HIGHWAY TRAFFIC NOISE ANALYSIS AND ABATEMENT

POLICY AND GUIDANCE

BY

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL HIGHWAY ADMINISTRATION

OFFICE OF ENVIRONMENT AND PLANNING

NOISE AND AIR QUALITY BRANCH

WASHINGTON, D.C.

JUNE 1995

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TABLE OF CONTENTS

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TABLE OF CONTENTS

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LIST OF FIGURES

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INTRODUCTION

Studies have shown that some of the most pervasive sources of noise in our environment today are those associated with transportation. Traffic noise tends to be a dominant noise source in our urban as well as rural environment. In response to the problems associated with traffic noise, the United States code of Federal Regulations Part 772 (23 CFR 772), "Procedures for Abatement of Highway Traffic Noise and Construction Noise," establishes standards for mitigating highway traffic noise.

The purpose of this document is to provide Federal Highway Administration (FHWA) policies and guidance for the analysis and abatement of highway traffic noise. A 3 1/2-day training course, sponsored by the National Highway Institute, is available for instructing FHWA field and State highway agency (SHA) staffs in the details of the policies and the technical procedures required for analyzing and abating traffic noise impacts.

I. LEGISLATION

Effective control of the undesirable effects of highway traffic noise requires that (1) land use near highways be controlled, (2) vehicles themselves be quieted, and (3) mitigation of noise be undertaken on individual highway 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.

A. Land Use Planning and Control

The Federal Government has essentially no authority to regulate land use planning or the land development process. The FHWA and other Federal agencies encourage State and local governments to practice land use planning and control in the vicinity of highways. The FHWA advocates that local governments use their power to regulate land development in such a way that noise-sensitive land uses are either prohibited from being located adjacent to a highway, or that the developments are planned, designed, and constructed in such a way that noise impacts are minimized.

Some State and local governments have enacted legislative statutes for land use planning and control. As an example, the State of California has legislation on highway noise and compatible land use development. This State legislation requires local governments to consider the adverse environmental effects of noise in their land development process. In addition, the law gives local governments broad powers to pass ordinances relating to the use of

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land, including among other things, 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 and documenting the existence of local land use controls to prevent the future need for noise barriers adjacent to freeways and expressways.

Although some other states and local governments have similar laws, the entire issue of land use is extremely complicated with a vast array of competing considerations entering into any actual land use control decisions. For this reason, it is nearly impossible to measure the progress of using land use to control the effects of noise.

B. Source Control

The Noise Control Act of 1972 gives the Federal Environmental Protection Agency (EPA) the authority to establish noise regulations to control major sources of noise, including transportation vehicles and construction equipment. In addition, 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 FHWA office of Motor Carrier Safety (OMCS) to enforce these noise emission standards. The EPA has established regulations which set emission level standards for newly manufactured medium and heavy trucks that have a gross vehicle weight rating (GVWR) of more than 4,525 kilograms and are 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 4,525 Kilograms

* Using the Society of Automotive Engineers, Inc. (SAE), test procedure for acceleration under 56 kph

For existing (in-use) medium and heavy trucks with a GVWR of more than 4,525 kilograms, the Federal government has authority to regulate the noise emission levels only for those that are engaged in interstate commerce. Regulation of all other in-use vehicles must be done by State or local governments. The EPA emission level standards for in-use medium and heavy trucks engaged in interstate commerce are shown in Table 2 and are enforced b the FHWA OMCS.

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Table 2: Maximum Noise Emission Levels as Required by EPA for In-Use Medium and Heavy Trucks with GVWR Over 4,525 Kilograms Engaged in Interstate Commerce

C. Highway Project Noise Mitigation

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

A more important Federal legislation which specifically involves abatement of highway traffic noise is the Federal-Aid Highway Act of 1970. This law mandates FHWA to develop noise standards for mitigating highway traffic noise.

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

The FHWA regulations for mitigation of highway traffic noise in the planning and design of federally aided highways are contained in 23 CFR 772. The regulations require the following during the planning and design of a highway project: (1) identification of traffic noise impacts; (2) examination of potential mitigation measures; (3) the incorporation of reasonable and feasible noise mitigation measures into the highway project; and (4) coordination with local officials to provide helpful information on compatible land use planning and control. The regulations contain noise abatement criteria which represent the upper limit of acceptable highway traffic noise for different types of land uses and human activities. The regulations do not

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require that the abatement criteria be met in every instance. Rather, they require that every reasonable and feasible effort be made to provide noise mitigation when the criteria are approached or exceeded, compliance with the noise regulations is a prerequisite for the granting of Federal-aid highway funds for construction or reconstruction of a highway.

II. NOISE FUNDAMENTALS

As we all know, sound is created when an object moves; the rustling of leaves as the wind blows, the air passing through our vocal chords, the almost invisible movement of the speakers on a stereo. 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 defined as unwanted sound. Sound is produced by the vibration of sound pressure waves in the air. Sound pressure levels are used to measure the intensity of sound and are described in terms of decibels. The decibel (dB) is a logarithmic unit which expresses the ratio of the sound pressure level being measured 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 must be filtered out when measuring highway noise levels. Sound-level meters are usually equipped with weighting circuits which filter out selected frequencies. It has been found that 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 dBA.

In addition to noise varying in frequency, noise intensity fluctuates with time. In the past few years, there has been a definite trend toward the use of the equivalent (energy-average) sound level as the descriptor of environmental noise in the U.S. 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 1 hour, the descriptor is the hourly equivalent sound level, L_eq(h), which is widely used by SHAs as a descriptor of 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.

A few general relationships may be helpful at this time in understanding sound generation and propagation. First, as already mentioned above, decibels are logarithmic units. Consequently, sound levels cannot be added by ordinary arithmetic means. A chart for decibel addition is shown in Table 1. From this table it can be seen that the sound pressure level from two equal sources is 3 dB greater than the sound pressure level of just one source. Therefore, two trucks producing 90 dB each will 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 detectable by the human ear.

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Table 3: Decibel Changes, Loudness, and Energy Loss

Table 4: Rules for Combining Sound Levels by "Decibel Addition"

For noise levels known or desired to an accuracy of +/- 1 decibel (acceptable for traffic noise analyses:

Secondly, an increase or decrease of 10 dB in the sound pressure level will be perceived by an observer to be a doubling or halving of the sound. For example, a sound at 70 dB will sound twice as loud as a sound at 60 dB.

Finally, 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 dBA for each doubling of distance. Sound levels for a highway line source vary differently with distance, because sound pressure waves are propagated all 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 dBA 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.), the most suitable dropoff rate to use is not 3 dBA but rather 4.5 dBA per distance doubling. This 4.5 dBA dropoff rate is usually used in traffic noise analyses.

