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.
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.