Staying Safe As Days Darken

Now that fall has arrived and our precious daylight hours dwindle, those hoping to stay fit by walking, jogging, or cycling outdoors will have to contend with the dangerous road conditions imposed by the darkening days. In the United States, pedestrians make up 11 percent of all vehicle-crash fatalities. That's roughly 5,000 pedestrian deaths per year, or one pedestrian fatality every 105 minutes. If you include cyclists, the numbers are higher by nearly another 700 fatalities per year. Not surprisingly, the chances of ending up on the wrong end of a car bumper only increase as the days grow shorter. A 2008 report by the National Highway Traffic Safety Administration fingers autumn as the most dangerous part of the year for pedestrians, accounting for 29 percent of pedestrian-related fatalities. The most dangerous hours of the day? From about 6 p.m. to 9 p.m., the time immediately before, during, and after dusk.

Fortunately, here are ways to make us safer in any light by understanding the science of visual perception, and some companies are eager to charge a premium for clothing engineered for maximum visibility. But before you open your wallet, it helps to understand how we perceive light and colors, and what it takes to make us safest on the road.

Last month, Saucony unveiled a bright orange jacket that they claim is the most visible on the market, while specialty retailer Brooks has been selling bright yellow jackets for years. The reason most high-visibility clothing is yellow or green—and that fewer fire trucks come in red these days—is that those colors are easier to see than others, says Janet Sparrow, a professor of ophthalmology and pathology at Columbia University.

To understand why this is, consider a few basics from college physics: light can be thought of as waves of energy, where different distances between the peaks of the waves—a.k.a. wavelengths, measured in billionth's of a meter, or nanometers—correspond to different colors. And biology: of the multiple types of the color-sensitive cone photoreceptor cells in the human eye, some are in greater abundance, and have a higher sensitivity, than others. "People who have measured our wavelength sensitivity under lighted conditions"—known as photopic conditions—"find that our maximum is at about 550 nanometers," said Sparrow. "And this is yellow."

But as daylight turns to dusk, the burden of vision shifts from cone cells to rod cells—and our sensitivity to certain wavelengths of light also shifts, says Sparrow. While rod cells are supersensitive to luminance but not color, they are best at picking up light with a wavelength around 500 nanometers, better known to the layperson as green. That's why even as your ability to distinguish colors in the dark is dramatically reduced, green materials continue to stand out.

The most visible clothing appears to glow, or fluoresce, under daylight conditions, says Frank Schieber, professor of psychology, and head of the Visual Performance Laboratory and Driving Research Laboratory at the University of South Dakota. Traditional fluorescent materials do this by performing a clever trick: when sunlight hits them, a portion of the invisible, ultraviolet light (the same stuff we wear suntan lotion to protect our skin against) is absorbed and reemitted—but at the longer wavelengths humans can see. Newer fluorescent materials work the same way, but reemit low-visibility, high-energy blue light as yellow or green light. That's why a fluorescent jacket or vest appears so bright—almost as if lit up from within. And at dusk, as the visible spectrum of light shifts toward shorter wavelengths, and normal colors become less visible, fluorescent materials appear even brighter as they retain their luminance and compete against a generally dull landscape.

Fluorescent materials use short-wavelength light (UV and blue) to emit longer-wavelength yellow hues. At dusk, competing yellows and reds are less brilliant because the visible spectrum shifts away from them, and the remaining colors are not particularly sensitive to us. That means most of the colors we see at dusk are dull; powered by UV and bluish light that isn't normally very visible to us, fluorescents have no competition at this hour.

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