For the purpose of highway traffic noise analyses, motor vehicles fall into one of three categories: (1) automobiles - vehicles with two axles and four wheels, (2) medium trucks - vehicles with two axles and six wheels, and (3) heavy trucks - vehicles with three or more axles. The emission levels of all three vehicle types increase as a function of the logarithm of their speed.

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The level of highway traffic noise 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, 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. Any condition (such as a steep incline) that causes heavy laboring of motor vehicle engines will also increase traffic noise levels. 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 than 150 meters from heavily traveled freeways or more than 30 to 60 meters from lightly traveled roads.

III. FHWA NOISE REGULATIONS

The current FHWA procedures for highway traffic noise analysis and abatement are contained in 23 CFR 772, "Procedures for Abatement of Highway Traffic Noise and Construction Noise." These procedures specify the requirements that SHAs must meet when using Federal-aid funds for highway projects.

This discussion will address those requirements and point out the most important issues related to the requirements. Each paragraph of 23 CFR 772 will be presented in boldface type and followed by a discussion of that paragraph. Some parts are self-explanatory and need only a sentence or two of discussion. Other, more complicated paragraphs will have greater discussion.

772.1: PURPOSE. To provide procedures for noise studies and noise abatement measures to help protect the public health and welfare, to supply noise abatement criteria, and to establish requirements for information to be given to local officials for use in the planning and design of highways approved pursuant to Title 23, United States Code (U.S.C.).

The protection of the public's health and welfare is an important responsibility that FHWA helps to accomplish during the planning and design of a highway project. The U.S. Congress has directed that this be done when the 1970 Federal-Aid Highway Act was passed. Concerned citizens and States encouraged Congress to provide this protection.

772.3: NOISE STANDARDS. The highway traffic noise prediction requirements, noise analyses, noise abatement criteria, and requirements for informing local officials in this directive constitute the noise standards mandated by 23 U.S.C. 109(i). All highway projects which are developed in conformance with this directive shall be deemed to be in conformance with the Federal Highway Administration (FHWA) noise standards.

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This paragraph makes the whole 23 CFR 772 the FHWA noise standard. The standard is required by 23 U.S.C. 109 (i). Some people mistake the noise abatement criteria for the FHWA standard. Early on, FHWA did not want to be restricted to specific noise levels that may not be achieved in most highway projects. So, a standard was developed that would best serve the public in terms of protection and reasonable cost.

772.5: DEFINITIONS

a. Design Year - the future year used to estimate the probable traffic volume for which a highway is designed. A time, 10 to 20 years, from the start of construction is usually used.

b. Existing Noise Levels - the noise, resulting from the natural and mechanical sources and human activity, considered to be usually present in a particular area.

c. L₁₀ - the sound level that is exceeded 10 percent of the time (the 90th percentile) for the period under consideration.

d. L₁₀(h) - the hourly value of L₁₀.

e. L_eq - the equivalent steady-state sound level which in a stated period of time contains the same acoustic energy as a time-varying sound level during the same period.

f. L_eq(h) - the hourly value of L_eq.

g. Traffic Noise Impacts - impacts which occur when the predicted traffic noise levels approach or exceed the noise abatement criteria (Table 5), or when the predicted traffic noise levels substantially exceed the existing noise levels.

h. Type I Projects - a proposed Federal or Federal-aid highway project for the construction of a highway on new location or the physical alteration of an existing highway which significantly changes either the horizontal or vertical alignment or increases the number of through-traffic lanes.

i. Type II Projects - a proposed Federal or Federal-aid highway for noise abatement on an existing highway.

Most of these definitions are self-explanatory. However, the definition for "Traffic Noise Impacts" warrants further attention. A traffic noise impact occurs when the predicted levels approach or exceed the noise abatement criteria (NAC) or when predicted traffic noise levels substantially exceed the existing noise level, even though the predicted levels may not exceed the NAC. This definition reflects the FHWA position that traffic noise impacts can occur under either of two separate conditions: (1) when noise levels are

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unacceptably high (absolute level); or (2) when a proposed highway project will substantially increase the existing noise environment (substantial increase). In order to adequately assess the noise impact of a proposed project, both criteria must be analyzed. While the FHWA noise regulations do not define "approach or exceed, all SHAs must establish a definition of "approach" that is at least 1 dBA less than the NAC for use in identifying traffic noise impacts in traffic noise analyses.

Table 5: Noise Abatement Criteria (NAC) Hourly A-Weighted Sound Level in Decibels (dBA)*

* Either L_eq(h) or L₁₀(h) (but not both) may be used on a project.

NOTE: These sound levels are only to be used to determine impact. These are the absolute levels where abatement must be considered. Noise abatement should be designed to achieve a substantial noise reduction - not the noise abatement criteria.

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In developing the NAC contained in the noise regulations the FHWA attempted to strike a balance between that which is most desirable and that which is feasible. Factors such as technical feasibility, the unique characteristics of highway-generated noise, cost, overall public interest, and other agency objectives were important elements in the process of setting a standard. Establishing values for the NAC was approached by attempting to balance the control of future increases in highway noise levels and the economic, physical, and aesthetic considerations related to noise abatement measures. Numerous approaches were considered in establishing the criteria, including (1) hearing impairment, (2) annoyance, sleep, and task interference or disturbance, and (3) interference with speech communication. The first deals in terms of very loud noises seldom encountered for a highway project beyond the roadway proper. The second approach was desirable in principle but was insufficiently researched to be useful in practice. However, the third approach - speech interference - was usefully applied to the problem of highway traffic noise. Thus, it should be remembered that the NAC are based upon noise levels associated with interference of speech communication and that the NAC are a compromise between noise levels that are desirable and those that are achievable. FHWA believes that our regulations provide a well-balanced approach to the problem of highway-traffic-generated noise.

The NAC are not magical numbers. Traffic noise impacts can occur below the NAC. The NAC should not be viewed as Federal standards or desirable noise levels; they should not be used as design goals for noise barrier construction. All of the regulations contained in 23 CFR 772 constitute the standards mandated by the Federal-Aid Highway Act of 1970. Noise abatement should be designed to achieve a substantial noise reduction, which SHAs have defined in practice to be in the range of 5-10 dBA. The NAC should only be used as absolute values which, when approached or exceeded, require the consideration of traffic noise abatement measures.

The 23 CFR 772 purposefully provides the SHAs with flexibility to establish their own definition of "substantial increase." A 10 dBA increase in noise levels is a doubling of the perceived loudness. A 15 dBA increase in noise levels represents more than a doubling of the loudness. Factors such as available resources, the public's attitudes toward highway traffic noise, and the absolute noise levels may influence a state's definition. The FHWA will accept a well-reasoned definition that is uniformly and consistently applied. Several SHA definitions have evolved and are shown in Table 6.

