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Analysis and Abatement Guidance

Appendix C: Highway Traffic Noise Abatement Measures

Abatement Measures in 23 CFR 772

Early in the planning stages of most highway improvements, highway agencies prepare a highway traffic noise study. The purpose of this study is to determine whether the project will result in highway traffic noise impacts. If the predicted highway traffic noise levels cause an impact, the highway traffic noise study must consider highway traffic noise abatement measures to reduce the highway traffic noise levels. If an FHWA approved highway traffic noise abatement measure is determined to be feasible and reasonable, then the highway agency must incorporate the noise abatement measure in the project design. The FHWA approved highway traffic noise abatement measures include creating buffer zones, constructing barriers, installing noise insulation in buildings, and managing traffic. With the exception of noise insulation, the highway agency must maintain the noise abatement measure in perpetuity.

Noise Barriers

Technical Considerations and Barrier Effectiveness

Noise barriers are the most commonly used form of noise abatement and are the only form of noise abatement required for consideration on Federal or Federal-aid projects in accordance with 772.13(c)(1).

Noise barriers are solid obstructions built between the highway and the receivers along the highway. Effective noise barriers can reduce noise levels by 10 decibels, cutting the loudness of traffic noise in half. Barriers come in the form of:

  1. Earthen mounds along the road, called earth berms
  2. High, vertical barriers, called noise barriers or
  3. A combination of earth berms and noise barriers

Earth berms have a very natural appearance and are usually attractive. However, due to their large footprint, very tall berms require large amounts of land. Noise barriers require less space, but may have height restrictions because of structural requirements and aesthetic considerations. Noise barriers are of wood, stucco, concrete, masonry, metal, and other materials. Some States also include aesthetic requirements for color and texture applications on noise barriers to improve their appearance.

Noise barriers 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 dB additional noise level reduction for each 2 feet of height after it breaks the line of sight (with a maximum theoretical total reduction of 20 dB(A)). To avoid undesirable end effects, a good general rule 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 barriers for driveway connections or intersecting streets reduce the effectiveness of barriers. In some areas, homes are scattered too far apart to permit construction of noise barriers at a reasonable cost.

Noise barriers can be quite effective in reducing highway traffic noise for receivers within approximately 200 feet of a highway. Table 8 summarizes barrier attenuation.

Table 8: Barrier Attenuation
Reduction in Sound
Level
Reduction in Acoustic Energy Difficulty to Obtain Reduction
5 dB(A) 70% Simple
10 dB(A) 90% Attainable
15 dB(A) 97% Very Difficult
20 dB(A) 99% Nearly Impossible

Noise Barrier Material Types

There are no Federal requirements or FHWA regulations related to the selection of material types in the construction of highway traffic noise barriers. Individual highway agencies select the material types to use when building their barriers. Highway agencies normally make this selection based on a number of factors such as aesthetics, durability, maintenance, cost, public comments, etc. The FHWA does not specify the type of material to use for noise barrier construction, but the material type chosen must meet State specifications approved by the FHWA. The material chosen should be rigid and of sufficient density (approximately 4 pounds/square foot minimum) to provide a k loss of 20 dB(A) greater than the expected reduction in the noise diffracted over the top of the barrier.

Shadow Zone

Noise barriers and earthen berms create a shadow zone. The vertical nature of a noise barrier or earthen berm causes an area of decreased sound energy on the non-highway side due to diffraction, reflection and transmission loss. Receivers that are located in the shadow zone (see Figure 2), will benefit the most from the noise barrier or earth berm.

Figure 2: Noise Barrier Shadow Zone

This figure shows a residence on the top of the hill and a residence at the bottom of the hill. The residence at the bottom of the hill has an earthen berm next to it, which is reducing the noise from the highway located on the other side of the earthen berm. The figure is depicting that the earthen berm is generating a "shadow zone" for the residence at the bottom of the hill resulting it a lower noise level. The residence at the top of the hill is outside of the "shadow zone" generated by the earthen berm and therefore not experiencing a lower noise level.

Shadow Effect of Noise Barrier

The noise barrier protects the shielded house, but leaves the unshielded house unprotected.

Public Perception

Overall, public reaction to 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 improved; they have a more relaxing environment, open windows more often, and use yards more in the summer. Other perceived benefits include: increased privacy, cleaner air, improved view and a rural sense, 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. Residents near a barrier seem to feel that barriers effectively reduce highway traffic noise and that the benefits of barriers outweigh the disadvantages of the barriers.

