Some similar information is at http://www.fhwa.dot.gov/environment/htnoise.htm
What are noise barriers?
Noise barriers do not completely block all noise - they only reduce overall noise levels. Effective noise barriers typically reduce noise levels by 5 to 10 decibels (dB), cutting the loudness of traffic noise by as much as one half. For example, a barrier which achieves a 10-dB reduction can reduce the sound level of a typical tractor trailer pass-by to that of an automobile.
How Does a Noise Barrier Work?
A noise barrier must be tall enough and long enough to block the view of a highway from the area that is to be protected, the“receiver.” Noise barriers provide very little benefit for homes on a hillside overlooking a highway or for buildings which rise above the barrier.
Therefore, a barrier that is effective for someone standing on the ground might do nothing to block sound from someone at the 2nd or 3rd floor of a home where you would have a clear view over the barrier wall to the source of the noise.
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 to the barrier.
Can Trees Be Planted to Act as Noise Barriers?
Vegetation, if it is high enough, wide enough, and dense enough that it cannot be seen over or through, can decrease highway traffic noise. A wide strip of trees with very thick undergrowth can lower noise levels. 30 meters (100 feet) of dense vegetation can reduce noise by five decibels.
However, it is not feasible to plant enough trees and other vegetation along a highway to achieve such a reduction. Trees and other vegetation can be planted for psychological relief but not to physically lessen noise levels.