Q: I often see motor vehicles being driven in the dark, or near dark, without headlights on. My understanding is that multiple studies have shown that even during the daytime having headlights on increases safety. Why aren’t headlights required at all times?
A: If you start a petition, I’ll sign it. For the past few years I’ve been trying to make a habit of turning my headlights on every time I drive, no matter what the time of day. But I’m not perfect at it, and I’d be eternally grateful to the person who convinces the National Highway Traffic Safety Administration (NHTSA) to make this a rule for car manufacturers. (Also, I don’t think NHTSA makes decisions based on citizen petitions.)
Turning on your headlights during the day doesn’t help you see better, but it helps other people see you. When we can spot other vehicles earlier, it gives us more time to respond and avoid a collision.
That’s why using your headlights during daylight reduces your odds of a crash. Multiple studies evaluating the effectiveness of daytime vehicle lighting found reductions in crashes for vehicles that use headlights or Daytime Running Lights (DRLs). The decreases range from about five to fifteen percent for multi-vehicle daytime crashes. There was an outlier; one of two studies by NHTSA found that the reductions were not “statistically significant.” It’s a tricky thing to evaluate, but if 25-plus studies spread over several countries and decades get similar results and one doesn’t I’m inclined to go with the group.
I’ll make a note here about DRLs. They’re kind of like extra-dimmed always-on headlights, but with a caveat. When you turn on your headlights, your taillights come on too. With most DRLs the taillights remain off. At night in well-lit areas drivers might not realize they’re operating only on DRLs. Other drivers will though, when they are surprised by the rear of an unlit vehicle in front of them.
We could make a law that requires drivers to use their headlights day and night, but why create a law that then requires enforcement, when we could make it automatic? If the goal is greater visibility, requiring humans that consistently forget things to do another thing can’t compete with making headlights operational at all times. That’s also the choice that Canada and over half of European countries have made.
On a related tangent, we could also build cars that never exceed the speed limit. I know there would be a lot more resistance to that idea, compared to always-on headlights, but it’s a rational idea. If you’re part of the 50 percent of adults that don’t like that idea, keep in mind that the positive data I’m about to share pales compared to the impact of eliminating speeding.
Returning to headlights, let’s look at the costs (both economic and human) of not using daytime lighting. I was well on my way to turning this into one big math problem, so I’ll leave out the equations and get to the results: a five percent reduction in multi-vehicle daytime crashes (the most conservative estimate in the studies) would save about $8.5 billion a year. And that’s not the important part. That reduction works out to over 300 deaths and 30,000 injuries prevented. That’s a pretty good reason to turn on your headlights during the day. To anyone who counters, “yeah, but driving with your lights on uses more gas.” That’s true, but just barely. The typical driver would spend about an extra penny a day on fuel costs. Until they all come on automatically, let’s turn on those headlights.
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