Commercial property owners may oppose noise barrier construction because the barrier may block the line of site to the property.

Highway agencies should inform all affected residents and property owners that noise barriers do not eliminate highway traffic noise. Some noise will remain, even with the construction of highly effective barriers.

Receiver Locations for Noise Barrier Design

Highway agencies have options for receiver locations for barrier design:

  1. At or near a building in residential or commercial areas, and
  2. At an area between the right-of-way line and a building where frequent human activity occurs.

Either of these locations is acceptable, as long as a highway agency chooses one location and applies it uniformly and consistently in all its analyses It is important to note that using an area at or near the highway right-of-way line as a receiver location for barrier design will produce an inappropriate amount of noise reduction and should, therefore, be avoided.

Design Considerations

A successful design approach for noise barriers should be multidisciplinary and include architects/planners, landscape architects, roadway engineers, acoustical engineers, and structural engineers. Receiver locations and 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.

The designers should consider the psychological effect on the passing motorist; designing barriers within the context of the setting. This means different design considerations for dense, urban settings than for open suburban or rural areas. The design should also avoid monotony for the motorist. At normal roadway speeds, visual perception of noise barriers will tend to be of the overall design of the barrier and its color and surface texture. Due to the scale of barriers, a primary objective is to achieve a visually pleasing design by avoiding a tunnel effect with major variations in material type and surface treatment (texture and color). Some localities may desire installation of special icon panels depicting works of art or perhaps emblems significant to the area. Highway agencies are encouraged to work with local governments to help improve the appearance of noise barriers using context sensitive solutions.

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 when compared to a roadway configuration with a wide 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 employ a range of design alternatives to develop a visually pleasing and effective barrier.

From both a visual and a safety standpoint, noise barriers should not begin or end abruptly. There are several alternatives to achieve a gradual transition from the ground plane to the desired barrier height. One concept is to begin or terminate the barrier in an earth berm or mound. Other possibilities include adding a slope to the top of the barrier, curving the barrier in a transition form, stepping the barrier down in height, or terminating the barrier in a vegetative planter. The concept of terminating the barrier in a vegetative planter in areas where climatic conditions are conducive to continued vegetative growth.

Visual Impact

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.

Carefully consider the visual character of noise barriers in relationship to the environment. 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, consider the relationship of material, surface texture, and color 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.

Preserve aesthetic views and scenic vistas to the extent possible. However, the highway agency cannot reject feasible and reasonable noise barrier based on visual impacts without justification. Local governments cannot arbitrarily veto and/or restrict the length or height of an abatement measure determined feasible and reasonable based on visual quality concerns. In this case, the FHWA will not authorize the Federal-aid project unless the recommended noise abatement is included in the project design, plans and specifications.

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 and may provide a distraction for motorists.

Graffiti

Graffiti on noise barriers can be a potential problem. A possible solution to this problem is applying an anti-graffiti coating or using materials. Landscaping and plantings near barriers can discourage graffiti as well as to add visual quality.

Reflection of Noise from a Noise Barrier

Construction of a noise barrier on the opposite side of the highway from a receiver will not result in a substantial increase in highway traffic noise levels. If 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 dB(A), due to a doubling of energy from the noise source. In practice, however, not all of the acoustical energy reflects 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 rarely show an increase of greater than 1-2 dB(A), 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 10 feet tall should be at least 100 feet apart.

Highway agencies must include provisions in their noise policy for use of absorptive treatment on roadside structures. This includes noise barriers, retaining walls, bridges and any other structure the highway agency may consider for application of a sound absorptive material.

Noise Barrier Structural and Safety Design Criteria

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 and 2002. These specifications allow for more consistency and less conservatism in barrier design. Highway agencies 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 on the AASHTO homepage: http://www.aashto.org/aashto/organization.nsf/homepage/overview.

There are several safety considerations to keep in mind when designing a noise barrier. The designer must consider the effect on site distance for drivers. There AASHTO Green Book provides design requirements for Stopping Sight Distance (SSD) Decision Sight Distance (DSD), and the Horizontal Sightline Offset (HSO).

Designers must also consider the safety of the traveling public and those on adjacent properties when considering possible vehicle impacts with noise barriers. Several States use specially designed noise barriers on bridges to guard against dislodging of the barrier onto roads below the bridge. Another factor to consider is the presence of a noise barrier within the clear zone and the need for safety barriers in these circumstances.

