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To Dim the Headlights or Not to Dim: What's in It for Me?

A reader named Linda Cass asks an interesting question that pokes into the self-interest/fairness/altruism area we’ve been writing about lately:

I’m an Englishwoman living in France but I should think the system of dipping one’s headlights for an oncoming vehicle is pretty much common in the U.S. as well.
I keep wondering why so few people fail to do this. There is no incentive to do this except for joining in the habit and hoping that everyone coming towards you does it too. There is no punishment if you don’t do it — I can’t really see a police car doing a U-turn and chasing you.
So why do 99% of people do it?

Let’s assume that Linda is correct, that the vast majority of drivers (if not quite 99%) do dim their brights. I do not have the world’s best nighttime vision, despite a lifetime of heavy carrot intake. (According to, the carrot/vision belief is pure myth, but a fantastically interesting one, derived from a British military lie during World War II meant to cover up the use of radar to intercept German planes.) So when I drive at night, especially on an unfamiliar road during the rain, e.g., there’s nothing I’d rather do than not dim my lights. And yet I do. Why?
Am I worried the oncoming car is a police car that will make a U-turn? Am I trying to avoid the little burst of shame that arrives when the oncoming driver flicks his brights at me? And if so, isn’t it amazing that a social convention like this one can survive on such a fragile sentiment?
The fact is that driving at night is dangerous. (If you ask Google to find you “nighttime auto accidents,” you’ll get lots of injury legal firms; but if you ask Google Scholar, you’ll learn about the proliferation of drunks, car thieves, and drag racers at night.) What I’d like to know is whether the benefit of dimming your headlights – that is, the benefit of not blinding the oncoming driver – is indeed larger than the benefit of keeping your own brights burning?