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Table 6: Criteria Used by States to Define "Substantial"

The use of subjective descriptors to describe traffic noise impacts is not required. Traffic noise impacts "occur based upon the definition contained in 23 CFR 772. This definition does not contain subjective descriptors. If impacts are identified, noise abatement measures must be considered and implemented if found to be reasonable and feasible. When analyzing the reasonableness of abatement, SHAs should consider the relationship between the absolute noise levels and the extent of the increase over existing noise levels for a given situation. A small increase at a higher absolute level (e.g., 70 dBA to 75 dBA) can be more important and justify greater consideration than a similar increase at a lower absolute level (e.g, 50 dBA to 55 dBA). Likewise, a large increase at a lower absolute level (e.g., 40 dBA to 55 dBA) can be less important and justify less consideration than a similar increase at a higher absolute level (e.g., 55 dBA to 70 dBA).

772.7: APPLICABILITY:

a. Type I Projects. This directive applies to all Type I projects unless it is specifically indicated that a section applies only to Type II projects.

b. Type II Projects. The development and implementation of Type II projects are not mandatory requirements of 23 U.S.C. 109(i) and are, therefore, not required by this directive. When Type II projects are proposed for Federal-aid highway participation at the option of the highway agency, the provisions of paragraphs 6, 8, and 11 of this directive shall apply.

The regulation applies to all Type I and Type II projects. The implementation of a Type II program is optional and not mandatory.

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772.9: ANALYSIS OF TRAFFIC NOISE IMPACTS AND ABATEMENT MEASURES

a. The highway agency shall determine and analyze expected traffic noise impacts and alternative noise abatement measures to mitigate there impacts, giving weight to the benefits and cost of abatement, and to the overall social, economic and environmental effects. (1)

b. The traffic noise analysis shall include the following for each alternative under detailed study:

(1) identification of existing activities, developed lands, and undeveloped lands for which development is planned, designed and programmed, which may be affected by noise from the highway;

(2) prediction of traffic noise levels;

(3) determination of existing noise levels;

(4) determination of traffic noise impacts; and

(5) examination and evaluation of alternative noise abatement measures for reducing or eliminating the noise impacts.

c. Highway agencies proposing to use Federal-aid highway funds for Type II projects shall perform a noise analysis of sufficient scope to provide information needed to make the determination required by paragraph 772.13a of this directive.

Paragraph 772.9a is the major requirement for doing noise analyses on all Type I projects. However, this requirement includes the evaluation of noise reduction benefits, abatement cost, and social, economic, and environmental (SEE) effects. This evaluation requires a balancing by the SHA of benefits versus disbenefits. This can be a difficult task because very little guidance exists on this topic. Noise reduction benefits and abatement cost will be discussed in detail in paragraph 772.ll. The process of balancing noise abatement and the SEE effects of the mitigation is strongly influenced by the public involvement process. The people who live next to the highway project can best evaluate if the abatement benefits will outweigh the SEE effects. The SHAs should not do this evaluation without public involvement. It is also important to remember that noise abatement consideration should be an inherent project consideration that is not handled separately but is incorporated and considered in the total project development decision.

Paragraph 772.9b lists the minimum requirements needed to adequately evaluate the impacts and abatement for each alternative under detailed study for the proposed highway project. The analysis should present the noise impacts and evaluation of alternative abatement measures in a comparative format. In this way, the potential noise impacts and likely abatement measures associated with

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the various alternatives, including the "no-build" alternative, are clearly defined. Detailed procedures on how to do the analysis exists in the text of the National Highway Institute noise training course, "Fundamentals and Abatement of Highway Traffic Noise."

Paragraph 772.9b(1) requires the identification of existing activities and developed lands. This identification includes not only the type (e.g., residential, commercial), but the number or extent of activities. This quantification is often overlooked in the analysis. The extent of the noise impact on the people living near the highway project cannot be evaluated correctly without the quantification of the existing activities.

Paragraph 772.9b(1) also requires noise analysis for undeveloped lands for which development is "planned, designed, and programmed." The terms "... planned, designed, and programmed ..." mean that: 1) a definite commitment has been made to develop the property in question, and 2) there is also official knowledge (such as through a public agency) that such development has been "planned, designed, and programmed." A definite commitment means that a developer has shown a definite interest to develop the land within a reasonable period of time and has reached a point where he can no longer practically change his plans.

The exact date for determining when undeveloped land is "... planned, designed, and programmed ..." for development is not specified in 23 CFR 772. Each SHA and accompanying FHWA Division office should establish a mutually acceptable specific date that is appropriate for the development process in their respective State. One specific date that has evolved is the date of issuance of a building permit, other dates used by States include the date of final approval of the development plan and the date of recording of the plat plan. . Any of these dates are in conformance with FHWA policy.

772.ll: NOISE ABATEMENT

a. In determining and abating traffic noise impacts, primary consideration is to be given to exterior areas. Abatement will usually be necessary only where frequent human use occurs and a lowered noise level would be of benefit.

b. In those situations where there are no exterior activities to be affected by the traffic noise, or where the exterior activities are far from or physically shielded from the roadway in a manner that prevents an impact on exterior activities, the interior criterion shall be used as the basis of determining noise impacts.

In most situations, if the exterior area can be protected, the interior will also be protected. The selection of the exterior area where "frequent human use occurs" is very important. This requires a site visit to determine whether people are using the entire exterior area or only a small portion, like a patio or porch. Some States choose the right-of-way line (a point farthest away from a house) to be on the conservative side when doing the noise impact analysis. Interior use applies mostly to hospitals and schools.

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Interior noise level predictions may be computed by subtracting from the predicted exterior levels the noise reduction factors for the building in question. If field measurements of these noise reduction factors are obtained or the factors are calculated from detailed acoustical analyses, the measured or calculated reduction factors should be used. In the absence of such calculations or field measurements, the noise reduction factors may be obtained from the following table:

Table 7: Building Noise Reduction Factors

NOTE: The windows shall be considered open unless there is firm knowledge that the windows are in fact kept closed almost every day of the year.

c. If a noise impact is identified, the abatement measures listed in paragraph 772.13c of this directive must be considered.

This self-explanatory paragraph requires consideration of noise abatement when noise impacts occur. As noted in paragraph 772.5g, noise impacts occur when noise levels approach or exceed the noise abatement criteria or when predicted levels substantially exceed existing levels. Consequently, this paragraph requires consideration of noise abatement for both of these types of noise impacts.

d. When noise abatement measures are being considered, every reasonable effort shall be made to obtain substantial noise reductions.

Abatement must provide at least a 5 dBA reduction in highway traffic noise levels in order to provide noticeable and effective attenuation. When noise abatement is proposed, it is recommended that an attempt be made to achieve the greatest reduction possible. SHAs have generally defined substantial reduction to be in the range of 5-10 dBA.

