Adapted from Guide on Evaluation and Attenuation of Traffic Noise. AASHTO. 1974.
In conclusion, as the total number of vehicles on a roadway increases, it requires more and more additional vehicles to cause a noticeable change in the overall noise level. This condition is often manifested in the results of noise impact studies for projects to widen existing highways, especially those with already substantial traffic volumes.
Source: SHA Travel ForecastingBack to Top
Sound barriers reduce the noise which enters a community from a highway by absorbing, reflecting, or forcing the sound to take a longer path over and around the barrier. Sound is energy that decreases in intensity as it travels away from its point of origin. Sound waves travel out in all directions from vehicles on the roadways. When the sound waves encounter an obstacle or barrier, some of the sound will bounce off the barrier's surface (reflective surface). If the surface is porous, where there are small cavities or holes that extend into the interior of the barrier material, a portion of the sound waves will travel inside the cavities when they reach the surface. The waves will bounce around and eventually expend all their energy. This process is called absorption. A noise barrier can achieve a 5 decibel (dBA) noise level reduction, when it is tall enough to break the line-of-sight from the highway to the home or receiver. After it breaks the line-of-sight, it can achieve approximately 1.5 decibels of additional noise level reduction for each meter (appr. 3 feet) of barrier height. To effectively reduce the noise coming around its ends, a barrier should be at least eight times as long as the distance from the home or receiver to the barrier.
Source: FHWA WebsiteMaryland SHA determines the height of proposed barriers using acoustic profiles (noise measurement equipment is used to collect noise data in such a way that it can be displayed, mapped, and then worked with) so that a 7 to 10 decibel noise level reduction can be achieved. There are no standard barrier heights since each project site has different topography that needs to be accounted for. Back to Top
Sound barriers do have limitations and are not always an effective noise abatement method. To determine if the construction of a sound barrier will provide a reasonable noise level reduction, both the distance of the impacted community from the roadway and the topography of the area are considered. Role of Distance Typically, the primary impacted residences (which we also refer to as first row residences) are within 100 to 200 feet of the roadway. Second row residences, which are also quite often impacted, are usually a next door neighbor or located across the street from a first row residence. As noise impacts and potential noise abatement methods are evaluated past second row residences, it becomes increasingly difficult to provide effective abatement. The construction of a sound barrier is not likely to reduce noise levels for residents who live far away from the roadway. Role of Topography To work effectively, the barrier must be high enough and long enough to block the view of the road from the area that is to be protected. Sound barriers do very little for homes on a hillside overlooking a road. Source: FHWA Website
As seen above, the house at the bottom of the hill is protected by the sound barrier, but the one on top of the hill (overlooking the roadway) is not. In addition, buildings higher than barriers, homes scattered too far apart, and openings in noise barriers for driveway connections or intersecting streets are not good areas for sound barriers. In some cases, SHA can offer alternatives to help reduce noise levels. These alternatives are evaluated on a case-by-case basis consistent with Federal guidelines.
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