The A-weighted decibel scale begins at zero. This represents the faintest sound that can be heard by humans with very good hearing. However, based on many tests of large numbers of people, a sound level of 70 is twice as loud to the listener as a level of 60. This principle is illustrated an the next page. Generally, the loudness of traffic noise is increased by heavier traffic volumes, higher speeds, and greater numbers of trucks.
Vehicle noise is a combination of the noises produced by the engine, exhaust, and tires.
The loudness of traffic noise can also be increased by defective mufflers or other faulty equipment on vehicles. In addition, there are other more complicated factors that affect the loudness of traffic noise. For example, as a person moves away from a highway, traffic noise levels are reduced by distance, terrain, vegetation, and natural and manmade obstacles. Traffic noise is not usually a serious problem for people who live more the.500 feet from heavily traveled freeways or more than l00 to 200 feet from lightly traveled roads. Impact Highway traffic noise is never constant.