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This paragraph does not say to reduce the noise abatement criteria; it says "substantial noise reductions." Consequently, a projected noise level of L_eq 69 for a Category B activity (see Table 5) should not be abated merely to the noise abatement criterion of L_eq 67, but rather a substantial reduction should be obtained (at least 5 dBA). The choice of what minimum reduction to strive for is certainly a subjective one and is probably related to data found in technical literature, such as the following table.

Table 8: Relationship Between Decibel, Energy, and Loudness

A reduction of 10 dBA (say 75 dBA to 65 dBA) will be perceived by the public as a halving of the loudness. This is an easily recognizable change. 5 dBA and 7 dBA changes can also be recognized, but to a lesser degree. Two points should be kept in mind: (1) any reduction will improve the noise environment in such areas as annoyance, speech interference, task interference, etc., and (2) no matter what the reduction, until the level reaches a very low level (about L_eq = 55 dBA), the noise environment will continue to be dominated by traffic noise that is clearly audible.

e. Before adoption of a final environmental impact statement or finding of no significant impact, the highway agency shall identify:

(1) noise abatement measures which are reasonable and feasible and which are likely to be incorporated in the project, and

(2) noise impacts for which no apparent solution is available.

This paragraph ties the noise regulation to the NEPA requirements. An important point is that the requirements for the draft environmental impact statement (EIS) are the same as the final. Therefore, the information for both 772.lle(1) and 772.lle(2) are needed in the draft EIS and the final EIS. The choice of the word "likely" was deliberate. If a decision maker is to make an informed decision and if the public is to be made aware of the impacts, the State must make its intentions known. If the State later decides that mitigation is not warranted, the decision should have strong support. If the

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State would like to qualify the word "likely," this is acceptable. When a project involves consideration of more than one barrier, a statement of “likelihood” for each barrier should be included in the environmental document. The following is an illustration of some appropriate words.

Based on the studies so far accomplished, the State intends to install noise abatement measures in the form of a barrier at______________________________. These preliminary indications of likely abatement measures are based upon preliminary design for a barrier cost of $____________ that will reduce the noise level by ________ dBA for ___________ residents. If it subsequently develops during final design that these conditions have substantially changed, the abatement measures might not be provided. A final decision of the installation of the abatement measure(s) will be made upon completion of the project design and the public involvement processes.

f. The views of the impacted residents will be a major consideration in reaching a decision on the reasonableness of abatement measures to be provided.

The views of the impacted residents should be a major consideration if determining the reasonableness of traffic noise abatement measures for proposed highway construction projects. The views should be determined and addressed during the environmental phase of project development. The will and desires of the general public should be an important factor in dealing with the overall problems of highway traffic noise. SHAs should incorporate traffic noise consideration in their on-going activities for public involvement in the highway program, i.e., the residents' views on the desirability and acceptability of abatement need to be reexamined periodically during project development.

g. The plans and specifications will not be approved by FHWA unless those noise abatement measures which are reasonable and feasible are incorporated into the plans and specifications to reduce or eliminate the noise impact on existing activities, developed lands, or undeveloped lands for which development is planned, designed, and programmed.

This is a summary statement of the requirements in the 1970 Federal-Aid Highway Act [23 U.S.C. 109(i)].

The key words in this paragraph are "reasonable" and "feasible." For a thorough explanation of reasonableness and feasibility of abatement, see the discussion on pp. 50-56.

772.13: FEDERAL PARTICIPATION

a. Federal funds may be used for noise abatement measures where:

(1) a traffic noise impact has been identified,

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(2) the noise abatement measures will reduce the traffic noise impact, and

(3) the overall noise abatement benefits are determined to outweigh the overall adverse social, economic, and environmental effects and the costs of the noise abatement measures.

Paragraph 772.13a identifies the simple rules that guide the funding of noise abatement on highway projects. These rules apply to both Type I and Type II projects.

Federal-aid highway funds may not be used as payment or compensation for a traffic noise impact through the purchase of a noise easement from a property owner. The FHWA noise regulations clearly indicate that Federal funds may only be used to reduce traffic noise impacts and provide noise abatement benefits. Monetary compensation accomplishes neither of these requirements.

Federal-aid funds may be used in compensation paid during right-of-way negotiations for a partial taking of property. Noise, air quality, access, visual quality, etc. are frequently considered jointly in determining this compensation, which is regarded as part of right-of-way acquisition, not environmental mitigation.

b. For Type II projects, noise abatement measures will not normally be approved for chose activities and land uses which come into existence after May 14, 1976. However, noise abatement measures may be approved for activities and land uses which come into existence after May l4, 1976, provided local authorities have taken measures to exercise land use control over the remaining undeveloped lands adjacent to highways in the local jurisdiction to prevent further development of incompatible activities.

Paragraph 772.13b limits funding participation for retrofit barriers on existing highways because in 1976 FHWA publicly stated that local governments must help control noise impacts through noise-compatible land-use planning and zoning. However, it is important to remember that this paragraph does not prohibit the approval of Type II barriers after 1976. It says that the land use activity (housing development) built near a highway after 1976 usually cannot get a Type II barrier unless the local government has an active land use control program to prevent future incompatible activities (e.g., zoning requirements, noise-sensitive growth and development procedures, local ordinances). The FHWA has not rigidly applied this requirement in the past. However, after the date of issuance of this guidance Type II abatement projects for new activities and land uses which come into existence may only be approved if an active local land use control program was adopted prior to existence of the new activities and land uses. EXAMPLE: A Type II noise barrier is requested for homes that were constructed prior to a local community's adoption of an active noise-compatible land use control program. Type II abatement may not be approved for this location. SHAs should be certain to make local officials aware of this requirement (see paragraph 772.15 on page 18).(2)

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c. The noise abatement measures listed below may be incorporated in Type I and Type II projects to reduce traffic noise impacts. The costs of such measures may be included in Federal-aid participating project costs with the Federal share being the same as that for the system on which the project is located, except that Interstate construction funds may only participate in Type I projects.

(1) traffic management measures (e.g., traffic control devices and signing for prohibition of certain vehicle types, time-use restrictions for certain vehicle types, modified speed limits, and exclusive land designations),

(2) alteration of horizontal and vertical alignments,

(3) acquisition of property rights (either in fee or lesser interest) for construction of noise barriers,

(4) construction of noise barriers (including landscaping for aesthetic purposes) whether within or outside the highway right-of-way. Interstate construction funds may not participate in landscaping,

(5) acquisition of real property or interests therein (predominately unimproved property) to serve as a buffer zone to preempt development which would be adversely impacted by traffic noise. This measure may be included in Type I projects only, and

(6) noise insulation of public use or nonprofit institutional structures.

