Removing Your Sunglasses Can Save Your Life
Learn why driving with shades is a really bad idea
Posted Feb 14, 2018
Have you ever slammed on the brakes to avoid hitting a car that abruptly braked in front of you, or even a pedestrian suddenly darting into the road?
Ever swerved to avoid a deer or piece of junk on the highway?
I have, plenty of times. Almost always, I have stopped or swerved in time to avoid disaster.
But not always. Once, driving through Kansas, not one, but two deer leapt in front of my car, shattering my windshield, totaling my car and nearly killing me. Another time, driving home in a thick fog from a ski trip in Lake Tahoe, I plowed into a stopped car, which had in turn plowed into another stopped car that had hit a semi rig in a massive, 40 car pile up. Naturally, the car behind me crashed into me as well. Then there was the night that a wooden loading pallet shook loose from its bindings and fell off of a truck in front of me. I swerved almost in time, but ended up just clipping the pallet with my right front tire, blowing the tire out, losing control of the car, and smashing head first into a pylon.
Thank god for airbags!
Looking back, the thing that has struck me (as it were) about all of these near misses and disasters was how incredibly thin the margin between a collision and a non-collision was. When I did manage to react in time, the margin of safety was usually just a few inches. Similarly, when I didn’t avoid trouble, as with the wooden pallet, an inch or two would have saved me, if only I’d had that inch or two.
This brings me directly to the subject of wearing sunglasses while driving.
Don’t ever do it.
Well, whenever you wear sunglasses, which typically throw away about 90% of incoming light, you are slowing down the conduction velocity of visual signals traveling from your eye to your brain.
It turns out that throughout your nervous system, the less intense a neural stimulus is, the slower it will travel from Point A to Point B. For instance, sunglasses that block 90% of incoming light slow the propagation of visual information from your eye to your brain by about 15 milliseconds.
If you don’t believe me, grab a friend, a ruler and a pair of sunglasses, and perform the simple, fun experiment at the end of this blog.
Although 15 milliseconds may not sound like much, some quick math reveals that this span of time equates to a travel distance of about 1.66 feet at a speed of 75 miles/hour. That’s right, when you drive at 75 miles an hour—as I was when I hit the wooden pallet on the freeway--you are covering 110 feet every second, so a 15 millisecond delay equates to 1.66 feet.
I estimate that, when I hit the ill-fated pallet, only a couple inches of my right tire clipped it, so if I had been able to subtract those inches from my reaction time, I would have missed the pallet entirely and kept my airbag in the steering column where it belonged. The accident was at night, when the brightness of the road and pallet were much reduced, so, if the pallet had fallen during daylight hours, I almost certainly would have missed it.
So here’s the bottom line. Sunglasses look cool and reduce glare of strong sunlight. But if you wear them while driving they can also reduce your lifespan.
Have your friend drop the yardstick between your outstretched palms, as shown below and record how many inches of the yardstick pass through your hands before you grasp it. Your hands must be separated the same distance before each trial, and the yardstick must start at the same height. After three trials, put on your sunglasses (or lower the room lights until you can just barely see the yardstick.) Then repeat the experiment while wearing your sunglasses (or in the dark). More of the yardstick should pass through your hands before you grasp it in the dark, because visual signals travel to the brain more slowly in the dark than in full brightness.
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The magnitude of the Pulfrich stereo-phenomenon as a function of binocular differences of intensity at various levels of illumination. Am. J. Psychol. 62:159-181.
Rogers B.J. Anstis S.M. (1972) Intensity versus Adaptation and the Pulfrich Stereophenomenon Vision Res. 12:909-928.
Williams JM, Lit A. (1983) Luminance-dependent visual latency for the Hess effect, the Pulfrich effect, and simple reaction time. Vision Res. 23(2):171-9.