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Analysis and Abatement Guidance

Three-Part Approach to Highway Traffic Noise Abatement

Effective control of highway traffic noise requires (1) control of land use planning adjacent to highways, (2) quieter vehicles, and (3) when feasible and reasonable, abatement of highway traffic noise for individual projects.

The first component is traditionally an area of local responsibility. The other components are the joint responsibility of private industry and of Federal, State, and local governments.

Noise Compatible Planning

The Federal government has no authority to regulate land use planning or the land development process on non-Federal lands. The FHWA and other Federal agencies encourage State and local governments to practice land use planning and control near highways. The FHWA advocates that local governments use their regulatory authority to prohibit incompatible development adjacent to highways, or require planning, design and construction of developments that minimize highway traffic noise impacts.

Some State and local governments have enacted statutes for land use planning and control. For example, California requires local governments to consider the adverse environmental effects of highway traffic noise in their land development process. Additionally, the law gives local governments broad powers to pass ordinances relating to the use of land, including the location, size, and use of buildings and open space. Wisconsin has a State law, which requires formal adoption of a local resolution supporting the construction of a proposed noise barrier that documents the existence of local land use controls to prevent the future need for noise barriers adjacent to freeways and expressways. State or local governments may not use this type of legislation to override construction of a noise barrier deemed feasible and reasonable. It is FHWA's position that per 772.13 (d)(2)(i) only the residents and property owners at benefiting receptors can make a determination on desirability of feasible and reasonable noise abatement on public right-of-way.

Other States and local governments have similar laws, but the entire issue of land use is extremely complicated. Many competing considerations enter into land use control decisions, making it unlikely that land use planning and control will eliminate incompatible land development near highways.

Source Control

The Noise Control Act of 1972 authorizes the U.S. Environmental Protection Agency (EPA) to establish noise regulations to control major sources of noise, including transportation vehicles and construction equipment. Additionally, this legislation requires EPA to issue noise emission standards for motor vehicles used in interstate commerce (vehicles used to transport commodities across State boundaries) and requires the Federal Motor Carrier Safety Administration (FMCSA) to enforce these noise emission standards. The EPA established regulations, which set emission level standards for newly manufactured medium and heavy trucks with a gross vehicle weight rating (GVWR) greater than 10,000 pounds and capable of operating on a highway or street. Table 1 shows the maximum noise emission levels allowed by the EPA noise regulations for these vehicles.

Table 1: Maximum Noise Emission Levels as Required by EPA for Newly Manufactured Trucks with GVWR Over 10,000 Pounds
Effective Date Maximum Noise Level 50 Feet from Centerline of Travel*
January 1, 1988 80 dB(A)
* Using the Society of Automotive Engineers, Inc. (SAE), test procedure for acceleration under 35 mph

The Federal government also has authority to regulate noise emission levels for existing (in use) medium and heavy trucks with a GVWR of more than 10,000 pounds that are engaged in interstate commerce. Table 2 shows the EPA emission level standards for in use medium and heavy trucks engaged in interstate commerce. The FMCSA enforces these standards. State or local governments have regulatory authority over all other vehicles.

Table 2: Maximum Noise Emission Levels as Required by EPA for In Use Medium and Heavy Trucks with GVWR Over 10,000 Pounds Engaged in Interstate Commerce
Effective Date Speed Maximum Noise Level 50 Feet from Centerline of Travel
January 8, 1986 < 35 mph 83 dB(A)
> 35 mph 87 dB(A)
Stationary 85 dB(A)

Highway Traffic Noise Abatement

The National Environmental Policy Act (NEPA) of 1969 provides broad authority and responsibility to Federal agencies for evaluating and mitigating adverse environmental effects, including highway traffic and construction noise. NEPA directs the Federal government to use all practical means and measures to promote the general welfare and foster a healthy environment.

The Federal-Aid Highway Act of 1970 (23 USC §109(i)) specifically addresses the abatement of highway traffic noise. This law mandates FHWA to develop highway traffic noise standards.

The law requires promulgation of highway traffic noise level criteria for various land use activities.The law further provides that FHWA not approve the plans and specifications for a Federal-aid highway project unless the project includes adequate highway traffic noise abatement measures to implement the appropriate noise level standards. The FHWA has developed and implemented regulations for the analysis and mitigation of highway traffic noise in Federal-aid highway projects.

The FHWA highway traffic noise regulation is 23 CFR 772. The regulation requires the following during the planning and design of a highway project: (1) identification of highway traffic noise impacts; (2) examination of potential abatement measures; (3) the incorporation of reasonable and feasible highway traffic noise abatement measures into the highway project; (4) coordination with local officials to provide helpful information on compatible land use planning and control; and (5) identification and incorporation of necessary measures to abate construction noise.

