Noise from roads and highways has long been considered a serious form of pollution. Transportation workers and even people that live close to busy roads not only face the day-to-day nuisance of loud noise but are also at risk for losing their hearing over time. How loud road noise is depends mainly on traffic volume and how fast the vehicles are moving. Trucks are notable for making much more noise than cars, so how many trucks are traveling in relation to other vehicles raises the noise level.
The volume of traffic is one factor that determines how loud road noise is. According to the Federal Highway Administration, if 2,000 vehicles per hour are traveling on a given stretch of road, the noise level will be twice as much as if 200 vehicles were counted in the same time. Speed is another significant factor. Traffic moving generally at 65 miles per hour will generate noise that is twice as much as traffic going along at 30 miles per hour.
Scientists conducted a study at Purdue University's Institute for Safe, Quiet, and Durable Highways with the idea that vehicle tires were responsible for a lot of road noise. Researchers proposed four hypotheses on how tires generated noise on the road, which include air being pushed out from between tire treads, the force of treads hitting pavement, compression of the tread block on the road, and friction. A machine was built, based on this research, which can test different types of concrete and tires to study the noise created by each combination.
Trucks are a major factor in road noise. Tires, along with engines and exhaust noise, combine to generate the sounds heard on a busy road. A single truck moving at 55 miles per hour is as loud as ten cars at the same speed, according to the Federal Highway Administration. Defective components on cars, such as mufflers and other parts, contribute to road noise. Road conditions, such as inclines, can affect noise since vehicle engines have to work harder.
Construction is also a major contributor. Construction projects on highways, streets and in buildings create noise from pneumatic hammers, air compressors, and large vehicles such as bull dozers and dump trucks. All of these factors combine in proximity to a busy road to build the noise level. The closer someone is to a heavily traveled road, the more noise they will experience. At a distance of 500 feet from a freeway, noise is typically not a big problem, while 100 to 200 feet is enough to escape much of the noise on lightly traveled roads. Any barriers, such as sound walls, buildings, and even trees and terrain will lessen the noise.
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