Flipping the Bird
As any driver of freeways in California or New Jersey knows, an extended middle finger is a gesture of recognition from one driver to another, a way of saying, "Thank you for sharing this short time on the path of human individuation with me."
In fact, this heartwarming gesture has roots in ancient Rome, where the digitus impudicus, or "impudent finger," was a sign of disdain, much as it is today. Specifically, the Roman historian Martial writes, "Laugh loudly, Sextillus, when someone calls you a queen and put your middle finger out."
So, if you were somehow transported back to the time of Julius Caesar, rest assured that you would have at least one means of communication.