Traffic sounds from highways are another major form of modern-day noise pollution that scientists are seeking to control. At the Institute for Safe, Quiet, and Durable Highways, a U.S. agency created in 2000 at Purdue University in West Lafayette, Indiana, researchers are attempting to improve on the wooden barriers that are used along many highways to cut down on noise. Such barriers cost as much as $1 million per mile to construct, but they block only a fraction of the noise generated by cars and trucks.
Highway noise is caused largely by the vehicles' tires. Air becomes trapped between a tire and the road, then bursts from the confines of the treads, producing a variety of pops and whistles. Also, the blocklike shapes in the tread design strike against the road surface like hammers. Adding to the din, the tire treads and the steel belts inside the tires vibrate as the tire rotates, creating even more noise.
One technique for quieting highway noise is to use a coarser, more porous pavement surface. Compared with conventional pavement, the porous variety—made with larger chunks of rock—is less smooth. The irregular surface actually dampens sound waves by trapping them in the pavement. Porous road surfaces, already in wide use in parts of Europe, have been shown to reduce traffic noise by about 6 to 10 decibels, compared with typical pavement.
The sound-deadening qualities of coarse pavement can be destroyed, however, by the dirt, sand, and other debris that inevitably accumulate on a road surface. These drifting materials fill in the gaps of the irregular surface, making it as smooth—and as noisy—as conventional pavement. To counteract this problem, highway engineers in the United States have been developing pavement consisting of a layer of fine-grained porous pavement 2.5 centimeters (1 inch) deep on top of a thicker layer of coarser-grained porous material. The sound waves penetrate the fine layer and are trapped in the coarse pavement below, preventing them from reflecting up from the road. Since the coarse layer would not be exposed to weather, the engineers expected that it would retain its noise-reducing properties longer.
Rubberized asphalt is also being studied for its noise-reducing properties. This type of pavement is made of asphalt mixed with rubber from recycled tires. Initial test results show that rubberized asphalt is about 50 percent quieter than conventional asphalt. Each year about 190 million tires are discarded in the United States and are piled up in huge mounds in landfills. According to the Rubberized Asphalt Concrete Technology Center, an industry trade group, each kilometer (0.6 mile) of a four-lane highway with a rubberized-asphalt surface 5 centimeters (2 inches) thick would require an amount of rubber equal to more than 5,000 waste tires.
(Source: How stuff works?)