If they’re not moving when there’s no reason not to move, I say that they have anal digit syndrome.
Asspipe
Most sources say this term was originated by professional wrestler Fred Blassie, so your origin theory seems unlikely.
Thank you for your support.
For people driving slow, grandma/grandpa or Gomer.
Otherwise just the generic “fuck-nut”.
Lately it’s been “jerkwad.” “Fartknocker” is a favorite alternate, and has been for as long as I can remember.
Fartknocker also works as a general expletive for a driving-related situation that is not caused by another driver. Running late and the railroad crossing lights up just before you get there? “Fartknocker!”
The epithet reserved for the most egregious situations is “asshole.” Oh, I swear like a sailor most of the time, but for some reason, when I’m genuinely angry and I want to call someone the worst thing I can think of, “asshole” is what comes out.
Sounds weird but…“f**king Rita”. Back story: after becoming fans of Dexter, my husband and I developed a hatred for his wife, Rita. It became our own cuss word after using it a few times.
I hope I may be permitted this mini-hijack but you’ve helped me remember something that’s been hidden in a corner of what passes for my mind.
In the movie Jungle Fever (1991) Wesley Snipes and Annabella Sciorra are involved (at least their characters are) in an interracial affair. Annabella’s character is Angie Tucci. (If you Google Angela Tucci you’ll find several others.) But the fact that the A. Tucci in the movie is Snipes’s sex object made my wife and me decide that Snipes is “Fucking A. Tucci.” And Spike Lee can’t possibly have been unaware of the slang counterpart of that phrase. Not quite as out front as Gus Grissom’s (as played by Fred Ward in The Right Stuff (1983)) “Fuckin’ A, Bubba” but in the same ballpark.
End of hijack.
I was on a weekend getaway with some longtime friends recently. I guess these two have gotten kind of zen recently and they were frankly appalled at my freestyle cursing at pedestrians, other drivers, etc. I just do it for fun, not out of borderline road rage or anything.
Just to get my girls relax a bit (and shut the fuck up about my vocabulary), I started saying things like PARDON ME, SIR and OH NO, YOU FIRST DEAR in a chilly, bloodless tone. They seemed to think it was a big improvement and it was actually almost as fun as swearing (but not quite).
Example:
Certainly, Mr Zeldar…I don’t mind waiting for you. Please. Take your time, I’ve got all day.
How sweet of you. No, please, you have the right of way. Live long and prosper. Asshole!
(That sort of thing? 
)
“Asshole,” normally said a la Otto from A Fish Called Wanda, so “Asshoooooooole!”
My usual name for annoying drivers is “turkeytrot” but if they REALLY get on my nerves I call them fartblossom.
Nostril-fucks
I always picture that they gather together, form a line, and hump the nose of some huge nosed hive-boss at the end of the day before crawling back under their rocks.
I usually start with “Oh sir, oh sir…” (or ma’am, as the case may be) and then escalate from there as necessary.
Fuckwad is a favorite, and then there is bluehair, grandpa, Sparky, dillweed, asswipe, asshole, dickface, and fucking Einstein.
My absolute favorite is for tailgater: “hey, asshole, if you are going to ride my ass like that, the LEAST you can do is pull my hair.”
Jagoff. I’m guessing this is a particularly Chicago-centric term. (ETA: Although it seems to be popular in Pittsburgh, too.)
SOB and M’F**ker. Have no need for more.
“Fucktard” is my epithet of choice
My mother used to say “Charlie”. That is, she didn’t simply fire “Charlie” as an insult, but would say “you’re (doing something wrong), Charlie”. My father didn’t drive much. I don’t drive at all.
Idiot. Simple, consice and to the point.