What Those Ugly Concrete Walls Along The Highway Are Really Designed To Do

Ever noticed those 8-to-20-foot-high concrete walls that stretch almost endlessly along busy patches of highways? They may seem like barriers meant to prevent your car from careening uncontrollably past, but that's not their primary purpose. Although sometimes made from wood or composite materials besides concrete, their main function has more to do with shielding nearby residential areas, office buildings, and wildlife from harmful noise pollution.

In other words, highway walls are actually sound barriers to deflect or block the sounds of honking, revving, and tire or wheel bearing noises from speeding vehicles. And while they may seem like ordinary walls, there's a bit of science behind their dreary existence.

American highways got their first taste of sound buffers in the 1970s, but there's a bit of controversy regarding their effectiveness in keeping the peace, with some people contending they can actually make the sound louder in adjoining areas. However, the Federal Highway Administration (FHWA) says field studies have disproven those claims. It adds that noise barriers made of wood, concrete, composite, masonry blocks, exposed aggregates, metal, and acrylic or plexiglass panels are all effective in filtering unwanted noise.

According to the Center for Environmental Excellence, a joint venture of the FHWA and the American Association of State Highway and Transportation Officials, traffic noise boils down to three factors: volume, vehicle speed, and the number of trucks at any given time. The highway is perceivably noisier if more vehicles and trucks are going at higher speeds, and the noise typically emanates from the engines, exhaust, and tire roar.

For instance, the Center says, vehicles traveling at 65 mph are twice as loud as at 30 mph, and a single truck at 55 mph makes more highway noise than 28 cars at 55 mph. It's why highway walls are placed on high-speed, high-volume sections of the highway.

It may not seem like it behind the wheel of your trusted jalopy, but highways can get as noisy as standing beside a leaf blower, which is around 70 to 80 decibels during the noisiest times, just before or after rush hours. According to the FHWA, noise barriers can reduce noise levels by 5 to 10 decibels, which is enough to make a tractor-trailer sound like a smaller vehicle from the other side of the wall. 

Thankfully, Americans and the rest of the civilized world are not like the French, who resorted to building cement walls to protest the construction of new highways instead of making the place quieter. 

Unsurprisingly, these buffers along the highway have other purposes than reflecting, diffracting, and absorbing noise, thus masking the sound of tire roar, screaming engines, and iconic exhaust notes. Their height and width could inhibit people or wildlife from crossing over, which makes the stretch of road safer for motorists. 

While the walls aren't designed to serve as crash barriers, they may prevent debris from flying over in the event of a crash. And from the other side, those walls block visual pollution while enabling homeowners to have a quieter living space, all while helping retain the value of their real estate.

Of course, you should keep your eyes on the road even if you come across some visually interesting highway walls  with murals, fancy textured patterns, or plants. What's clear is they're here to stay, and we'll see more of them as the urban landscape expands.