If you’re a commuter or drive on the highway often, you’ve probably seen those tall wall-like structures alongside the road. You may think they’re there to prevent cars and trucks from going off the road, or to provide privacy to nearby neighborhoods. But that’s actually not their main purpose.
It all started back when the 1958 Hollywood Bowl was drowned out by nearby highway noise. Ever since, tall barriers made out of noise-cancelling materials were constructed along the busiest areas of America’s highways to prevent the noise from interfering with surrounding areas. There are now 2,748 miles of these walls across the U.S.
According to the Federal Highway Administration, barrier walls can reduce highway noise by almost half. The structures are typically made out of wood, concrete or brick—materials that absorb or deflect sound.
One critical aspect of the construction of these wall is their height. They’re built high enough to be over the line of sight, usually much taller than any car or truck. Building them up this high reduces sound waves by about 5-10 decibels. Human ears can’t usually notice a difference below 3 decibels, so the taller the better!
The barriers are most effective when they are built within 61 meters of the road, which is why they can seem like safety barriers. They also cannot have any openings, or else the noise will leak through and the wall will be ineffective.
The FWHA also puts aesthetic standards on the noise reducing walls. They “must be designed to be visually appealing [and] must be designed to preserve aesthetic values and scenic vistas,” according to the organization’s website. This is done to keep the natural landscape that typically surrounds highways intact and make them pleasant for surrounding residential communities as well.