The Twilight Zone: Headlights

Q: It’s getting dark earlier now, and I’m seeing drivers without their headlights on, especially when it’s not all the way dark, but more like dusk. At what point are you required to turn on your headlights?

A: If you’re going to the beach to watch the sunset, it’s not critical to know the exact moment that the sun sets. You just want to be there to see the colors. But when complying with the law is tied to the sunset, it would help to be precise. So let’s nerd out on atmospheric science.

First, the law (or more accurately, the first part of the headlight law): Drivers are required to turn on their headlights from 30 minutes after sunset until 30 minutes before sunrise. When I first read that I thought there must be some mistake. Once the sun has set, isn’t it dark? Why would the law allow us to drive in the dark for 30 minutes?

There was no mistake – I just didn’t understand what sunset really is. Officially, It’s the moment that the top edge of the sun disappears below the horizon. From that moment until six degrees below the horizon we’re in civil twilight. During that time the sun’s light is still reaching the sky above us, so it stays bright enough to see without artificial light. Civil twilight in Washington lasts for 30 to 40 minutes, depending on the time of year.

The end of civil twilight marks the beginning of dusk. Or more accurately, civil dusk. At this point, we start needing additional light to see clearly. We move through nautical twilight (and nautical dusk), astronomical twilight (and astronomical dusk), finally reaching full-on darkness. If you’re driving in an area with streetlights, headlights, and other artificial light, you might not realize you need your headlights until well into dusk. And even if those sources provide enough light to see where you’re going, you still need headlights. They’re not just for you to see others; they also help others see you.

But, and this probably obvious, you don’t have to wait until the law requires it to turn on your headlights. For safety, keep them on all the time. A study by the National Highway Traffic Safety Administration (NHTSA) found that daytime running lights (DRLs), which are like headlights but a bit dimmer, reduce head-on fatal crashes by five percent, fatal crashes with pedestrians and cyclists by 12 percent, and fatal crashes with motorcyclists by 23 percent.

The second part of the headlight law requires drivers to turn on their lights “at any other time when, due to insufficient light or unfavorable atmospheric conditions, persons and vehicles on the highway are not clearly discernible at a distance of one thousand feet ahead.” This time of year, unfavorable atmospheric conditions are a normal part of driving. And similar to dusk, you might not notice the need to turn on your headlights in a rainstorm.

For many newish cars daytime running lights solve that problem, but introduce two new ones. Because DRLs are on whenever you’re driving, you might not notice the need to turn on headlights when it gets dark, and you’ll end up driving with reduced visibility. The arguably bigger problem (at least with some cars) is that while your DRLs are on, your taillights are not. You might be able to see marginally alright, but drivers behind you won’t see you.

Where does that leave us? You could wait until nearly dusk to turn your headlights and hope you don’t forget, but the better option is driving with them on all the time.