A funny thing happened on the way to the mall the other day. Coming down a two-lane rural highway, an approaching driver flashed his headlights at me. My first instinct was that my high beams were on—but it was a sunny day. A few seconds later, it dawned on me that the burst of illumination was meant as a warning: Just around the upcoming bend sat a local trooper on the grassy shoulder.
As it happened, I was more or less going the limit, so this was moot. But the experience stayed with me as the miles wore on. Firstly, I couldn't recall the last time I had seen such a warning, which I thought another victim of the Internet age, given over to things like Trapster.
Secondly, I was curious as to why anyone would bother: It takes time and effort to warn strangers about a speed trap—all for the sake of helping someone whom you will not see again and who cannot return the favor. It's an interesting question for evolutionary biologists; a form, perhaps, of what Ernest Fehr and his colleagues have called "strong reciprocity" (PDF)—one of those curious "forms of human behavior involving interaction among unrelated individuals that cannot be explained in terms of self-interest." This makes sense when each person's sacrifice in time and effort goes toward the collective (and thus his own) good. But what about when the helping behavior comes at a potential cost to group fitness—by ensuring a higher supply of speeding drivers on local roads? (One also wonders about those people who are tweeting DUI checkpoint locations.)
What really intrigued me about the episode, however, was the existence of this informal language of road signals—either creative adaptations of the simple communicative tools available on an automobile (lights, turn/brake signals, horn) or, often, some gesture by the driver himself (a wave, "the finger," etc.). How do these things emerge, how are they transmitted, and how are they understood?
The headlight flash for the speed trap seems, like most informal signals, to be of rather obscure origin. One source speculates that the gesture came into widespread practice only with the advent of a steering-column-mounted headlight control in the late 1960s, which made it simpler to "flash" than the old floor-positioned switch. Another idea is that headlight flashes (as well as some other signals) are introduced by people—like truckers—for whom the road is a job site, and so they are born as a kind of workplace lingo. Take, for instance, this account of a Greyhound bus driver in the New York Times in 1975: "Traveling west, the sun was level with the windshield, yet Al spotted a radar signal from an oncoming truck. This section of the turnpike was notorious for speed traps. 'Did you catch that?' I hadn't. 'To signal radar, first flash your headlights then make a circular motion with your hand.' " Informal road signals, then, are seen as a kind of secret code, like gang signs or Masonic hand symbols, which only certain groups understand. (The paranoid conclusion of this may be the "gang initiation killing" rumors of a few years back, in which motorists were warned that flashing their lights at another car would trigger a homicide.)
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