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| Environment |
Highway Traffic Noise in the United States
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| Owner | Linear Kilometers * | Percent Of All Roads |
| Federal | 193 | 3 |
| State | 1,241 | 20 |
| Local | 4,855 | 77 |
* Rounded to the nearest thousand
The Federal-aid highway program is a federally assisted, State administered grant program which provides Federal funds to State and local governments to construct and improve highways. The Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) set the course for future roles of federal, state, and local government in maintaining the country's highways, bridges, and mass transit facilities, and in strengthening highway safety programs. The ISTEA called for linking "...all forms of transportation in a unified, interconnected manner..economically efficient and environmentally sound...[to] move people and goods in an energy efficient manner." The ISTEA restructured the Federal-aid highway program and established two Federal-aid systems - the National Highway System (NHS) and the Interstate System, which is a component of the NHS. The revised Federal-aid program includes about 1.5 million kilometers of the 6.3 million kilometers of roads in the United States. As can be seen in Table 2, urban roadways currently comprise a small portion of total roadways, yet carry a large portion of all highway travel.
| System | Linear Kilometers * | Percent of All Roads | Percent of Vehicle Kilometers Traveled |
| Urban | 1,355 | 21.5 | 60.6 |
| Rural | 4,934 | 78.5 | 39.4 |
| TOTAL | 6,289 | 100.0 | 100.0 |
* Rounded to nearest thousand
The FHWA is the designated Federal government agency for administering the Federal-aid highway program. The FHWA mission is to aid States in providing safe and efficient surface transportation for the movement of people and goods by all modes. The FHWA is responsible for providing guidance to State highway agencies and metropolitan planning agencies (MPOs) and for reimbursing the States and MPOs for the Federal share of projects. The States, in cooperation with MPOs, initiate, plan, design, construct, operate, and maintain the highways on the Federal-aid systems.
With a few exceptions, the FHWA does not pay for the entire cost of the projects it funds. Federal funds are normally "matched" with State and/or local government funds to account for the necessary dollars to complete the project. The Federal share is specified in legislation. Interstate System projects are typically funded 90 percent Federal and 10 percent State, but most other types of projects are funded at a slightly lower Federal share (80 percent).
Effective control of the undesirable effects of highway traffic noise requires that land use near highways be controlled, that vehicles themselves be quieted, and that 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.
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 Federal Government 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 land, including among other things, the location, size, and use of buildings and open space.
Although 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.
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 Federal Motor Carrier Safety Administration (FMCSA) 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 3 shows the maximum noise emission levels allowed by the EPA noise regulations for these vehicles.
| Effective Date | Maximum Noise Level 15 Meters from Centerline of Travel* |
| January 1, 1988 | 80 dBA |
* 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 4 and are enforced by the FMCSA.
| Effective Date | Speed | Maximum Noise Level 15 Meters from Centerline of Travel |
| January 8, 1986 | Less than 56 kph | 83 dBA |
| January 8, 1986 | Greater than 56 kph | 87 dBA |
| January 8, 1986 | Stationary | 85 dBA |
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 Title 23 of the United States Code of Federal Regulations Part 772 (attached). The regulations require the following during the planning and design of a highway project: 1) identification of traffic noise impacts; examination of potential mitigation measures; 2) the incorporation of reasonable and feasible noise mitigation measures into the highway project; and 3) 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 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.
The FHWA noise abatement procedures are codified in the Code of Federal Regulations (23 CFR 772). The procedures are described in the following sections.
Noise descriptors are used to describe the time-varying nature of noise. The L10 and Leq noise descriptors are used in the abatement procedures. The former is the noise level exceeded 10% of the time in the noisiest hour of the day. The latter is the constant, average sound level, which over a period of time contains the same amount of sound energy as the varying levels of the traffic noise. The L10 is a statistical descriptor that is easy for most people to determine and understand. While the Leq descriptor is harder for inexperienced people to understand, it has the advantages over L10 of being more reliable for low-volume roadways and of permitting noise levels from different sources to be added directly to one another for inclusion in noise analyses. Leq for typical traffic conditions is usually about 3 dBA less than L10 for the same conditions.
A traffic noise impact occurs when either of the following conditions exist:
The projected traffic noise levels approach or exceed the noise abatement criteria (NAC) shown in Table 5, or
The projected traffic noise levels substantially exceed the existing noise levels in an area.
| Activity Category | Leq(h) | L10(h) | Description of Activity Category |
| A | 57 (Exterior) | 60 (Exterior) | Lands on which serenity and quiet are of extraordinary significance and serve an important public need and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose. |
| B | 67 (Exterior) | 70 (Exterior) | Picnic areas, recreation areas, playgrounds, active sports areas, parks, residences, motels, hotels, schools, churches, libraries, and hospitals. |
| C | 72 (Exterior) | 75 (Exterior) | Developed lands, properties, or activities not included in Categories A or B above. |
| D | -- | -- | Undeveloped lands. |
| E | 52 (Interior) | 55 (Interior) | Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals, and auditoriums. |
* Either L10(h) or Leq(h) (but not both) may be used on a project.