Two important points about this paragraph are: (1) the participating share is the same as that for the system on which the project is located; (2) buffer zones can only be used in Type I projects. The potential use of buffer zones applies to predominantly unimproved property. This authority is not used to purchase homes or developed property to create a noise buffer zone. It is used to purchase unimproved property to preclude future noise impacts where development has not yet occurred.

Although most noise mitigation has been implemented along Interstate highways, Federal funds may be used for mitigation measures along other types of highways if the noise impacts exist and the criteria in 772.13a are met.

The most-used abatement measure is the noise barrier; however, paragraph 772.llc requires consideration of all the abatement measures listed in paragraph 772.13c. Noise insulation may only routinely be considered for public use or nonprofit institutional structures, e.g., churches, schools, hospitals, libraries, etc. Private dwellings may only be noise-insulated under the provisions of section 772.13d.

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The purchase of a noise easement, in locations where traffic noise impacts are expected to occur or already exist, should not be considered as a noise abatement measure. It does not reduce noise levels or abate the impacts. It only provides monetary compensation and is, thus, not eligible for Federal-aid participation.

d. There may be situations where (1) severe traffic noise impacts exist or are expected., and (2) the abatement measures listed above are physically infeasible or economically unreasonable. In these instances, noise abatement measures other than those listed in paragraph 771.13c of this directive may be proposed for Type I and II projects by the highway agency and approved by the Regional Federal Highway Administrator on a case-by-case basis when the conditions of paragraph 772.13a of this directive have been met.

This paragraph allows the States the flexibility to propose innovative noise abatement measures when severe traffic noise impacts are anticipated and normal abatement measures are physically infeasible or economically unreasonable. In these instances, the Regional Federal Highway Administrator may approve a state's request for unusual or extraordinary abatement measures on a case-by-case basis. When considering extraordinary abatement measures, the state must demonstrate that the affected activities experience traffic noise impacts to a far greater degree than other similar activities adjacent to highway facilities, e.g., residential areas with absolute noise levels of 75 dBA L_eq(h) or more, residential areas with noise level increases of 30 dBA or more over existing noise levels. Examples of extraordinary abatement measures would be the noise insulation of private residences or the purchase of private dwellings from willing sellers.

772.15: INFORMATION FOR LOCAL OFFICIALS In an effort to prevent future traffic noise impacts on currently undeveloped lands, highway agencies shall inform local officials within whose jurisdiction the highway project is located of the following:

a. The best estimation of future noise levels (for various distances from the highway improvement) for both developed and undeveloped lands or properties in the immediate vicinity of the project,

b. Information that may be useful to local communities to protect future land development from becoming incompatible with anticipated highway noise levels, and

c. eligibility for Federal-aid participation for Type II projects as described in paragraph 772.13b of this directive.

The prevention of future impacts is one of the most important parts of noise control. The compatibility of the highway and its neighbors is essential for the continuing growth of local areas. Both development and highways can be compatible. But, local government officials need to know what noise levels to expect from a highway and what techniques they can use to prevent future impacts, states can help by providing this information to local governments; such information should be made available for disclosure in real estate transactions.(3)

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Highway traffic noise should be reduced through a program of shared responsibility. Thus, the FHWA encourages state and local governments to practice compatible land use planning and control in the vicinity of highways. Local governments should use their power to regulate land development in such a way that noise-sensitive land uses are either prohibited from being located adjacent to a highway, or that the developments are planned, designed, and constructed in such a way that noise impacts are minimized. Local officials should be made aware of the requirement for the adoption of an active noise- compatible land use control program for approval of Type II abatement (see paragraph 772.13b on page 16).

772.17: TRAFFIC NOISE PREDICTION

a. Any traffic noise prediction method is approved for use in any noise analysis required by this directive if it generally meets the following two conditions:

(1) The methodology is consistent with the methodology in the FHWA Highway Traffic Noise Prediction Model (Report No. FHWA-RD-77-108).

(2) The prediction method uses noise emission levels obtained from one of the following:

(a) National Reference Energy Mean Emission Levels as a Function of Speed (Figure 1).

(b) Determination of reference energy mean emission levels in "Sound Procedures for Measuring Highway Noise: Final Report," Report No. DP-45-lR.

b. In predicting noise levels and assessing noise impacts, traffic characteristics which will yield the worst hourly traffic noise impact on a regular basis for the design year shall be used.

Most states use the FHWA highway traffic noise prediction model (FHWA model) with its national emission levels. If a state uses different emission levels, documentation must be provided to the FHWA Division Office to justify its use. Paragraph 772.17a(2)(b) specifies that the method in Report No. DP-45-lR be used to obtain these emission levels. The FHWA Division Office should forward the proposed emission levels to FHWA Headquarters for review and comment. Some states have modified computer versions of the FHWA model to change input/output characteristics to suit the state's design process.

Traffic characteristics used in predicting future noise levels could make a substantial difference in the results. "Worst hourly traffic noise impact" occurs at a time when truck volumes and vehicle speeds are the greatest, typically when traffic is free-flowing and at or near level of service C conditions. The numbers of medium and heavy trucks are very important.

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SHAs should use either the posted speed limit or the operating speed (highest overall speed at which a driver can travel on a given highway under favorable weather conditions and under prevailing traffic conditions, without at any time exceeding the safe speed as determined by the design speed on a section-by-section basis) to predict traffic noise levels. SHAs are required to use the operating speed if it is determined to be consistently higher than the posted speed limit. In determining the operating speed along an existing highway, the first step is to identify the time period during which the worst traffic noise impacts are expected to occur. Then, the speed may be determined by actually driving a vehicle in the traffic stream and recording the average speed. It may also be determined by using radar meters or other devices to measure speeds at a point along the highway (making no adjustments to the actual instrument measurements). Such measured speeds are then arithmetically averaged to calculate a time mean speed (as defined in Highway Capacity Manual: Special Report 209). Either the "traffic stream" speed or the time mean speed can be used to represent the operating speed.

772.19. CONSTRUCTION NOISE

The following general steps are to be performed for all Types I and II projects:

a. Identify land uses or activities which may be affected by noise from construction of the project. The identification is to be performed during the project development studies.

b. Determine the measures which are needed in the plans and specifications to minimize or eliminate adverse construction noise impacts to the community. This determination shall include a weighing of the benefits achieved and the overall adverse social, economic, and environmental effects and the costs of the abatement measures.

c. Incorporate the needed abatement measures in the plans and specifications.

