If you've ever driven past those colorful balls strung up along a power line, you might have wondered about what purpose they serve—a thought that usually disappears as soon as the balls have faded from your rearview mirror. Though we sort of wish they were rogue holiday decorations local governments forgot to take down, the truth is that they’re actually used for aircraft safety.
According to electric utility company Edison International, the balls are called visibility marker balls (or just marker balls, for short), and they help make power lines more obvious to low-flying aircraft like planes and helicopters. Though you might not have realized it, you usually spot marker balls near mountain passes, deep valleys, major freeway crossings, and airports—all locations where aircraft tend to fly at low altitudes, running the risk of getting tangled up in the hard-to-see cables.
The Federal Aviation Administration regulates these marker balls and details their specifications in the Advisory Circular No. 70/7460-1L [PDF]. The diameter of the balls must be at least 36 inches on wires that cross canyons, lakes, and rivers, but the FAA allows 20-inch spheres on power lines fewer than 50 feet above ground level and within 1500 feet of an airport runway end. They should be spaced evenly at roughly 200-feet intervals along regular wires, and with less space (30- to 50-feet) intervals on wires near runway ends.
If there are fewer than four marker balls on a given wire, they should all be “aviation orange,” the fluorescent hue you probably associate with some communication towers. Otherwise, they should alternate between orange, white, and yellow to provide the highest level of visibility to approaching aircraft.
According to a 1983 article from United Press International, the marker balls first gained popularity in the early 1970s, when Arkansas’s then-governor Winthrop Rockefeller noticed electric wires whiz by as his aircraft started to land and decided something should be done to make them more obvious to pilots. The article also notes that the benefits go beyond aviation—the markers also help geese and boats steer clear of inconspicuous cables.
If you’re wondering by what magic the marker balls get installed on hard-to-reach power lines, it sometimes takes one helicopter and a very plucky technician. Check out the video below from T&D World to see exactly what that looks like. (The especially thrilling bit starts around 2:10.)
Now that one road-related mystery has been solved, find out the function of those black tubes sometimes stretched across the road.
[h/t Edison International]