There is no mandated definition for what constitutes a substantial increase over existing noise levels in an area. Most State highway agencies use either a 10 dBA increase or a 15 dBA increase in noise levels to define a "substantial increase" in existing noise levels. Several State highway agencies use a sliding scale to define substantial increase. The sliding scale combines the increase in noise levels with the absolute values of the noise levels, allowing for a greater increase at lower absolute levels before a substantial increase occurs.
The location of existing activities in the vicinity of various study alternatives for a highway project are identified by individual land uses, or by broad categories of land use for which a single NAC level may apply. In some cases, lands which are undeveloped at the time of the project may be known to be under consideration for development in the future. If this is the case and definite commitments have been made to develop the land, then, these lands are treated as developed and the highway noise impacts assessed accordingly. Primary consideration for highway traffic noise analysis is normally given to exterior areas where frequent human use occurs.
The FHWA regulation makes a distinction between projects for which noise abatement is considered as a feature in a new or expanded highway and those for which noise abatement is considered as a retrofit feature on an existing highway. The former are defined as Type I projects, the latter as Type II. For Type I projects, the consideration of noise abatement as part of the highway construction project is mandatory if Federal-aid funds are to be used and if a traffic noise impact is expected to occur. Type II projects are, however, completely voluntary on the part of the individual States, and such projects compete for funds with all the other construction needs of the States. It should be noted that the National Highway System Designation Act of 1995 (NHS) restricted Federal participation in Type II noise barriers to those projects that were approved before November 28, 1995 or are proposed along lands where land development or substantial construction predated the existence of any highway.
Analysis of the traffic noise impacts expected from construction of a highway involves a number of technical steps. The traffic noise analysis includes the following for each alternative under detailed study:
identification of existing activities, developed lands, and undeveloped lands for which development is planned, designed and programmed, which may be affected by traffic noise from the highway;
determination of existing noise levels;
prediction of traffic noise levels;
determination of traffic noise impacts; and
examination and evaluation of alternative noise abatement measures for reducing or eliminating the traffic noise impacts.
If potential traffic noise impacts are identified, noise abatement is considered and implemented, if it is found to be both reasonable and feasible. The views of the impacted residents are a major consideration in reaching a decision on the reasonableness of abatement measures to be provided. When noise abatement measures are being considered, every reasonable effort is made to obtain substantial noise reductions. Substantial noise reductions have been defined by State highway agencies to typically range from 5 to 10 dBA.
Federal funds may be used for noise abatement measures where:
a traffic noise impact has been identified,
the noise abatement measures will reduce the traffic noise impact, and
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.
The Federal share of the abatement costs is at the same participating ratio as for the system on which the project is located.
If traffic noise impacts are identified, various noise abatement measures are considered to mitigate the adverse impacts. The construction of a noise barrier is the mitigation measure most often associated with the concept of noise abatement. For this reason a special section on noise barriers, which begins on page 12, has been included in this report to discuss this subject in more detail.
Other possible noise abatement measures include traffic management measures, creating buffer zones, planting vegetation, installing noise insulation in buildings, and relocating the highway.
Traffic management measures can sometimes reduce noise problems. For example, if acceptable alternative truck routes are available, trucks can be prohibited from certain streets and roads, or they can be permitted to use certain streets and roads only during daylight hours. Traffic lights can be changed to smooth out the flow of traffic and to eliminate the need for frequent stops and starts. Speed limits can be reduced; however, about a 32 kilometer-per-hour reduction in speed is necessary for a readily noticeable (5 dBA) decrease in noise levels.
Buffer zones are undeveloped, open spaces which border a highway. Buffer zones are created when a highway agency purchases land, or development rights, in addition to the normal right-of-way, so that future dwellings cannot be constructed close to the highway. This prevents the possibility of constructing dwellings which would otherwise experience an excessive noise level from nearby highway traffic. An additional benefit of buffer zones is improvement of the roadside appearance. However, because of the tremendous amount of land which must be purchased and because in many cases dwellings already border existing roads, creating buffer zones is often not possible. While Federal-aid highway funds may be used on a highway project to create buffer zones, this measure has not been used very often.
