Noise Walls
Most noise barriers in North Carolina are made of concrete or masonry, which are durable, require little maintenance and can be designed for aesthetics. (Municipal governments can provide funding for additional enhancements.)
The N.C. Department of Transportation examines several factors when determining if a noise wall would be an effective way to reduce the noise level. These considerations involve answering several very important questions.
Was the development (home, school, etc.) permitted before the project's Date of Public Knowledge?
| The Date of Public Knowledge is both the date that the public and local government are officially notified of the future path of the road and also the date the project's final environmental document is approved.
A structure with a building permit dated after the Date of Public Knowledge is not eligible for noise abatement.
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| Will a noise wall
reduce the predicted noise level? | A noise wall might not always
decrease the noise level enough to justify its construction. |
How many people will hear a difference in noise? Does the number of people justify the cost of a noise wall?
| Sometimes, the state just cannot afford to build a wall because the cost outweighs the benefit. |
| Is a noise wall technically realistic? | Every road is different – sometimes the terrain makes building a noise wall very difficult or too costly. |
| Do most benefited property owners and tenants want a noise wall? | Commercial areas typically prefer visibility and accessibility over noise reduction. In other cases, a benefited property owner or tenant might prefer the existing view over having a noise wall in their yard. |
Qualifying for a Noise Wall
A noise wall would more likely be built in an area of high-density development than in an area of low-density development since it could reduce the noise level for many houses. A low-density area would be less likely to get a wall because its cost would outweigh the number of people who would benefit from it.
Distance is another factor NCDOT considers when determining where to build noise walls. An area farther from a road – even if it is heavily populated – would be less likely to qualify since a wall would have a minimal effect on the noise level.
Earth Berms
Earth berms are a low-cost, aesthetically pleasing alternative to noise walls that not only have the advantage of blending in with the surrounding environment but also provide slightly more noise reduction than walls.
But berms require a large amount of soil and need a lot of available land. For example, a berm 10 feet tall might be more than 4 feet wide. Most highway projects – especially in urban areas – don't have that amount of space available and buying additional right of way is expensive.
Vegetation & Landscaping
Vegetation and landscaping are not acceptable means of noise reduction and generally do not provide adequate support for reducing traffic noise levels. Vegetation must be very dense, very tall and very deep to make a difference.
Generally, it takes at least 100 feet of depth to get a 5-decibel reduction. In highly populated areas, such thick vegetation is rare.
Although vegetation does not reduce noise levels, it can "soften" the harsh tones of traffic noise as well as improve aesthetics by screening or hiding a road.
After construction, NCDOT might provide landscaping to provide a visual buffer along the highway corridor.
Federal noise regulations also contain criteria for analyzing traffic noise and determining the threshold at which a traffic noise impact occurs for different types of land uses and human activities.
The regulations do not require providing noise reduction in every instance, but they do require highway agencies to make every feasible and reasonable effort to provide reduced noise levels when the threshold noise levels are approached or exceeded.
Compliance with the federal noise regulations is a prerequisite for receiving federal-aid funds for highway construction.