This is where things can get murky.
For example, if you're driving and pass a speed trap, you might flash your headlights at drivers coming in the other direction to let them know. But flashing headlights can also mean "your high beams are too bright," "you forgot to put your headlights on" or "go ahead" in situations where it's unclear who has the right of way. In order to interpret the meaning, a person will consider the context: the time of day, the type of road, the weather. But how would an autonomous vehicle react?
There are other forms of communication to help us navigate, ranging from honks and sirens, to hand signals and even bumper stickers.
Of course, humans use all sorts of hand gestures – waving a car in front of them, indicating that another driver needs to slow, and even giving the finger when angry. Sounds can communicate love, anger, arrivals, departures, warnings and more. Drivers can express total disapproval with a hard, extended hit of the horn. Of course, emergency sirens encourage drivers to make way.
But specific meaning can vary by region or country. For example, a few years ago, Public Radio International ran a story about the language of honking in Cairo, Egypt, which is "spoken" primarily by men. These honks can have complex constructions; for example, four short honks followed by a long one mean "open your eyes" to warn someone who is not paying attention.
In Pittsburgh, people tend to honk before going through a short, narrow or curvy tunnel. In Morocco, where I'm originally from, drivers perform varied honks when passing; they'll honk once before passing to secure cooperation, again as they pass (to signal progress), and lastly after they pass to say, "thank you." Yet this might be confusing – or even perceived as rude – to drivers in the U.S.
Written communication also plays a role between cars and drivers. For example, signs such as "Baby on Board" or "Students on Board" are supposed to encourage the drivers following these vehicles to be even more careful. Bumper stickers like "Caution: Wide Right Turn" or "This Vehicle Makes Frequent Stops" can be critical to safety.