I never understood road rage.
The rage comes from the fact the number 1 killer of people under the age of 35 is car crashes, not to mention crippling or severely fucking someone up for life because of driver stupidity. There is so much incompetence out there, along with the usual sense of entitlement that is so prevalent in society any more. Couple that with the fact, so many folks of all shapes, sizes and ages have the audacity to take an "aggressive" stance towards complete strangers all because they feel protected in their bubbles of steel.
Would that soccer mom or old man be aggressive and disrespectful with body language or words in any other setting in life? Of course they wouldn't, but because they are in the safety(?) of their car they think they are suddenly invincible and can actually 'take on" anyone and everyone. Its comical and insulting all at the same time.
As for the question the OP raised, I could write a a book about lousy drivers and some of my altercations with them. Besides using expletives, I rely on the "jack off" sign (although not on my forehead like the other poster)....because I feel it sums up their behavior quite succinctly....just concentrating on themselves completely oblivious to the rest of the world. Its also a more offensive gesture than the middle finger, which most kids already understand...and its my hopes that at the very least a couple of Johnny Dickhead Drivers are having to have that awkward conversation with their children when they ask..."What does that mean daddy, and why does it have you so upset?".
Edit to add: When I was in my early to mid 20's and a lot more wild and belligerent I actually created a couple of signs to flash to drivers.....2 inch block letters with a sharpie on some posterboard I kept right in the passenger seat.
They were..."Your driving sucks!", "Nice turn signal", "Asshole", and "You're HOT!".