Urban landscape design handbook

From chapter IV: Creating the illusion of privacy and tranquility

Tip #6: If you build a gigantic wall alongside the highway, the residential neighborhood closest in proximity to the highway will feel protected and private and will therefore believe that the roaring sound of the highway below will somehow stay below the wall and if it intermittently creeps over the wall, it will resemble an ocean beach with crashing waves simply because one doesn't have a visual of the origin of the noise and can therefore use one's imagination.