Vegetation, which is so high, wide, and dense that it cannot be seen over or through, can decrease highway traffic noise. However, it requires a 61-meter width of such vegetation to reduce noise by 10 decibels, which cuts in half the loudness of traffic noise. It is not feasible to plant enough vegetation along a road to achieve such reductions. If vegetation already exists, it can be saved to maintain a psychological relief, if not an actual lessening of traffic noise levels. If vegetation does not exist, it can be planted for psychological relief, not to reduce traffic noise levels.
Insulating buildings can greatly reduce highway traffic noise, especially when windows are sealed and cracks and other openings are filled. Sometimes noise-absorbing material can be placed in the walls of new buildings during construction. However, insulation can be costly, because air conditioning is usually necessary once the windows are sealed. Federal-aid highway funds may be used for noise insulation of public-use or non-profit institutional structures. Such funds may also be used for noise insulation of residences and other private-use buildings where noise impacts are severe and no other type of abatement is possible. Very few private-use buildings have been noise insulated with Federal-aid highway funds. The majority of Federal-aid highway funds used for noise insulation has been spent to noise insulate schools. In many parts of the country, highway agencies do not have the authority to insulate buildings; thus, in those States, noise insulation cannot be included as part of a highway project.
A noise attenuation measure which should always be considered is the possibility of altering the highway location to avoid those land use areas which have been determined to have a potential traffic noise impact. Since sound intensity decays with distance from the source, increased distance between the noise source and receiver will reduce the noise impact. It may also be possible to obtain attenuation by depressing the roadway slightly to produce a break in the line of sight from the source to the receiver. Potential noise reduction should be considered with the many other factors which influence the selection of roadway alignment.
The FHWA noise regulation requires coordination with local officials whose jurisdictions are affected. The primary purpose of this coordination is to promote compatibility between land development and highways.
Highway agencies furnish the following information to appropriate local officials:
Estimated future traffic noise levels at various distances from the highway improvement.
Locations where local communities should protect future land development from becoming incompatible with anticipated highway traffic noise levels.
The FHWA has developed a model to accurately predict future highway traffic noise levels. State highway agencies either use the FHWA model for highway traffic noise analysis or a model based upon the same methodology as that contained in the FHWA model.
The FHWA has also developed national averages of vehicle emission levels to be used in the FHWA prediction model. State highway agencies either use the national average levels or measure their own levels based upon FHWA measurement procedures.
Highway construction noise is often viewed by the public as being short term and a necessary price for growth and improvement. Highway construction noise should generally be addressed in a qualitative, rather than quantitative, manner commensurate with the scope of the highway project. Construction noise levels may be predicted, if warranted. If potential construction noise impacts are identified, a common sense approach should be utilized to incorporate appropriate abatement measures into the highway project.
Noise barriers are solid obstructions built between the highway and the homes along the highway. Effective noise barriers can reduce noise levels by ten to fifteen decibels, cutting the loudness of traffic noise in half. Barriers can be formed from earth mounds along the road (usually called earth berms) or from high, vertical walls. Earth berms have a very natural appearance and are usually attractive. However, an earth berm can require quite a lot of land, if it is very high. Walls take less space. They are usually limited to eight meters in height because of structural and aesthetic reasons. Noise walls can be built out of wood, stucco, concrete, masonry, metal, and other materials. Noise barriers are designed and constructed to be visually pleasing and blend with their surroundings.
The Federal-aid highway program has always been based on a strong State-Federal partnership. At the core of that partnership is a philosophy of trust and flexibility, and a belief that the States are in the best position to make investment decisions that are based on the needs and priorities of their citizens. The FHWA noise regulations give each State department of transportation (DOT) flexibility in determining the reasonableness and feasibility of noise abatement and, thus, in balancing the benefits of noise abatement against the overall adverse social, economic, and environmental effects and costs of the noise abatement measures. The State DOT must base its determination on the interest of the overall public good, keeping in mind all the elements of the highway program (need, funding, environmental impacts, public involvement, etc.). Congress affirmed and extended the philosophy of partnership, trust, and flexibility in the enactment of ISTEA.
The flexibility in noise abatement decisionmaking is reflected by data indicating that some States have built many noise barriers and some have built none. Through the end of 1998, forty-four State DOTs and the Commonwealth of Puerto Rico have constructed over 2,610 linear kilometers of barriers at a cost of over $1.4 billion ($1.9 billion in 1998 dollars). Six States and the District of Columbia have not constructed noise barriers to date. Table 6 lists the ten States that have constructed the most noise barriers, in terms of area, length, and cost. The cost data in Table 6 give a general indication of trends. However, the data should not be used for exact comparisons, since precise, uniform individual barrier costs are very difficult to obtain. Table 7 shows total noise barrier areas by material type. Table 8 lists the six States that have not constructed noise barriers to date.