Traffic Management

Controlling traffic can sometimes reduce highway traffic noise problems. Possible ways to achieve this are:

  1. Prohibiting trucks from certain streets and roads,
  2. Permitting trucks to use certain streets and roads only during daylight hours,
  3. Timing traffic lights to achieve smooth traffic flow and to eliminate the need for frequent acceleration and deceleration,
  4. Reducing speed limits reduces highway traffic noise levels; however, an approximate reduction of 20 mph is necessary for a readily perceptible decrease in noise levels.

Alteration of Horizontal and Vertical Alignments

A change in the horizontal or vertical alignment of the highway may reduce noise levels at noise sensitive receivers. Suppressing the highway's vertical alignment to create a natural berm between the highway and receivers or shifting the highway's horizontal alignment away from noise sensitive receivers and closer to less sensitive receivers are two methods to accomplish this measure. Usually, this approach is limited to use on projects on new alignment as a means of avoiding impacts rather than as an abatement measure. It is may be very expensive to alter the alignment of a highway to reduce noise levels.

Acquisition of Property Rights for Noise Barrier or Buffer Zones

The highway agency may acquire property rights to allow for the construction of a noise barrier. Include the cost of property purchased by the highway agency in the barrier's reasonableness determination. Buffer zones can only be used in Type I projects. The potential use of buffer zones applies to predominantly unimproved property; not to purchase homes or developed property to create a noise buffer zone. Highway agencies may purchase unimproved property to preclude future highway traffic noise impacts.

Buffer zones are undeveloped, open spaces that border a highway (as defined by this policy). Buffer zones occur when a highway agency purchases land or development rights, in addition to the normal right-of-way, to prohibit construction of future dwellings close to the highway. This prevents the possibility of exposing new dwellings to 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 needed land and because in many cases dwellings already border existing roads, creating buffer zones is often not possible. The intention of this provision is for purchase of currently undeveloped land. The highway agency should not consider purchase of developed land to create buffer zones.

The purchase of a noise easement is not eligible for Federal-aid participation.

Noise Insulation

Highway agencies may only consider noise insulation for public use or nonprofit institutional structures, e.g., places of worship, schools, hospitals, libraries, etc. "Public use or nonprofit institutional structures" means the facility is open for public use, owned by the public or that a nonprofit organization owns the facility.

Insulating buildings can greatly reduce highway traffic noise. Sometimes this involves installation of sound absorbing material in the walls of a new building during construction. However, insulation can be costly because air conditioning is usually necessary once the windows are sealed. In some 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 places of worship, schools and hospitals.

The highway agency should consider entering into a legal agreement with the owners of a building that will receive noise insulation specifying the noise insulation requirements, such as the sound transmission class (STC) of windows and doors used for noise insulation, and ensuring the owners understand that they bear all post installation expenses such as utilities and maintenance. The State noise policy should also cover these issues.

Visual Screening

Vegetation

Vegetation, if it is high enough, wide enough, and dense enough and opaque may reduce highway traffic noise. A 200-foot width of dense vegetation can reduce noise by 10 decibels. It is usually impossible, however, to plant enough vegetation along a road to achieve such reductions. See Figure 3.

Roadside vegetation may create a psychological effect, if not an actual lessening of highway traffic noise levels. Since a substantial noise reduction does not occur until vegetation matures, the FHWA does not consider the planting of vegetation to be a highway traffic noise abatement measure. The planting of trees and shrubs provides psychological benefits and by providing visual screening, privacy, or aesthetic treatment, but not highway traffic noise abatement.

Figure 3: Vegetation

This figure shows two pictures of vegetation and noise reduction. The first figure illustrates a 200 foot area of dense vegetation from understory to the top of the trees between the highway and a residence . With this type of vegetation, noise levels can potentially be reduced by 10 dBA. The second picture illustrates a single row of planted trees between the highway and a residence. In this situation there may be a psychological effect, but there is no noise reduction.

Privacy Fencing

Privacy fencing provides a visual screen between the source and receptor, but is unlikely to provide a discernible reduction in noise levels. Like vegetation, this screening may provide psychological relief, but not highway traffic noise abatement.

Flexibility in Decision Making

The basis for the Federal-aid highway program is 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 on the needs and priorities of their citizens. The FHWA highway traffic noise regulations give highway agencies flexibility to determine the feasibility and reasonableness of highway traffic noise abatement; balancing the benefits of highway traffic noise abatement against the overall adverse social, economic and environmental effects and costs of the highway traffic noise abatement measures. The highway agency 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.).

Updated: 01/27/2012
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