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Traffic Noise
The Problem with Traffic Noise
In the past few decades the problem of noise in our environment has increased to the point that it has become a major concern to both public citizens and state and federal officials. Highway traffic noise has become a dominant noise source in both our urban and rural environments.

In response to this concern, the West Virginia Division of Highways is committed to providing effective technology and solutions in dealing with the undesirable noise problem associated with highway traffic.

In 1969, the National Environmental Policy Act (NEPA) provided some legal authority for dealing with highway traffic noise. A more important Federal law that specifically involved mitigation of highway traffic noise is the Federal-Aid Highway Act of 1970. This law authorized the Federal Highway Administration (FHWA) to develop noise level standards and regulations for highway traffic noise in federally funded highway projects. The FHWA regulations for the mitigation of highway traffic noise are contained in Title 23 of the United States Code of Federal Regulations Part 772 (23 CFR 772), entitled "Procedures for Abatement of Highway Traffic Noise and Construction Noise."

Fundamentals of Sound and Noise
Sound is all around us. It can take the form of leaves rustling in the wind, a ringing alarm clock or the near invisible movement of a speaker in a radio. Sound is produced when an object moves or vibrates. If you throw a pebble into a pool of water, you will notice small ripples or circular waves form and travel outward from the point where the pebble struck the water. Sound also travels in waves. When an object vibrates back and forth, it causes the vibration of tiny particles in the surrounding air to ripple outward in waves called sound waves. When these vibrations (or sound waves) reach our ears, we hear what we commonly call sound. The rate at which the object vibrates is called its frequency. The frequency of the vibrating object determines the frequency or pitch of the sound. Human ears can only hear sound waves with a frequency or pitch between approximately 20 to 15,000 cycles per second.

Our environment surrounds us with sound, but most of us would probably not say that we are surrounded by noise. So what is the difference between sound and what we call noise? Noise can be defined as unwanted or undesirable sound. Since highway traffic sound is normally unwanted, highway traffic sound is usually called highway traffic noise. Traffic noise is a combination of noises produced by the engines, exhausts and tire-road interactions from the various vehicles using a highway. There are other factors that can affect the level of traffic noise. Distance from the highway, ground terrain, vegetation (trees) and interference from natural and man-made objects all play a role in determining the level of traffic noise at a given location.

Scientists monitor sound by using instruments called sound level meters or SLM's. These meters measure the different sound levels over a specified period of time. Sound is usually described in units called decibels (dB.) Often, an adjustment of the high and low pitched sounds is made to approximate the way we typically hear sounds. These adjusted sounds are referred to as "A-weighted levels" (dBA.) Many highway agencies (including WVDOH) use a traffic noise descriptor known as the hourly equivalent sound level or Leq(h.) The dBA scale ranges from 0 dB, which represents the quietest sound that can be heard by people with normal hearing, to about 120 dB, which for most humans is the loudest sound that can be heard without pain. Some examples of common indoor and outdoor sound levels are shown below:

Common Indoor & Outdoor Noise Levels
Subjective Evaluations

Source: Michael Baker Jr., Inc.

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