There are many factors that affect sound levels. Sound levels are influenced by the distance and the path traveled between the source of the sound and receptor (person). There is a natural reduction of sound levels with increasing distance between the source and the receptor. The type of ground cover between the source and the receptor also affects sound levels by disrupting or deflecting the sound wave. Highway-generated sound levels will decrease with the distance from the roadway. This reduction will vary depending upon the ground type (hard or soft). Hard ground (e.g., pavement, dirt, or water) will decrease sound levels by 3 decibels for each doubling of distance. Soft ground (e.g., grass, trees, or vegetation) will decrease sound levels by 4.5 decibels for each doubling of distance. Obstacles between the source and the receptor, such as buildings, hills, and trees, will result in additional noise reductions depending upon their size, density, and location.
Sound can best be expressed on a logarithmic basis. Consequently, sound levels cannot be added by ordinary arithmetic means. Sound levels from two equal sources will result in a total increase of 3 decibels. For example; two cars, each generating 60 decibels of sound, will result in 63 decibels of sound at a receptor, not 120 decibels.
Additionally, highway noise levels constantly vary with the number, type, and speed of the vehicles that produce the noise. When conducting measurements of highway noise, it is necessary to account for these varying noise levels. The most common way to account for the time-varying nature of sound is through a measurement known as Leq. Leq averages background sound levels with short-term transient sound levels to provide a uniform method for comparing sound levels over time.
As characterized in the figures below, noise levels from highway traffic are affected by three factors: (1) the volume of the traffic, (2) the speed of the traffic, and (3) the number of trucks in the flow of traffic. Generally, the loudness of traffic noise is increased by heavier traffic volumes, higher vehicle speeds, and greater numbers of trucks. Vehicle noise is a combination of the noise produced by the engine, exhaust, and tires.
Other conditions will also increase traffic noise levels. For example, a steep incline along a roadway will cause heavy laboring of motor vehicle engines.
How Speed Affects Traffic Noise
 Traffic at 65 miles per hour sounds twice as loud as

traffic at 30 miles per hour
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How Traffic Volume Affects Noise
 2000 vehicles per hour sound twice as loud as

200 vehicles per hour
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How Trucks Affect Traffic Noise

One truck at 55 miles per hour sounds as loud as

28 cars at 55 miles per hour
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Source: http://www.fhwa.dot.gov/environment/noise/index.cfm
The time period used for highway noise analyses is typically one hour. Usually, the noisiest hour of the day or night along a highway occurs just before or after the peak hours when the vehicle volumes, speeds, and the truck to auto ratio are in a combined optimum condition to yield the highest hourly noise level. The noisiest hour typically does not occur during the peak traffic hour because the peak hour will have the highest traffic volumes resulting in slower speeds and, therefore, lower highway sound levels. In accordance with FHWA Noise Policy and Guidance, a one-hour Leq is used for assessing highway noise impacts on different land uses.
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