The impact of construction noise does not appear to be serious in most instances. FHWA Technical Advisory T 6160.2, "Analysis of Highway Construction Noise," outlines procedures for the analysis of highway construction noise. The following items should be considered to ensure that potential construction noise impacts are given adequate consideration during highway project development:

a. Calculation of construction noise levels is usually not necessary for traffic noise analyses. If a construction noise impact is anticipated at a particular sensitive receptor, use of the model

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contained in "Highway Construction Noise: Measurement, Prediction, and Mitigation" to predict construction noise levels should be sufficient. The computerized prediction model HICNOM is quite sophisticated and requires considerable input, and, therefore, should be used only on highly complex or controversial major urban projects.

b. Potential impacts of highway construction noise should be addressed in a general manner for traffic noise analyses. The temporary nature of the impacts should be noted. An indication of the types of construction activities that can be anticipated and the noise levels typically associated with these activities can be obtained from existing literature and presented in the noise analysis.

c. Utilizing a common-sense approach, traffic noise analyses should identify measures to mitigate potential highway construction noise impacts. Low-cost, easy-to-implement measures should be incorporated into project plans and specifications (e.g., work-hour limits, equipment muffler requirements, location of haul roads, elimination of "tail gate banging," reduction of backing up for equipment with alarms, community rapport, complaint mechanisms).

d. Major urban projects with unusually severe highway construction noise impacts require more extensive analyses. Sensitive receptors should be identified, existing noise levels should be measured, construction noise levels should be predicted, and impacts should be discussed so as to properly indicate their severity. Mitigation measures likely to be incorporated into these projects may be quite costly and should be thoroughly discussed and justified in the analyses. The use of portable noise barriers and special quieting devices on construction equipment have been used for construction noise mitigation.

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Figure 1: Reference Energy Mean Emission Levels

LEGEND:

1. AUTOMOBILES: ALL VEHICLES WITH TWO AXLES AND FOUR WHEELS

2. MEDIUM TRUCKS: ALL VEHICLES WITH TWO AXLES AND SIX WHEELS

3. HEAVY TRUCKS: ALL VEHICLES WITH THREE OR MORE AXLES

National Reference Energy Mean Emission Levels as a Function of Speed

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IV. HIGHWAY TRAFFIC NOISE ANALYSIS AND DOCUMENTATION

The major objectives of a noise study for new highway construction or a highway improvement are:

• To define areas of potential noise impact for each study alternative
• To evaluate measures to mitigate these impacts
• To compare the various study alternatives on the basis of potential noise impact and the associated mitigation costs

Traffic noise studies thus provide useful information, directed primarily to two distinctly different audiences - the government decisionmaker and the lay public. For the government decisionmaker, the study should provide a portion of the data needed for the informed selection of a satisfactory project alternative and appropriate Mitigation measures. For the lay public, the study should provide discussion of potential impacts in any areas of concern to the public.

The final product of a highway traffic noise study should be a clear, concise written discussion of the study. There should be a stand-alone discussion, a noise study report, that gives the reader a detailed description of all the elements of the analysis done for the study, including information on noise fundamentals and regulatory requirements. The environmental document for Type I projects, i.e., Categorical Exclusion (CE), Environmental Assessment/Finding of No Significant Impact (EA/FONSI), Environmental Impact Statement (EIS), should contain a brief summary of the important points found in the noise study report. The project development records should fully document the traffic noise analysis level-of-effort expended, strategies considered, adjacent residents' views and opinions, and a final decision on the reasonableness and feasibility of abatement. (4)

The key elements of a highway traffic noise study are as follows:

• Definition of impact criteria and identification of noise-sensitive land uses
• Determination of existing noise levels
• Prediction of future traffic noise levels for study alternatives
• Identification of traffic noise impacts for study alternatives
• Identification and consideration of abatement
• Consideration of construction noise
• Coordination with local government officials

A. Definition Of Impact Criteria and Identification Of Noise-Sensitive Land uses

The first step in the highway noise study is the definition of criteria for noise impact. With this definition established, the location of noise-sensitive land uses in the vicinity of the various study alternatives can be identified.

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A noise impact occurs (1) when the projected highway noise levels approach or exceed the noise abatement criteria in 23 CFR 772 or (2) when the projected highway noise levels substantially exceed existing noise levels in an area. Based upon this, noise-sensitive land uses in the vicinity of each of the study alternatives that may be impacted by future highway noise levels should be identified. Noise-sensitive areas may be identified by individual land uses, or by broad categories of land use for which a single criterion level may apply. In some cases, lands that are undeveloped at the time of the project may be known to be under consideration for development in the future. Depending upon the certainty of development in accordance with Paragraph 772.9b(1) of 23 CFR 772 (see page 11), these lands should be treated as "planned, designed, and programmed," and the severity of highway noise impact should be assessed accordingly.

A brief categorization of land-use types follows:

Example In this study, all land along the project is considered to fall in activity category B.

Somewhat more detail is provided by the following:

Example The region is primarily residential, although it is zoned for general business as well. Two apartment complexes and 50 residences are east of Airport Drive, at the south end. The nearest facade of these buildings is approximately 21 meters from the road centerline, and the farthest facade is roughly 122 meters away. The apartments house about 200 families.

B. Determination of Existing Noise Levels

In general, existing noise levels should be established by field measurements for all developed land uses and activities. Field measurements should be made, since existing background noise is usually a composite from many sources, and noise prediction models are applicable only to noise originating from a specific source. If it is clear that existing noise levels at locations of interest are predominantly due to a highway, then the existing noise levels may be calculated using the FHWA highway traffic noise prediction model.

When making existing noise measurements, a number of factors need to be considered: 1) time of day, e.g., peak hour vs. any other time of day; 2) day of week, e.g., weekend day vs. work day; 3) week of year, e.g., tourist season vs. non-tourist season; and 4) representativeness of the noise. The noise measurement should yield the worst hourly noise level generated from representative noise sources for that area. The period with the highest sound levels may not be at the peak traffic hour but instead, during some period when traffic volumes are lower but the truck mix or vehicle speeds are higher.

Measurements should be made at representative locations - that is, residential neighborhoods, commercial and industrial areas, parks, churches, schools, hospitals, libraries, etc. Measurements are normally restricted to exterior areas of frequent human use; interior measurements are only made when there

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are no outside activities, such as churches, hospitals, libraries, etc. Measurements are usually taken in one of three exterior locations: (1) at or near the highway right-of-way line; (2) at or near buildings in residential or commercial areas; and (3) at an area between the right-of-way line and the building where frequent human activity occurs, such as a patio or the yard of a home.

Representativeness relates to the noise typically found in a given location. Aircraft noise is usually representative near an airport but not in areas having no airport; the noise from barking dogs is usually representative near kennels but not in a residential neighborhood; and the noise from ambulance or police sirens is usually representative near hospitals or police stations but not in other locations.

Measurements are made to represent an hourly equivalent sound level, L_eq(h). For statistical accuracy, a minimum of approximately eight minutes of measurements must be made. Most SHAs have automated measrement equipment and typically measure 15-minute time periods to represent the L_eq(h). This is acceptable if nothing unusual is expected to occur during the noisiest hour. Measurements along low-volume highways may require longer measurement periods (e.g., 30-60 minutes) to attain desirable statistical accuracy. If information is not available to identify the noisiest hour of the day or if there is public controversy at a specific location, 24-hour measurements are sometimes taken.

