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.

Fluorescent materials are highly visible, and a wise choice for the autumnal fitness buff who wants to stay safe. But be warned: fluorescent materials appear bright during the day and even brighter near dusk, but are ineffective for nighttime use. "As soon as the limiting performance is from the headlights instead of the sun, then you're going to loose all of the benefits of fluorescent colors," said Schieber. The problem is that headlights and street lamps don't provide enough of the right sort of energy to get fluorescents to glow.

At night, says Schieber, pedestrians and cyclists have to rely on artificial sources of light, such as blinking LEDs or car headlights. The latter are easy to take advantage of thanks to specially engineered, retroreflective surfaces.

Normal materials, like wool or cotton, either scatter light, or reflect it at an angle based on where the light source is coming from, much the way the angle a billiard ball bounces off the rails of the table depends on its incoming trajectory. But retroreflective materials, like bicycle reflectors and the reflective strips included on road-worker's vests, use thousands of tiny glass beads or optical prisms to reflect light back to its source—regardless of where the light came from.

"With normal headlights you can see retroreflective markings at 1,000 feet," said D. Alfred Owens, a professor of psychology at Franklin and Marshall College. That gives a driver 10 times the distance he would normally have to react to and avoid a pedestrian. But some outerwear for running, cycling, or roadside work only has retroreflective strips across the chest, back, and other relatively static locations, which isn't ideal for getting the attention of nearby drivers. If you're looking for a better solution the next time you got shopping, consider the approach that Saucony and some other companies employ: adding retroreflective material or LEDs to parts of the jacket that are constantly moving—and very catchy to a driver—like the wrists and elbows.

The key to improved safety in the future may not depend on technological breakthroughs but rather on how we use what we've already got. In the 1970s, a Swedish scientist named Gunnar Johansson made a startling discovery: because of the way our minds have evolved to quickly make sense of our surroundings, a dozen strategically placed lights, moving together in just the right way, are perceived as a human in motion. The concept, called biological motion, or biomotion, is particularly well suited for use in pedestrian safety, says Owens. (The ideal placement for these lights are seen via the animation on the Bio Motion Lab's Web site.

"If they are stationary, they are meaningless," said Owens, but as soon as the lights begin to move, "and by that I mean within 100 milliseconds—you perceive a person, you know what that person is doing. Many times you know if it is a man or a woman, if the person is young or old." In numerous studies conducted by Owens and others, drivers spotted pedestrians with strips of retroreflective tape attached to points known to invoke the perception of biomotion significantly better than pedestrians using any other approach to visibility.

Though most high-visibility clothing includes retroreflective material on the chest, and some has retroreflective material or small lights at the wrists, Owens says that he has yet to find a company manufacturing clothing designed to harness the full power of biomotion. "It could save lives," he said. And it would probably make an entrepreneur or athletics company a lot of money at the same time.

In the meantime, while psychologists, engineers, and entrepreneurs figure out how to use and market biomotion and other clever techniques to make pedestrians more visible to drivers, one of the best ways to improve your safety as a pedestrian is available immediately: changing your behavior.

Pedestrians, says Owens, dramatically overestimate the ability of drivers to spot people at night. This occurs, in part, because the pedestrian's environment is darker than the driver's field of vision—so the pedestrian's eyes are better adjusted to the darkness.

In an experiment that demonstrates the mismatch between what a driver sees and what a pedestrian thinks the driver sees, study subjects are asked to walk toward the headlights of a parked car at night, and to stop when they are sure that the driver can identify the subject as a person. "They typically estimate that they are visible at 300 or 400 feet," said Owens, adding that the drivers don't actually see the pedestrian until the pedestrian is within 100 feet. This leads to what Owens calls a doubly blind situation: "The drivers don't know what they can't see, and they can't see the pedestrians in time to do anything. The pedestrians feel as though they are highly conspicuous, so they are not as cautious, or fearful, as they should be."

The good news, said Owens, is that when Richard Tyrell, a professor of psychology at Clemsen University, included information about the disparity in driver versus pedestrian vision to lecture audiences (including teenagers in a drivers' education class), those who were exposed to the information made better estimates when given the walk-toward-the-headlights test. "The problem is cut in half: they are still overly optimistic, but they think they are visible at 200, rather than 400 feet," said Owens.

So if you're looking to pick up a new outfit for walking, running, or cycling in the darker half of the year—make sure it's made out of fluorescent material for daytime and dusk, and includes retroreflective strips (or small embedded lights)—located at as many of the dozen biomotion points as possible—for the night. And the next time you're on the road facing a car, simply having read this article may help you better estimate the distance at which the car's driver can see you—though you'll still have to divide that distance in two. And if math (or spatial relations) are not your strong suit? Maybe it's time to join the gym instead.