And wikipedia has this to say about Noise Barriers:

http://en.wikipedia.org/wiki/Noise_barrier
Noise barriers have been built in the United States since the mid-20th century, when vehicular traffic burgeoned. In the late 1960s, acoustical science technology emerged to mathematically evaluate the efficacy of a noise barrier design adjacent to a specific roadway. By the 1990s, noise barriers that included use of transparent materials were being designed in Denmark and other western European countries.^[1]Below, a researcher collects data to calibrate a roadway noise model for Foothill Expressway...