Highway Noise Policy and Abatement Guidelines
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Highway Noise Policy and Abatement Guidelines

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    This document contains HDOT’s noise policy on highway traffic noise and construction noise. This policy describes the HDOT’s implementation of the requirements of the Federal Highway Administration (FHWA) Noise Standard in 23 Code of Federal Regulations (CFR) Part 772 (see Section 10.1). This policy was developed by HDOT and by FHWA. The level of highway traffic noise primarily depends on three factors: 1) The volume of the traffic, 2) The speed of the traffic, and 3) The number of trucks in the flow of the traffic. Generally, heavier traffic volumes, higher speeds, and greater number of trucks increase the loudness of highway traffic noise. Vehicle noise is primarily a combination of noise produced by the engine, drivetrain, exhaust, and tires. For the purpose of highway traffic noise analyses, motor vehicles are grouped into five categories: 1) Automobiles ‐ vehicles with two axles and four tires; 2) Medium trucks ‐ all cargo vehicles with two axles and six tires; 3) Heavy trucks ‐ all cargo vehicles with three or more axles; 4) Buses ‐ all vehicles designed to carry more than nine passengers; and 5) Motorcycles – all vehicles with two or three tires and an open‐air driver/passenger compartment. Noise is unwanted sound. The vibration of sound pressure waves in the air produces sound. Sound pressure levels used to measure the intensity of sound are described in terms of decibels. The decibel (dB) is a logarithmic unit, which expresses the ratio of the measured sound pressure level to a standard reference level. Sound is composed of various frequencies, but the human ear does not respond to all frequencies. Frequencies to which the human ear does not respond are filtered out when measuring highway traffic noise levels. Sound level meters are usually equipped with weighting circuits, which filter out selected frequencies. The A‐scale on a sound level meter best approximates the frequency response of the human ear. Sound pressure levels measured on the A‐ scale of a sound meter are abbreviated dB(A). The most common descriptor of environmental noise in the U.S. is the equivalent (energy average) sound level. The equivalent sound level is the steady state, A‐weighted sound level which contains the same amount of acoustic energy as the actual time varying, A‐weighted sound level over a specified period of time. If the time period is one hour, the descriptor is the hourly equivalent sound level, Leq(h), which is widely used as a descriptor of highway traffic noise. An additional descriptor, which is sometimes used, is the L10. This is the A‐weighted sound level that is exceeded 10% of the time.
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