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.
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 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.
Noise barriers can be quite effective in reducing noise for receptors within approximately 61 meters of a highway. Table 10 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 |
Figure 2: Noise Barrier Examples
Figure 3: Noise Barrier Shadow Zone
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.
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.
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, and surface treatment.
The design approach for noise barriers may vary considerably depending upon roadway design constraints. For example, the design problem both from an acoustic and visual standpoint is substantially different for a straight roadway alignment with narrow right-of-way and little change in vertical grades than for a roadway configuration with a large right-of-way and variations in horizontal and vertical alignments. In the former case, the roadway designer is limited in the options of visual design to minor differences in form, surface treatment, and landscaping. In the latter case, the designer has the opportunity to vary the barrier type, utilize landscaped berming, and employ more extensive approaches to develop a visually pleasing barrier.
From both a visual and a safety standpoint, noise barriers should not begin or end abruptly. A gradual transition from the ground plane to the desired barrier height can be achieved in several ways. One concept is to begin or terminate the barrier in an earth berm or mound. Other concepts include bending back and sloping the barrier, curving the barrier in a transition form, stepping the barrier down in height, and terminating the barrier in a vegetative planter. The concept of terminating the barrier in a vegetative planter should only be utilized in areas where climatic conditions are conducive to continued vegetative growth and in areas where the planter edges will be protected from potential conflict with roadway traffic.
Graffiti on noise barriers can be a potential problem. A possible solution to this problem is the use of materials which can be readily washed or repainted. Landscaping and plantings near barriers can be used to discourage graffiti as well as to add visual quality.
Highway traffic noise levels are not substantially increased by construction of a noise barrier on the opposite side of a highway from a receiver. If both the direct noise levels and the reflected noise levels are not abated by natural or artificial terrain features, the noise increase is theoretically limited to 3 dBA, due to a doubling of energy from the noise source. In practice, however, not all of the acoustical energy is reflected back to the receiver. Some of the energy is diffracted over the barrier, some is reflected to points other than the receiver, some is scattered by ground coverings (e.g., grass and shrubs), and some is blocked by the vehicles on the highway. Additionally, some of the reflected energy to the receiver is lost due to the longer path that it must travel. Attempts to conclusively measure this reflective increase have never shown an increase of greater than 1-2 dBA, an increase that is not perceptible to the average human ear.
Multiple reflections of noise between two parallel plane surfaces, such as noise barriers or retaining walls on both sides of a highway, can theoretically reduce the effectiveness of individual barriers and contribute to overall noise levels. However, studies of the issue have not indicated problems associated with this type of reflective noise. Any measured increases in noise levels have been less than can be perceived by normal human hearing. Studies have suggested that to avoid a reduction in the performance of parallel reflective noise barriers, the width-to-height ratio of the roadway section to the barriers should be at least 10:1. The width is the distance between the barriers, and the height is the average height of the barriers above the roadway. This means that two parallel barriers 3 meters tall should be at least 30 meters apart.
To provide standard structural design criteria for the preparation of noise barrier plans and specifications, the American Association of State Highway and Transportation Officials (AASHTO) Subcommittee on Bridges and Structures developed "Guide Specifications for Structural Design of Sound Barriers," which was published in 1989 and amended in 1992.
These specifications allow for more consistency and less conservatism in barrier design. SHAs are encouraged to apply realistic noise barrier structural design practices and to avoid overly conservative design procedures, especially those related to wind load criteria.
AASHTO has also published a "Guide on Evaluation and Abatement of Traffic Noise: 1993 (code GTN- 3)." This report contains a good discussion of the problem of highway traffic noise and ways to address the problem in the United States. It presents a discussion very similar to that found in FHWA literature. Copies of the report are available from AASHTO, 444 North Capitol Street, N.W., Suite 429, Washington, DC, 20001, telephone (202) 624-5800.
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 SHA 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 SHA 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. From 1970 to 1992, forty SHAs and the Commonwealth of Puerto Rico have constructed over 1,486 linear kilometers of barriers at a cost of over $816 million ($875 million in 1992 dollars). Ten States and the District of Columbia have not constructed noise barriers to date.
Vegetation, if it is high enough, wide enough, and dense enough that it cannot be seen through, can decrease highway traffic noise. A 61-meter width of dense vegetation can reduce noise by 10 decibels, which cuts in half the loudness of traffic noise. It is usually impossible, however, to plant enough vegetation along a road to achieve such reductions.
Roadside vegetation can be planted to create a psychological relief, if not an actual lessening of traffic noise levels. Since a substantial noise reduction cannot be obtained for an extended period of time, the FHWA does not consider the planting of vegetation to be a noise abatement measure. The planting of trees and shrubs provides only psychological benefits and may be provided for visual, privacy, or aesthetic treatment, not noise abatement.
Figure 4: Vegetation and Noise Reduction
Controlling traffic can sometimes reduce noise problems. For example, 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 33 kilometer-per-hour reduction in speed is necessary for a noticeable decrease in 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. In many parts of the country, highway agencies do not have the authority to insulate buildings; thus, in those States, insulation cannot be included as part of a highway project. Noise insulation is normally limited to public use structures such as schools and hospitals.
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 that would otherwise have an excessive noise level from nearby highway traffic. An additional benefit of buffer zones is that they often improve the roadside appearance. However, because of the tremendous amount of land that must be purchased and because in many cases dwellings already border existing roads, creating buffer zones is often not possible.
Note: Open space can be left as a buffer zone between residences and a highway.
Pavement is sometimes mentioned as a factor in traffic noise. While it is true that noise levels do vary with changes in pavements and tires, it is not clear that these variations are substantial when compared to the noise from exhausts and engines, especially when there are a large number of trucks on the highway. Additional research is needed to determine to what extent different types of pavements and tires contribute to traffic noise.
It is very difficult to forecast pavement surface condition into the future. Unless definite knowledge is available on the pavement type and condition and its noise generating characteristics, no adjustments should be made for pavement type in the prediction of highway traffic noise levels. Studies have shown open-graded asphalt pavement can initially produce a benefit of 2-4 dBA reduction in noise levels. However, within a short time period (approximately 6-12 months), any noise reduction benefit is lost when the voids fill up and the aggregate becomes polished. The use of specific pavement types or surface textures must not be considered as a noise abatement measure.
