When is dusk and when do you need to turn on your headlights? Let’s discuss
Q: I recently found out that headlights are required 30 minutes after dusk. Wouldn’t it make more sense to require headlights before it gets dark instead of after? And if someone doesn’t have their headlights on at night is it OK to flash my lights to let them know?
A: You’re absolutely right about it making sense to turn headlights on before it gets dark. And based on the word count in your question, you’re roughly 95 percent right about the law. The law doesn’t use the word “dusk,” it uses “sunset.”
On a side note, wouldn’t it be great if our correctness was judged on how many words we got right instead of the overall level of accuracy of a statement based on one incorrect word? OK, maybe that’s a terrible idea, but anyone with good grammar could be factually wrong a lot and still have a high correctness score.
While we often use “sunset” and “dusk” interchangeably, they’re actually two different times. If you doubt that, just ask an astronomer, a photographer or the law. Sunset is the moment when the top edge of the sun meets the horizon. (Astronomers will tell you that sunset is when the geocentrical center of the sun is 50 minutes of arc below the horizon, but they’re just showing off their big, sciencey words.)
Dusk is when it actually gets dark. In between sunset and dusk is twilight. We have twilight because after sunset, sunlight reflects off our atmosphere to illuminate the earth until the sun has moved (I know it’s the Earth that’s actually rotating, but from our perspective) more than 18 degrees below the horizon. Twilight is also the name of a book (and movie) that caused the town of Forks to be inundated by teenage girls hoping to spot a vampire.
Twilight is broken down into three phases: civil twilight, when it’s still bright enough to see without additional illumination; nautical twilight, when we lose the ability to see details and objects look more like silhouettes; and astronomical twilight, which to most of us looks just like night but isn’t quite dark enough for astronomers yet.
There are also three kinds of dusk, each corresponding with the respective phases of twilight. Civil dusk, the transition from civil twilight to nautical twilight is what we normally think of as dusk. But if you ever overstay your welcome in a park the closes at dusk, tell the park ranger you thought the sign was ambiguous and that you interpreted it as astronomical dusk. Then tell me how that works out for you.
Around here, civil twilight lasts about 40 minutes, so the law requiring headlights to be on 30 minutes after sunset actually makes some sense. There’s more to the headlight law though. In addition to turning them on before dark, headlights are required any time conditions are such that you can’t see a person or vehicle from at least 1,000 feet away. Since people are harder to see than cars, I’d interpret that to mean that if you can’t see a pedestrian from 1,000 feet away, you need to turn on your headlights.
Now that I’ve given you a science lesson and an explanation of the law, let me tell you why that’s all semi-useless. First of all, who checks the time of sunset before driving in the evening? And who is a good judge of 1,000 feet while driving? It’s hard to judge 1,000 feet while standing still. Thirty minutes after sunset, seeing 1,000 feet — these are rules that don’t help much.
Instead, I’ll propose that we turn our headlights on all the time. We need headlights on at night so we can see where we’re going, but even during the day driving with lights on makes it easier for other people to see you. If we’re thinking holistically about driving we need to consider not only what helps us to drive safely, but also what helps other road users too.
Not OK to flash your lights
And what about part two of your question; is it OK to flash your lights at night to let someone know their headlights aren’t on? Washington doesn’t have a law that specifically prohibits flashing your high beams, but it’s illegal to have your high beams on within 500 feet of an approaching car, essentially making flashing of high beams illegal at any distance close enough to where the person flashed would understand that it was intended for them. Briefly turning your headlights off is also illegal because they’re required at night.
Should you do it anyway to help someone out? I’ll let you decide what rules you’re willing to break to help another driver. But practically speaking, I’ve tried it and it doesn’t generally work. As it turns out, headlights are great for illumination but lousy for communication.
This story was originally published January 27, 2020 at 5:00 AM.
