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Why must highway noise barriers be placed close to either the road or nearby residents?

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Highway noise barriers must be placed as close as possible to either the noise source (the road) or the observer (nearby residents) to maximize their effectiveness in mitigating noise pollution.1

This placement is critical because of the physical phenomenon of diffraction, which allows sound waves to bend around obstacles. Because road noise—particularly from large vehicles—is often dominated by lower frequencies that diffract more easily, a barrier must be positioned to force the sound to undergo the greatest possible degree of diffraction to reach the observer. By placing the barrier near the source or the observer, the path the sound must travel to bend over or around the barrier is lengthened, thereby reducing the noise level reaching the residential area.1

Additionally, for these barriers to provide significant noise reduction, they must be constructed to limit the amount of sound that is transmitted directly through the barrier material itself.1