Measurements should be made with noise meters of sufficient accuracy to yield valid date for the particular project (ANSI Sl.4-1983, TYPE II or better). Procedures should be adopted and followed so that measurements will have consistent and supportable validity. Traffic conditions, climatic conditions, and land uses at the time of measurement should be noted.

The following excerpt from an environmental impact statement shows how existing noise levels can be documented.

Example Figure _ is a plan map of the study area and shows the location of the noise measurement sites. The microphone was located 1.5 meters above the ground. Measurement Site Nos. 1, 2, and 4 are along the existing Airport Drive and near the apartment buildings closest to the project roadway. These locations were chosen to document existing noise levels and traffic conditions at the residential area where the potential for noise impacts due to the project exists. Sites 3 and 5 are located in residential areas near the location of the proposed extension of Airport Drive. In these areas, existing noise levels are expected to be the lowest in the project corridor. Sites 6 and 7 are near the other roadways in the study area that carry substantial traffic and connect to the proposed project.

The existing noise measurements were made during midday hours on June 12 and 13, 1988. The temperature varied from 18 degrees C to 27 degrees C, and winds were light and variable, having little effect on sound propagation over moderate distances.

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Noise measurements were obtained with the BBN Model 6l4 portable Noise Monitor, set to compute sound level distributions on a minute-by-minute basis. During each minute of analysis, the ambient noise sources were noted and local traffic counts were made. The duration of each measurement period was between 20 and 35 minutes.

C. Prediction of Future Traffic Noise Levels

The next step involved in the highway noise study is to analyze the noise levels expected to occur as a result of the proposed highway or highway extension. Noise levels should be estimated for each of the potential project alternatives, including the "do-nothing" case. The method used to predict traffic noise levels and traffic data for the various alternatives should be well documented.

Example Prediction of the Future Traffic Noise Levels For each of the seven alternatives under consideration, traffic noise at each receptor for the year 2000 was predicted using the FHWA Highway Traffic Noise Prediction Model, STAMINA 2.0. This model uses the number and type of vehicles on the planned roadway, their speeds, and the physical characteristics of the road, e.g., curves, hills, depressed, elevated, etc. In this regard it is to be noted that only preliminary alignment and roadway elevation characteristics were available for use in this noise analysis. Each alternative was modeled assuming no special noise abatement measures would be incorporated, only those existing natural or man-made barriers were included. The roadway sections were assumed to be at-grade, except where grade separation of intersections was necessary. Thus, the analysis represents "worst-case" topographic conditions. The traffic volumes used in the projections were obtained from the Metropolitan Council Regional Traffic Assignment Model. The noise predictions made in this report are highway-related noise predictions for the traffic conditions during the design year. It was assumed that the peak-hour volumes and corresponding speeds for trucks and automobiles result in the noisiest conditions. During all other time periods, the noise levels will be less than those indicated in this report.

D. Identification of Traffic Noise Impacts

The next step in the noise study involves a comparison of the predicted noise levels for each project alternative with the noise abatement criteria and existing noise levels. This comparison identifies the traffic noise impacts associated with each alternative in terms of the change in existing levels and the amount by which criteria may be approached or exceeded. The main purpose of this comparison is to contrast the noise impacts that are expected to occur as a result of the highway project, for each active alternative, with the existing noise impacts.

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The noise abatement criteria from 23 CFR 772 are listed in Table 5 (see page 8). Abatement must be considered when future noise levels approach or exceed these criteria. Traffic noise analyses should recognize and consider absolute noise levels as well as incremental increases in noise levels when identifying traffic noise impacts and when considering noise abatement measures.(5)

The following example illustrates a discussion of impact in an EIS:

Example A noise analysis has been conducted for the proposed actions. The greatest noise impact will be felt at residential sites which are near the proposed loop location. Table No. 7 shows the results of this analysis. The average impact on the selected noise sites is +12 dBA which will seem about 2 1/2 times as loud as the existing noise environment. The largest impacts (up to +25 dBA) will be felt at rural residences that are now on the less traveled backroads and will be close to the proposed highway.

For the recommended Alternate 3, 52 single-family residences,12 multiple-family residences and 2 churches equal or exceed the noise abatement criteria. Fifty-two single-family residences, 28 multiple-family residences, 2 businesses, and 2 churches will experience a substantial increase in existing noise levels, that is, an increase of 10 dBA or more.

E. Identification and Consideration of Abatement

The next step in the noise study is identification and evaluation of various noise abatement measures that could mitigate the adverse impacts predicted for the proposed highway project. For example, traffic management measures such as the following should be included in the evaluation:

• Prohibition of certain vehicle types
• Time use restrictions for certain vehicle types
• Modified speed limits
• Exclusive land use designations
• Traffic control devices
• Combinations of the above measures.

Additional noise abatement measures are discussed in detail in the Section V. For each abatement measure, the following information should be presented:

• Description of the measure
• Anticipated costs, problems, and disadvantages
• Anticipated benefits relative to the existing levels and other factors.

Examples The most likely method available to lessen the noise levels and thus alleviate noise impact from Airport Drive is to incorporate noise control into the highway design stage. Since the alignment and grade of Airport Drive have already been established, noise barriers beside the roadway are probably the most acceptable means of noise control.

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…The first location, for which a noise control barrier has been designed is along Airport Drive at the East Avenue-Fair Oaks apartment complex. The proposed barrier is located 3.6 meters from the edge of Airport Drive, is about 540 meters long, and runs from a point about 45 meters north of the edge of Niners Road at the Airport Drive intersection to about 21 meters north of the northernmost apartment building. If the top of the barrier is 3 meters above grade level, it will provide 9-11 dB reduction in the noise levels at the nearest building, first floor elevation (1.5 meters above ground). This will reduce the predicted exterior Leq noise levels near these buildings from 73-74 dB to 62-65 dB.

…The cost of noise barriers depends directly on the material used to build it. Depending upon material selection, barrier costs including installation may be as little as $50 per ,lineal meter or as great as $250 per lineal meter. If wooden barriers are erected along Airport Drive, the cost of the barrier for the apartments would be about $85,000, and the cost of the barrier for the three homes would be about $35,000.

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Table 9: Example of Abatement Information for an EIS Existing and Future Exterior Noise Levels (L_eq in dBA)

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F. Construction Noise Analysis

The consideration of construction noise must be addressed in an environmental document. The following example illustrates a construction noise discussion from an EIS:

Example It is difficult to predict reliable levels of construction noise at a particular receptor or group of receptors. Heavy machinery, the major source of noise in construction, is constantly moving in unpredictable patterns. Daily construction normally occurs during daylight hours when occasional loud noises are more tolerable. No one receptor is expected to be exposed to construction noise of long duration; therefore, extended disruption of normal activities is not anticipated. However, provisions will be included in the plans and specifications requiring the contractor to make every reasonable effort to minimize construction noise through abatement measures such as work-hour controls and maintenance of muffler systems.

