By Joe Nasvik
Editor's note:We always thought National Work Zone Awareness Week was aimed at the driving public.
And it is. But a coworker is just as likely as a motorist to hurt or kill someone working inside a barrier zone. More than half of jobsite fatalities are caused by equipment operators and others on the jobsite. This article explains the science — of human anatomy and brain function — that was used to develop and promulgate requirements for high-visibility safety apparel.
Not being aware of what's in front of you or to your side is the root cause of most worker versus vehicle accidents on construction sites. Part of the problem can be addressed with technology; there are many luminescent and retroreflective safety clothes and products on the market to help people and vehicles stand out more noticeably. But the physiology of how we see and perceive also plays a key role.
How the eye works
What you see in sharp focus, as well as the perception of color, is processed by a small area at the back of the eye called the fovea. This is where most of the cone receptors — sensors of color and fine detail — are located. The large area around the fovea is where peripheral information is gathered by rod sensors, which specialize in luminance differences, general shapes, and movement (the fovea also recognizes these details). This is referred to as peripheral vision.
Cones function best when lighting is bright, while rods capture detail when lighting is poor. Overall, we see much more under well-lit conditions and much less under dim lighting.
The fovea processes only about 2 degrees of our visual field. You can visualize this by holding your thumb up at arm's length: The invisible area behind your thumbnail is what the fovea perceives. Also, every eye has a blind spot where the optic nerve leaves the back of the eye. We compensate for this by moving our eyes quickly over a scene so our brain can build an image. Because of this physiology, however, we sometimes are prevented from getting an accurate view of reality.
Imagine you're driving down a road or highway, either during the day or at night, and looking at the scenery: a billboard or some movement more than 15 degrees from the road's shoulder. At that moment, your foveal vision (color and detail) of the roadway disappears — along with any construction workers or flaggers now in your path who think you see them.
The color of safety
Brian Linzie is a senior vision scientist at 3M in St. Paul, Minn., who studies how the brain works and how people think and see. He says the fovea processes color information. The color of safety garments and devices is perceived under lighted conditions.
“The two most popular colors for this use are fluorescent red-orange and yellow-green, because fluorescent surfaces appear lighter or more luminescent and these colors also contrast well with other colors,” he says. “The natural world doesn't have fluorescent colors and our attention focuses on color that stands out.”
But if everything on a jobsite were the same color, nothing in particular would stand out. “If luminescent red-orange is the color used to identify hazards, yellow-green might be a better choice for personnel — you focus attention on what stands out,” he adds.
Researchers are trying to find out if there's one color that stands out more than all the others. So far the best advice is to use colors that contrast with the background. That's why fluorescent yellow-green “pops” in rural settings, while fluorescent red-orange works well in brighter urban areas.
Night protective clothing
Just as your eyes are drawn to colors that appear more intense in the light, they're attracted to intense light under nighttime conditions. Reflectors accomplish this goal by returning much more of the light from a car's headlights to the driver than the surrounding surfaces. This makes these areas appear brighter and thus helps make pedestrians more noticeable.
There are two types of reflectors: beaded and prismatic. Beaded reflectors consist of very small glass beads that are silvered on the back side and oriented in the same direction to create a bright mirror. The beads are embedded in a plastic film to hold them in position. Companies such as 3M have been able to create bright reflectors that can be viewed from wide angles to the source of light.
Class 2 ANSI/ISEA 107 garments feature both horizontal and vertical bands of reflectors that are easily seen when headlights shine on them. But Class 3 garments perform even better because they combine reflectivity and emphasize pedestrian motion by placing reflectors on wrists and ankles — strong light in motion catches the eye the best.
If you're on the side of a road, it's natural to believe drivers will see you in their headlights. But they may not.
At night, wear reflectors on your wrists and ankles to reflect your movement. Don't rely on a white T-shirt to reflect more than a darker piece of clothing; in studies, 3M has proven that nonreflective fabrics offer little luminance at night regardless of color. In the daylight, wear colors that contrast with the environment.
Your absolute best defense, however, is to believe you aren't seen and act accordingly.
—This article is from the January 2010 issue of CONCRETE CONSTRUCTION, a sister publication of PUBLIC WORKS magazine.