Asshat or assclown.
Depends on the transgression.
This idea would make for a fun thread on its own. Toned-down cursing. Genteel swearing. Ladylike bullshit. Pick a name.
Another Idea I’ve had that may not even work at thread level has to do with when one of my kids would ask me if I knew somebody’s name and I would go on for a long time taking some unlikely surname (my go-to was Ducats) and list hundreds of possibilities like Elwood Ducats, Jasper Ducats, Clytemnestra Ducats, and just keep on until whichever kid asking the question would yell. “Uncle!” or “Stop!” or words to that effect. I was a bad parent. 
You just can’t go wrong with the all-purpose “asshole.”
Novelty songs generally have a very short shelf life for me, but I had to admit to getting a kick out of Adam Sandler’s “Ode to My Car,” in which his car’s lack of good brakes leads to him hearing (11 times a day) “Hey, watch it, asshole!”
It’s always asshole for me, unless there are children in the car. Then I say “Way to go, Bunky”. I don’t remember how or where I came up with this, but I know Bunky was an idiot.
Mine is racist, so I’ll put it in a spoiler.
Yackablackers, for when I’m stuck in a car lane at the drive-through or bank because the car ahead of me has a black woman blabbing with the black female attendant.
There was a Bunky on Big Brother several/many years ago. Him, maybe?
Then there is the douche family of putdowns. What is the order of offensiveness? Douche, douchebag, douchenozzle, douche pot pie, douche canoe, douchewad, douchewaffle, etc.
“Twit” if Celtling is in the car. “Absolute twat” when she isn’t.
For a slow-poke in the left lane it’s a sardonic “John Paul Jones,” “Roger Bannister” if he’s accompanied by a lane-blocker in the middle and/or right lanes.
A sarcastic “sweetheart.” As in, “Come on, sweetheart, move your ass!” or “Pick a lane, sweetheart!” Sometimes I go with “dumbass,” but usually it’s “sweetheart.”
(Note that I never use that with someone I’m dating. “Sweetie,” maybe, but I don’t believe I’ve ever sincerely called someone “sweetheart.”)
No, never watched that show. I think I picked the phrase up from a co-worker and who knows how he came up with it.
Jethro.
We reserve “Asshole” for the guy across the street. His last name is “Testosterone.”
So many great inspirations here! Can’t wait to start driving home today. heh.
I tend to rely on classics like You Dumb Fucking Idiot, Genius and Goddamned Moron, but a random one just popped out of my mouth recently - I referred to the annoying driver in question as a “turtleneck”. ???
This reminds me of the Far Side, “Why people named Buddy hate to drive.”
However I personally don’t use buddy. I like my buddies and don’t want to associate them with asshole drivers, even sarcastically. That’s why the word “dickhead” was invented.
Some movie or TV show (might have been Paladin – Have Gun, Will Travel – years ago) made reference to a pencil necked geek which became elevated in my own vocabulary for special occasions.
Glad to have been in on your being inspired. Just trying to help out as a good Christian. 
“Nice move, Zeldar!” has a ring to it. (Nothing personal, of course).
“Asshole” or “moron” typically. But if you’re a rural slow poke then you’re named “Clem.”
Just because.
Take a page from South Park: “God dammit, Butters!”
Just think of me when you use it. No problem for me – unless I’m that other guy – and I try hard not to be such a blight on people’s nice days.
I thought of a few I’ve used that are G-rated:
Mr. Midlife Crisis (tailgater in sports car)
Miss Can’tPutDownYourCellPhone (young woman drifting into my lane)
Useless mud flap (idiot merging slowly right in front of a semi)
This is a fun thread!
Cocksucking motherfucking slut
Dickweed
Fucking Buckeye (when car sports Ohio plates)
Granny Grunt