At night, it’s very dangerous for you to drive with one headlight. Do you know why?

It’s primarily not because you can’t see as well, although that does make it slightly more hazardous, especially if you drive too fast. At any speed over 100km/h, you are driving ‘beyond your headlights’. This means that, even if you have an excellent reaction time and good brakes, you will not be able to stop in time to avoid hitting anything that appears in your headlights; your vehicle is moving too fast to come to a stop in the distance illuminated by them. That’s one reason more deer are hit at night. Having only one headlight doesn’t decrease this distance, but it illuminate’s, less of the road making it less likely you’ll see the deer.

The primary reason why having only one headlight can be fatal is less obvious. It involves the way your brain perceives things it can’t make out clearly. For example, supposing you're driving at night, and you see light on the road in the distance:


As an experienced driver you are conditioned to interpret a vehicle approaching. The vehicle is in the distance because the two headlights appear as a single light source. Your eyes aren’t able to distinguish two lights, so your brain assumes they’re there, but still far away.

As the car gets closer, your eyes will confirm what your brain ‘expected’; the single light divides revealing a car getting closer.


Now let’s look at a different situation. Here’s a similar light in the distance:


Your brain assumes it’s a car in the distance but actually it is a car with one headlight, and it’s much closer than your brain assumes.


The danger lies in the fact that your brain interprets the single headlight as a pair of headlights at a distance. As a result, you will assume that you have plenty of time to make a left turn. In reality, the oncoming car is much closer than it appears, and if you were to stop and turn, you could easily collide.


So get that headlight fixed!