I use a term I picked up from my Dad, although I have no idea of it’s actual meaning, origin, or spelling. The word (my best guess at the spelling), is “Honyock,” and it’s pronounced phonetically. Honyock is only used for idiot drivers. Neither my Dad or I use it for other idiots. They’re just idiots.
Pigfuckers.
A guy I worked with likes to yell “Merry Christmas” or “Happy Birthday,” especially if people are already yelling at him. Confused people to no end.
I just breath deeply and utter the Johnny Carson curse:
“May the dandruff of a flying wombat lodge in your left earlobe!” 
ShitSuckingFuckDog and Ass Wipe get a lot of play when tooling around in the Hook Mobile, and that’s Ms Hook.
When I’m out on the bike and people pull out in front of me, etc, I usually content myself with fantisies of kicking their ass and other bikerly things like that.
Cockmaggot, or sometimes shitmold. Just making up new words keeps my mind off of the fact that every 9th person in my city appears to have been handed their driver’s license at random from a deranged gibbon.
“Fucknut” works for me most of the time.
My insult of choice this summer has been:
Douchebag!
Or, should the situation warrent:
Fucking douchebag!
Happy
Excuse me, I suppose some queen is on a schedule!
This is the part where we learn the meaning of the red light!
Another tragic victim of testosterone poisoning…
I’m usually in my car alone, so I don’t have to worry about insulting friends or being a bad influence on young ears. Few of my fav’s:
Fuck Head
Dip Shit
Asshole
Stupid son of a bitch
Cocksucker
Fucking cunt
I remember lots of stories while growing up of me learning many new words when riding with my Aunt. Bless her heart, she could swear with the best of them
Would you believe that it wasn’t until after I submitted my post that it occurred to me that my stream of , to me , random obscenities might be interpreted as anti-gay? I apologize. Without, I hope, sounding too flippant, I guess I should be called The Straight Dope.
Usually reserved for major offenses (but not always):
Bastard asshole!
I’m sorry to say that my standard insult against other drivers is “cocksucker.” While driving is the only time I use that particular epithet. I’ve occasionally resolved to switch to something less silly – I mean what kind of an insult is that anyway? I’ve sucked a few myself, right? I plan to switch to “pigfucker,” which has a similar ring. And I’ve never fucked any pigs (actual pigs, anyway), so that’s alright. But then I get cut off in traffic and “cocksucker” is just what pops out of my mouth.
Thumbsucking moron. I can even say it in front of my parents.
I swear far too much at other drivers - it’s something I’m working on (a constant project).
A while back I was using “cref.” It comes from TIAA-CREF (they’ve a commercial on NPR), and it was silly enough to make me smile, thus diffusing the situation. Sadly, I’m back to the obscenities.
I also have running conversations, like “The long skinny pedal. Push it” or “that’s right, take your time, it’s quite scary out.”
The best I’ve heard is “the turn signal is an indicator of intent, not a request,” so that gets used quite often.
Maybe I should take a page from Mr. Snicks - he likes to use “poopy” as warranted because it makes him smile.
When my wife and I are together in the car and we experience any of the tire squealing or engine racing common to penis compensators, I will turn to her and ask “Are you wet now?”
If I am behind someone in a work vehicle going slowly, I will mutter “Obviously you are being paid by the hour.”
The word “stupid” hissed through gritted teeth is also common for me, as is a bark of “pick a lane!” for those in need of help.
And there are many times I don’t say anything at all. If someone pulls across me in traffic, I don’t slow down and try to scare the hell out of them. I also like to fill the rearview mirror of those who jump out in front of me.
Your mother is your sister!!!
“What are you… from Knoxville?”
“Oh, goody. Another person with nowhere to go and all the time to get there.”
(Those are the only two ‘clean’ insults that I use. The rest of them aren’t even fit for “Talk Like a Sailor” boards. )
That’s mine too. “Goddamned pigfuckers” when they really piss me off.