The walls you see along highways serve several different purposes, but the most common ones are noise barriers designed to shield nearby neighborhoods from traffic sound. Others hold back soil on steep grades, protect wildlife from crossing into traffic, or manage water runoff. The type of wall depends on what problem engineers needed to solve at that specific stretch of road.
Noise Barriers Are the Most Common
The tall, flat walls running alongside highways through residential areas are almost always noise barriers. Highway traffic, especially at high speeds, generates a constant roar that can make life miserable for people living nearby. These walls work by blocking the direct path sound travels from the road to homes, yards, and schools.
The average noise barrier in the United States reduces traffic sound by about 7 decibels. That might not sound like much, but sound works on a logarithmic scale. A 5-decibel reduction removes 68 percent of the sound energy reaching you, which is immediately noticeable. A 10-decibel reduction, which some taller or better-positioned walls achieve, eliminates 90 percent of the sound energy and cuts perceived loudness roughly in half. Simply blocking your line of sight to the highway typically gets you that initial 5-decibel drop, which is why even modest walls make a real difference.
Federal regulations require highway agencies to consider noise barriers when a new or expanded road project would push noise levels past specific thresholds. For residential areas, that trigger point is 67 decibels, roughly the volume of a loud conversation. For hotels, offices, and restaurants, the threshold is higher at 72 decibels. A noise increase of just 5 to 15 decibels over existing levels from a highway project can also qualify as a significant impact requiring some form of noise reduction.
What Noise Walls Are Made Of
About 80 percent of noise barriers in the United States are made of concrete, typically precast panels that can be installed relatively quickly. Concrete is heavy and dense, which makes it excellent at blocking sound, and it holds up well against weather and time. The remaining 20 percent use materials like metal, wood, plastic, fiberglass, or even mounded earth (called berms).
You may have noticed that some walls have a rough, textured surface while others are smooth. This isn’t just decorative. Absorptive walls, often made with porous concrete or filled fiberglass panels, soak up sound energy rather than bouncing it back. Reflective walls with smooth surfaces simply deflect the noise, which can sometimes redirect it toward homes on the opposite side of the highway or increase noise levels for drivers. Adding sound-absorbing material to the upper portion of a barrier measurably improves its performance. In areas where highways are flanked by neighborhoods on both sides, absorptive walls are the better choice.
Occasionally you’ll spot transparent walls made of clear acrylic or polycarbonate panels. These are used where blocking the view would be undesirable, such as along scenic routes or where residents want to preserve sightlines. They block sound effectively but tend to be more expensive and require more maintenance to stay clear.
Retaining Walls Hold Back Earth
Not every wall along a highway is about noise. Many of the shorter, angled, or terraced walls you see are retaining walls, and their job is purely structural. Highways frequently cut through hills or rise above the surrounding terrain, creating elevation changes that would collapse without support. When there isn’t enough space to grade a gentle slope (common in cities where land is tight), engineers build a retaining wall to hold the soil in place.
Retaining walls are especially common near bridges and overpasses. The walls flanking a highway on-ramp as it rises to meet an overpass, for example, are typically holding back the fill material that supports the road surface. These walls also appear where highways run through hilly terrain and engineers had to carve a path through rock and earth. In urban areas with limited right-of-way, retaining walls let roads fit into spaces that would otherwise require buying and demolishing adjacent properties to create stable slopes.
Wildlife Fencing and Barriers
Some highway walls and tall fences exist specifically to keep animals off the road. Animal-vehicle collisions are a serious safety problem, particularly with large animals like deer, elk, and moose, where a collision can be fatal for both the animal and the driver.
Wildlife fencing is remarkably effective. Along the Trans-Canada Highway, fencing reduced collisions with large animals by 80 to 97 percent depending on the section studied. In Colorado, deer-vehicle accidents dropped by about 79 percent on fenced stretches, and in Wyoming, mule deer collisions fell by over 90 percent. Fencing in British Columbia that stood 8 feet high on both sides proved 97 to 99 percent effective at preventing large-animal crashes. Even smaller species benefit: along fenced sections of desert highway, tortoise deaths dropped by 93 percent.
These barriers work best when paired with wildlife crossings, either underpasses or overpasses that give animals a safe way to get to the other side. Fencing combined with underpasses reduced elk-vehicle collisions by nearly 87 percent in one study. Without a safe crossing option, animals may find gaps in the fence, which dramatically reduces effectiveness, sometimes cutting it by more than half.
How to Tell Which Type You’re Looking At
A few visual cues help you identify what a highway wall is doing. Noise barriers are typically tall (10 to 25 feet), vertical, and run in long stretches parallel to the road through populated areas. They often have decorative patterns or textures on the residential side. Retaining walls are usually shorter, angled slightly backward, and appear where there’s an obvious change in ground level between the road surface and the land beside it. Wildlife fencing looks more like heavy-duty chain link or mesh, often topped with outward-angled extensions, and tends to appear in rural or semi-rural stretches where forests or open land border the highway.
In many locations, walls serve double duty. A retaining wall that happens to sit between a highway and a neighborhood provides some noise reduction even if that wasn’t its primary purpose. And concrete noise barriers along elevated sections of highway may also function as safety barriers preventing vehicles from leaving the roadway. The walls you pass every day on your commute are quietly solving problems you might never have thought about.