G. Coordination with Local Government Officials

The final part of the noise study is coordination with local officials whose jurisdictions are affected. The primary purpose of this coordination is to promote compatibility between land development and highways.

The highway agency should furnish the following information to appropriate local officials:

• Estimated future noise levels at various distances from the highway improvement.
• Locations where local communities should protect future land development from becoming incompatible with anticipated highway noise levels.
• Information on the eligibility requirements for Federal-aid participation in Type II projects as described in paragraph 772.13b of 23 CFR 772 (see page 16).

V. HIGHWAY TRAFFIC NOISE ABATEMENT

Early in the planning stages of most highway improvements, highway agencies do a noise study. The purpose of this study is to determine if the project will create any noise problems. If the predicted noise levels cause an impact, the noise study must consider measures that can be taken to lessen these adverse noise impacts. There are a variety of things that a highway agency can do to lessen the impacts of highway traffic noise.

Some noise abatement measures that are possible include creating buffer zones, constructing barriers, planting vegetation, installing noise insulation in buildings, and managing traffic.

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A. Noise Barriers

1. Technical Considerations and Barrier Effectiveness

Noise barriers are solid obstructions built between the highway and the homes along the highway. Effective noise barriers can reduce noise levels by 10 to 15 decibels, cutting the loudness of traffic noise in half. Barriers can be formed from earth mounds along the road (usually called earthberms) or from high, vertical Walls. Earthberms have a very natural appearance and are usually attractive. However, an earthberm can require quite a lot of land if it is very high. Walls take less space. They are usually limited to 8 meters in height because of structural and aesthetic reasons. Noise walls can be built out of wood, stucco, concrete, masonry, metal, and other materials. Many attempts are being made to construct noise barriers that are visually pleasing and that blend in with their surroundings.

There are no Federal requirements or FHWA regulations related to the selection of material types to be used in the construction of highway traffic noise barriers. Individual SHAs select the material types to be used when building these barriers. The SHAs normally make this selection based on a number of factors such as aesthetics, durability and maintenance, costs, public comments, etc. The FHWA does not specify the type of material that must be used for noise barrier construction, but the material type that is chosen must meet state specifications which have been approved by the FHWA. The material chosen should be rigid and of sufficient density (approximately 20 kilograms/square meter minimum) to provide a transmission loss of 10 dBA greater than the expected reduction in the noise diffracted over the top of the barrier.

Noise barriers do have limitations. For a noise barrier to work, it must be high enough and long enough to block the view of a road. Noise barriers do very little good for homes on a hillside overlooking a road or for buildings which rise above the barrier. A noise barrier can achieve a 5 dB noise level reduction when it is tall enough to break the line-of-sight from the highway to the receiver and it can achieve an approximate 1.5 dB additional noise level reduction for each meter of height after it breaks the line-of-sight (with a maximum theoretical total reduction of 20 dBA). To avoid undesirable end effects, a good rule-of-thumb is that the barrier should extend 4 times as far in each direction as the distance from the receiver to the barrier. Openings in noise walls for driveway connections or intersecting streets destroy the effectiveness of barriers. In some areas, homes are scattered too far apart to permit noise barriers to be built at a reasonable cost.

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Noise barriers can be quite effective in reducing noise for receptors within approximately 61 meters of a highway. Table 10 summarizes barrier attenuation.

Table 10: Barrier Attenuation

Figure 2: Noise Barrier Examples

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Figure 3: Noise Barrier Shadow Zone

Shadow Effect of Noise Barrier

The lower house is protected by the barrier, but the upper one is not.

3. Public Perception

Overall, public reaction to highway noise barriers appears to be positive. There is, however, a wide diversity of specific reactions to barriers. Residents adjacent to barriers have stated that conversations in households are easier, sleeping conditions are better, a more relaxing environment is created, windows are opened more often, and yards are used more in the summer. Perceived non-noise benefits include increased privacy, cleaner air, improved view and sense of ruralness, and healthier lawns and shrubs. Negative reactions have included a restriction of view, a feeling of confinement, a loss of air circulation, a loss of sunlight and lighting, and poor maintenance of the barrier. Motorists have sometimes complained of a loss of view or scenic vistas and a feeling of being "walled in" when traveling adjacent to barriers. Most residents near a barrier seem to feel that barriers effectively reduce traffic noise and that the benefits of barriers outweigh the disadvantages of the barriers.

4. Design considerations

A successful design approach for noise barriers should be multidisciplinary and should include architects/planners, landscape architects, roadway engineers, acoustical engineers, and structural engineers. Noise reduction goals influence acoustical considerations and in conjunction with non-acoustical considerations, such as maintenance, safety, aesthetics, physical construction, cost, and community participation, determine various barrier design options.

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A major consideration in the design of a noise barrier is the visual impact on the adjoining land use. An important concern is the scale relationship between the barrier and activities along the roadway right-of-way. A tall barrier near a low-scale single family detached residential area could have a severe adverse visual effect. In addition, a tall barrier placed close to residences could create detrimental shadows. One solution to the potential problem of scale relationship is to provide staggered horizontal elements to a noise barrier to reduce the visual impact through introduction of landscaping in the foreground. This can also allow for additional sunlight and air movement in the residential area. In general, it is desirable to locate a noise barrier approximately four times its height from residences and to provide landscaping near the barrier to avoid visual dominance.

The visual character of noise barriers should be carefully considered in relationship to their environmental setting. The barriers should reflect the character of their surroundings as much as possible. Where strong architectural elements of adjoining activities occur in close proximity to barrier locations, a relationship of material, surface texture, and color should be explored in the barrier design. In other areas, particularly those near roadway structures or other transportation elements, it may be desirable that proposed noise barriers have a strong visual relationship, either physically or by design concept, to the roadway elements. Aesthetic views and scenic vistas should be preserved to the extent possible. In general, a successful design approach for noise barriers is to utilize a consistent color and surface treatment, with landscaping elements used to soften foreground views of the barrier. It is usually desirable to avoid excessive detail which tends to increase the visual dominance of the barrier.

The psychological effect on the passing motorist must be taken into consideration too. Barriers should be designed differently to fit dense, urban settings or more open suburban or rural areas and should also be designed to avoid monotony for the motorist. At normal roadway speeds, visual perception of noise barriers will tend to be of the overall form of the barrier and its color and surface texture. Due to the scale of barriers, a primary objective to achieve visually pleasing barriers is to avoid a tunnel effect through major variations in barrier form, material type,