The regulation contains highway traffic Noise Abatement Criteria (NAC) for different types of land uses and human activities. Highway traffic noise impacts occur when the predicted highway traffic noise levels approach or exceed the noise abatement criteria, or when the predicted highway traffic noise levels substantially exceed the existing highway traffic noise levels. The regulation does not require meeting the abatement criteria in every instance, and do not define the criteria as design standards for highway traffic noise abatement. Rather, the regulation requires that FHWA make every feasible and reasonable effort to provide substantial noise reduction when highway traffic noise impacts occur. Compliance with 23 CFR 772 is a prerequisite for granting Federal-aid highway funds for construction or reconstruction of a highway. Local zoning and design requirements, such as height limits on fencing and walls are not acceptable limitations on the configuration or design of noise abatement.

NOISE FUNDAMENTALS

Sound is when an object moves; the rustling of leaves as the wind blows, the air passing through our vocal chords, the almost invisible movement of speakers. The movements cause vibrations of the molecules in air to move in waves like ripples on water. When the vibrations reach our ears, we hear what we call sound.

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).

In addition to noise varying in frequency, noise intensity fluctuates with time. The most common descriptor of environmental noise in the United States of America 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 (see Figure 1). If the time period is one hour, the descriptor is the hourly equivalent sound level, Leq(h), which is widely used by highway agencies as a descriptor of highway traffic noise. An additional descriptor, which is sometimes used, is the L10. This is simply the A-weighted sound level that is exceeded 10 percent of the time.

Figure 1: Conceptualizing Equivalent Sound Level, LEQ
This figure shows the process of determining the equivalent (energy average) sound level for a given period of time. 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. The first graphic in this figure illustrates the varying sound level over time; the second graphic illustrates the acoustic energy associated with this varying sound level; the third graphic illustrates the same amount of acoustical energy, but averaged over a period of time; the fourth graphic illustrates the averaged sound level over that same period of time.

Decibel Addition

As mentioned above, decibels are logarithmic units and are not added arithmetically. Table 3 provides general procedures for decibel addition. This table shows that the sound pressure level from two equal sources is 3 dB greater than the sound pressure level of just one source. So, two trucks producing 90 dB each combine to produce 93 dB, not 180 dB. In other words, a doubling of the noise source produces only a 3 dB increase in the sound pressure level. Studies have shown that this increase is barely perceptible by the human ear.

Table 3: Rules for Combining Sound Levels by "Decibel Addition"
When two decibel values differ by Add the following amount to the higher value
0 or l dB 3 dB
2 or 3 dB 2 dB
4 or 9 dB 1 dB
10 dB or more 0 dB
*For noise levels known or desired to an accuracy or ±l decibel (acceptable for traffic noise analyses)

Decibel Changes, Loudness, and Energy Loss

Most observers perceive an increase or decrease of 10 dB in the sound pressure level as doubling or halving of the sound. For example, 70 dB will sound twice as loud as 60 dB. Table 4 shows the relationship between decibel changes and the corresponding relative loudness, as well as the actual loss in energy that occurs with each change.

Table 4: Decibel Changes, Loudness, and Energy Loss
Sound Level Change Relative Loudness Acoustic Energy Loss
0 dB(A) Reference 0
-3 dB(A) Barely Perceptible Change 50%
-5 dB(A) Readily Perceptible Change 67%
-10 dB(A) Half as Loud 90%
-20 dB(A) 1/4 as Loud 99%
-30 dB(A) 1/8 as Loud 99.9%

Sound Propagation

Sound intensity decreases in proportion with the square of the distance from the source. Generally, sound levels for a point source will decrease by 6 dB(A) for each doubling of distance. Sound levels for a highway line source vary differently with distance, because sound pressure waves propagate along the line and overlap at the point of measurement. A long, closely spaced, continuous line of vehicles along a roadway becomes a line source and produces a 3 dB(A) decrease in sound level for each doubling of distance. However, experimental evidence has shown that where sound from a highway propagates close to "soft" ground (e.g., plowed farmland, grass, crops, etc.), a more suitable drop-off rate to use is not 3 dB(A) but rather 4.5 dB(A) per distance doubling.

Vehicle Categories

For the purpose of highway traffic noise analyses, motor vehicles fall into one of 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

The emission levels of all five-vehicle types increase as a function of the logarithm of their speed. In other words, the highway traffic noise levels increases with increasing speed for all five vehicle types.

Variables Affecting Highway Traffic Noise

The level of highway traffic noise primarily depends on three things:

  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 numbers of trucks increase the loudness of highway traffic noise. Vehicle noise is primarily a combination of the noises produced by the engine, exhaust, and tires. Defective mufflers or other faulty equipment on vehicles can increase the loudness of highway traffic noise. Any condition (such as a steep incline) that causes heavy laboring of motor vehicle engines will also increase highway traffic noise levels. Additionally, other, more complicated factors affect the loudness of highway traffic noise. For example, as a person moves away from a highway, distance, terrain, vegetation, and natural and manmade obstacles reduce highway traffic noise levels. Highway traffic noise is not usually a serious problem for people who live more than 500 feet from heavily traveled freeways or more than 100 to 200 feet from lightly traveled roads. In quiet settings, however, such as rural areas, people notice highway traffic noise over greater distances. Pavement type can also affect noise generated at the tire/pavement interface.

Updated: 01/27/2012
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