| Square Meters (Thousands) | Linear Kilometers | Actual Cost (Millions) | 1998 Dollars (Millions) | ||||
| California | 2,849 | California | 777.2 | California | $399.6 | California | $487.2 |
| New Jersey | 806 | Virginia | 153.3 | New Jersey | 182.3 | New Jersey | 210.4 |
| Virginia | 790 | New Jersey | 142.1 | Maryland | 138.0 | Maryland | 153.2 |
| Ohio | 613 | Ohio | 138.2 | Virginia | 120.8 | Virginia | 143.0 |
| Maryland | 581 | New York | 110.7 | New York | 104.1 | New York | 116.5 |
| New York | 496 | Colorado | 104.4 | Pennsylvania | 74.8 | Pennsylvania | 88.3 |
| Minnesota | 491 | Minnesota | 101.8 | Ohio | 63.1 | Illinois | 71.0 |
| Illinois | 346 | Maryland | 99.6 | Illinois | 61.2 | Ohio | 68.1 |
| Pennsylvania | 333 | Illinois | 97.8 | Florida | 55.1 | Minnesota | 62.7 |
| Colorado | 329 | Pennsylvania | 83.5 | Michigan | 48.1 | Florida | 62.3 |
| 10 State Total | 7,634 | 1,808.6 | 10 State Total | $1,24.1 | $1,462.7 | ||
| Single Material Barriers | Combination Barriers | ||
| Material | Square Meters (Thousands) | Material | Square Meters (Thousands) |
| Concrete/Precast | 4292 | Berm/Wood | 279 |
| Block | 2731 | Concrete/Block | 166 |
| Wood/Post & Plank | 508 | Wood/Concrete | 155 |
| Concrete/Unspecified | 456 | Berm/Concrete | 138 |
| Berm Only | 344 | Berm/Metal | 134 |
| Wood/Glue Laminated | 294 | Metal/Concrete | 85 |
| Metal/Unspecified | 240 | Berm/Block | 72 |
| Wood/Unspecified | 239 | Concrete/Brick | 54 |
| Absorptive | 154 | Wood/Metal | 43 |
| Brick | 94 | Berm/Wood/Concrete | 27 |
| Other | 139 | Wood/Block | 26 |
| Berm/Wood/Metal | 16 | ||
| Other | 116 | ||
| Total | 9,491 | Total | 1,311 |
| Alabama | Mississippi | Montana |
| North Dakota | South Dakota | Rhode Island |
Noise barriers can be quite effective in reducing noise for receptors within approximately 61 meters of a highway. Table 9 summarizes barrier attenuation.
| Reduction in Sound Level | Reduction in Acoustic Energy | Degree of Difficulty to Obtain Reduction |
| 5 dBA | 70% | Simple |
| 10 dBA | 90% | Attainable |
| 15 dBA | 97% | Very Difficult |
| 20 dBA | 99% | Nearly Impossible |
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. Openings in noise walls for driveway connections or intersecting streets greatly reduce the effectiveness of barriers. In some areas, homes are scattered too far apart to permit noise barriers to be built at a reasonable cost.
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. 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.
Over the last two decades, much work has been done within the highway program to develop the basic tools necessary to analyze the impacts of highway traffic noise. Efforts have focused on the establishment of criteria for considering highway traffic noise, the measurement and prediction of noise levels, and the development and evaluation of feasible measures to abate highway traffic noise. Today, research efforts are continuing to assure that analysis tools reflect the current state-of-the-art in highway traffic noise, while meeting the program needs of State highway agencies.
Highway traffic noise research has been guided and continues to be guided by representatives of State DOTs, other State government agencies, local government agencies, Federal agencies, and the academic and private sectors. An important part of this cooperative effort is the work of the Transportation Research Board Committee on Transportation-Related Noise and Vibration, which has been instrumental in identifying and prioritizing research needs. Another important part of this effort is the research work that individual States conduct within their own highway programs.
Future research efforts will strive to produce more cost effective solutions and efficient allocation of resources to deal with the problems of highway traffic noise. Emphasis is anticipated in the areas of traffic noise prediction and abatement analysis.
The United States has undertaken a program which utilizes a three-part approach to the abatement of highway traffic noise. Noise-compatible development through effective land use planning and control is traditionally an area of local responsibility. Source control or control of noise emissions from the vehicles themselves is a joint responsibility of private industry and of Federal, State, and local governments. The FHWA has established noise standards for different types of land use activities adjacent to highways. These standards require that for certain types of federally-aided highway projects, States must conduct noise analyses to identify potential highway traffic noise impacts. If impacts are identified, noise abatement measures must be considered and implemented, if determined to be both reasonable and feasible. Among the various types of possible abatement measures, the construction of noise barriers is most commonly used.
