I mostly just smile back and continue on my merry way cruising along. All the while chuckling to myself at what a loser they are. Why would you lower yourself to their shenanigans? Darwins law will take care of them eventually.
I've been revved at while at a light, while driving, tail-gated, honked at, cut off... but in the end. No Fucks are given and that just infuriates them.
At the end of the day, I don't need to measure my penis when I drive.