dig that.
::melancholy arpeggio::
Honking relieves traffic congestion
However, I’ve been at a resort with a very sharp turn that had a sign “please sound horn” before going into the turn. In this case, people are expecting it, so it works.
I had a colleague who claimed he gave up his horn for Lent. 
He was from Cicero, so he had ample reason to use it at home.
I had to go to Cleveland to hear the proper use of the horn. I had a rental car, but it had NY plates so obviously I was an outsider. I was honked at (1) to alert me to the fact that the lane I was currently occupying suddenly and without warning was about to become a parking lane, complete with parked cars; (2) to alert me to the fact that even though I didn’t have an arrow and was making what seemed like a left turn, in fact I had the right-of-way and should proceed through said turn–in this case the guy passed me and yelled “Welcome to Cleveland” (3) to alert me to the fact that I was about to head the wrong way on a one-way street.
I appreciated it. In Denver people only use their horns to indicate extreme displeasure, and it’s not always, or even usually clear what you did that displeased them.
People don’t take well to being honked at, either. The last time I used my horn it was to alert a guy to the same situation as (3) above, only in Denver this time. He responded by sticking his arm out the window and waving his finger at me before proceeding to turn into oncoming traffic. I guess he figured out a couple of seconds later the purpose of the honk. Really it’s not very specific. It took me a couple of seconds to figure out the purpose of the honk in (1) above, but that was partly because I was so unused to the idea of a friendly, informative honk.
A bullhorn would be better. (“Way to park, Ace!”)
I’ve actually had my life saved once by a car horn when I dozed off and started to drift into an oncoming lane before someone honked at me to wake me up.
Aside from that, in College Station, TX, the car horn primarily exists so you can use it to tap out the first eight notes of the Aggie War Hymn (Hullabaloo, Caneck, Caneck!) while driving through campus or past the bars on North Gate. One time I saw a firetruck hauling ass lights a flashing and sirens a wailing, and they blasted out Hullabaloo with the firetruck horn. It was freaking awesome.
French cars used to have dual horns, a light tap sounded a nice discrete toot for town use, a sustained push sounded a huge bellow, for out of town use.
My first Citroen had one such but none of the subsequent ones, or the Renaults.
I’d rather like a destroyer klaxon, that old “Whoooop Whooop”
would shift just about anything from your projected path.
ETA: In Sweden, many years ago, it was the custom to give a gentle “Peep” on the horn and flash your headlights prior to overtaking.
Initially I thought this to be most impolite - a sort of GET OUT OF MY WAY MOTHER*$&^£)* thing. It was though just a cultural thing evolved from the very low traffic density, you could drive for hours without encountering another vehicle and be completely oblivious to someone behind wishing to overtake.
In DC Metro I’ve adoped the European (I think) method of encouraging people to let me pass in the left lane by putting my left signal on. I have discpvered it works pretty well. When I get hung up behind a car that is pacing other traffic and blocking flow, I’ll give 'em a minute to see if they move on then I politely apply my left turn signal. No honking or high beam flashing, it says “When you can, I would like to pass. Please.”
I have yet to get a negative reaction to it. In DC. It is to wonder.
Makes me wonder, do motorcycles have horns? I don’t think I’ve ever heard one.
They do.
At least in the UK a horn is a legal requirement and a vehicle (of any description) would fail its MOT test and be deemed unroadworthy without a satisfactorily functioning horn.
Another function of a horn is to scare the crap out of people. Just install a train horn on your car and blast away. Hilarity ensues.
or you can use a ship horn if train horns are too quiet for you…
My Honda Shadow does. I suspect any street bike is required to.