What Are Those Tall Concrete Walls Along The Highway Actually For?

The tall concrete walls that run along many of our highways are highway traffic noise barriers. According to the Federal Highway Administration (FHWA), these noise barriers can reduce traffic noise by up to half. The FHWA also states that these sound-reducing benefits will be most noticeable within a distance of 200 feet from the highway, roughly the distance of the initial row of homes that would otherwise face the highway. That said, while highway traffic noise barriers can most definitely make highways quieter, they won't eliminate traffic noise. Highway traffic noise barriers also should not be confused with the green things on highway dividers, which are something completely different.

The impetus for the building of barriers along our highways is the Noise Control Act of 1972, which, according to the EPA, "[e]stablishes a national policy to promote an environment for all Americans free from noise that jeopardizes their health and welfare." Highway noise is made up of three different elements. One is the sound of tires on the road, which can be affected by your tread pattern. Another is the engine and exhaust noises. Finally, there is noise produced by vehicles moving through the air. The states use complex models to calculate the present and future noise levels of a highway and determine whether noise mitigation is needed. If that turns out to be the case, then barriers will be built — but only if the local community wants them.

There are unresolved issues around highway traffic noise barriers. One is that the Federal Government transferred responsibility for noise regulation to state and local governments in 1982, even though the Noise Control Act of 1972 remains in effect. This leaves these decisions up to each state's transportation department, which must identify any potential noise-related impacts and reduce those noise levels where necessary.

Another problem with highway traffic noise barriers is the fact that they are costly to build, costing over $2 million per mile of highway. This is money that could be used for other transportation-related needs within a given state. There's also the problem that highway traffic noise barriers can't eliminate highway noise. While it is possible to reduce noise levels by 70 to 90%, anything more is just about impossible. A study published in Science of The Total Environment has also found that, under certain conditions of high perpendicular wind speeds, the air quality on the residential side of the barrier can worsen.

Highway noise barriers may not help everyone, either: homes in elevated locations may end up receiving more noise after the building of noise barriers. This is because the barriers can channel highway noise to homes at a higher elevation. Highway traffic noise barriers must be the right length and height to block highway noise from reaching all nearby homes. This is not always possible or economically feasible, depending on the homes' locations and how far apart they are.

People who live next to busy highways are exposed not only to highway noise but also to the air pollutants and the negative health effects that result from this exposure. A 2023 study in the Journal of Transportation Research estimated that around a quarter of the total U.S. population lives near highways with high traffic density, with Nevada recording the highest state total at 39.4%. The District of Columbia, which is not a state, came in even higher at 61.2%.

There are currently over 3,000 miles of U.S. highways that use high concrete walls to keep traffic noise away from adjoining neighborhoods. These can have a positive effect, with a working paper by the National Bureau of Economic Research finding that housing prices rise when traffic noise is reduced, with the largest increases noted for homes within 328 feet of a barrier. 

The benefits of highway noise barriers have led to new approaches to barrier construction, including a hybrid design that combines the durability of a solid concrete or steel barrier with the sound-absorbing properties of materials such as fiberglass, Rockwool, and foam. But there may be other saviors, too. Highway noise levels should decrease as the percentage of electric vehicles on the roads steadily increases, since one of the many advantages EVs have over combustion vehicles is that they don't produce any engine or exhaust noise.