As with any communication, context is key. The simple headlight flash is a bullying move for a tailgating Ferrari on the Italian autostrada, a gentle reminder (or a stern rebuke, if done repeatedly) on a rural road that an approaching driver has not dimmed his brights, or a quiet alternative to a horn-honk at a stalled driver when the light turns green—not to mention a number of other reported uses, including signaling to a truck driver that he has cleared one's vehicle and can safely merge. In all these cases, the meaning of the signal seems to grow, awkwardly and organically, out of a need to convey things amid the complex interactions of traffic for which we have no "official" signals: that moment, for example, when you are trying to tell another driver that it is OK to make a turn in front of you.
Before the advent of electric signals, hand signals were an integral part of the official driving experience—an arm extended and bent downward down meant the driver was slowing down, for example. This language is long forgotten by most drivers, but a variety of unofficial signals still flourish—the most prominent, "the finger," needs no explanation. Nor does its opposite, the "thanks" wave. (There are regional variants on the latter: On narrow Seattle streets, where one person pulls to the side to allow another to pass, the wave is considered a proper reply; in Hawaii, they give the "shaka.") But there are still other situations for which people have never quite settled on the appropriate signal. How does one say that he's sorry? A correspondent to a British newspaper suggested a finger pointed to the head as a gun — I should be shot for such an epic gaffe! —but as one letters-column respondent (self-identified as the chairman of the "Polite Society") aptly noted, the problem with this "is that it can be misconstrued as meaning 'you should be shot'."
There is one other class of gestures worth noting, what I shall call the vehicle affinity experience; namely, people driving the same vehicle exchanging a wave. One of the most oft-cited examples of this is for Jeep drivers, and one strain of speculation says it emerged from soldiers waving to another in passing vehicles. Or perhaps it's because Jeep drivers were more likely to have their windows open and tops down. But the wave is hardly limited to people in Jeeps: One hears it talked about among drivers of Corvettes, Saabs, Volkswagen GTIs, Subarus, Harley-Davidsons, and others.
There seem to be two things at work here. The first is the general human bias in favor of self-similarity. Studies have shown this effect in myriad ways; people are more likely to be drawn toward those whose surnames begin with a shared letter (e.g., "B" people gave more money to Bush in 2000, "G"s gave more to Gore) and more likely to look favorably on those who share their birthday. This appears to hold true on the road: Studies have found, for example, that drivers are less likely to honk at cars of similar perceived status as their own.
Not honking is one thing, but a true vehicle affinity experience probably requires a second condition: Two drivers will exchange an outright wave only if their car in common is exceedingly rare or if it has a strong distinguishing feature or personality. (Think of the tweedy, imported cache of early Saabs or the rough-riding, free-spirited appeal of Jeep.) So, while you don't get a lot of Camry drivers waving at one another (they'd be busy, for one), the Prius has been highly wave-able. In a scene from Curb Your Enthusiasm,Larry David turns to Jeff Garlin and exclaims, "See that? I waved to a guy in a Prius and he didn't wave back!" Jeff responds: "I don't wave to people in the same car as me." Larry's reply, perhaps harkening to that idea of kinship-based altruism alluded to earlier: "We're Prius drivers, we're a special breed." But even they might have trouble trying to apologize to one another in traffic.