Category HIGHWAY ENGINEERING HANDBOOK

9.9.1 Noise Barrier Design Loads

Wind Loads. In most cases, the wind load represents the main load. The design pressure depends upon the wind velocity, which should be based upon a 50-year mean recur­rence interval (Fig. 9.6). The wind pressure is applied perpendicular to the wall surface to develop the design wind load. On the basis of AASHTO Guide Specifications for Structural Design of Sound Barriers, the pressure may be calculated from

U. S. Customary units: P = 0.00256(1.3 V)2 Cfc (9.1a)

SI units: P = 0.613(1.3V)2 CdCc (9.1b)

where P = wind pressure, lb/ft2 (N/m2)

V = wind velocity, mi/h (m/s)

Cd = drag coefficient = 1.2 for noise walls

Cc = combined height, exposure, and location coefficient

The factor of 1.3 in Eq. (9.1) provides for wind gusts. Values of Cc and calculated wind pressures are given in Table 9.3 A and B. The following four conditions with increasing levels of wind pressure are included:

1. Noise barriers not located on structures and having exposure B1. This includes urban and suburban areas with numerous closely spaced obstructions having the size of single-family dwellings or larger that prevail in the upwind direction from the noise wall for a distance of at least 1500 ft (457 m).

2. Noise barriers not located on structures and having exposure B2. This includes urban areas with more open terrain that does not meet exposure B1.

3. Noise barriers located on bridge structures, retaining walls, or traffic barriers (exposure C). This is based on open terrain with scattered obstructions.

4. Noise barriers not located on structures and having exposure D. This includes coastal regions.

The interpretation of the surrounding terrain and identification of local conditions that may have increased effect on wind loads are left to the design engineer.

TABLE 9.3A Design Wind Pressures on Noise Walls

Pressure for indicated wind velocity, lb/ft2

Pressure for Indicated wind velocity, N/m2

Seismic Loads. AASHTO requires that, where structures are designed for seismic load, noise walls also be designed for such. They define the seismic load (EQD) as

EQD = A X f X D (9.2)

where A = acceleration coefficient (varies from 0.05 to 0.40 depending on geographical location; see AASHTO Guide Specifications, Fig. 1-2.1.3)

D = dead load

f = dead load coefficient (2.50, on bridges; 0.75, not on bridges; 8.0, connections of prefabricated walls to bridges; 5.0, connections of prefabricated walls to retaining walls)

The product of A and f must not be taken as less than 0.10.

Other Loads. In addition to dead load, other loads that might be encountered include earth load, live load surcharge, and ice and snow load. When encountered, these loads can be developed from information in the AASHTO Standard Specifications for Highway Bridges. Increased allowable stress levels may be used for certain combinations, as discussed below.

Preliminary Engineering. During the preliminary engineering step, the following actions should take place:

• Develop a basic noise abatement plan, and determine barrier height and location.

• Develop alternative methods of abatement such as walls, earth berms, berm-wall combinations, etc.

• Develop alternative locations for abatement facilities.

• Develop alternative material types such as concrete, timber, masonry, or steel.

• Develop a conceptual landscaping plan for each alternative.

• Develop cost estimates for alternatives.

• Develop a general environmental plan.

• Make preliminary arrangements for public informational meetings.

Items to be considered in selecting proposed alternatives include aesthetics, traffic safety, sight distance, drainage, maintenance, existing utilities, lighting, signing, potential soil problems, compatibility with surrounding terrain and land use, and restrictions imposed by available right-of-way. Consider any requirements for snow storage, future construction of sidewalks, trails, etc.

Layouts, cross-sections, and wall profiles should be prepared for each alternative. Aerial photography contour maps should provide sufficient accuracy for determining ground elevations. Supplementary field information may be required in problem areas. Drainage away from both sides of the noise barrier should be provided, with a minimum slope of 0.04. Ditches or culverts may be required where walls or berms alter natural drainage patterns.

Public and Municipal Involvement. Local officials and the affected public should be informed of the scope of the proposed work and the alternative methods being con­sidered to achieve noise abatement. Work through these groups to achieve a consensus. Provide sketches, renderings, plan drawings, and other visual aids to assist in the process. With this input, a public corridor plan should be developed with a consistent theme that considers aesthetics and avoids conflicts with adjacent barriers.

Preparation of Preliminary Plans. Preliminary plans must be prepared for design and safety review. The plans should include a layout with the wall placement and profiles of the ground line and the top of the barrier. Supplemental layouts for sight distance requirements may be required.

Preliminary Approvals. Local approval of the preliminary plan developed is sought at this time. Where applicable, municipal acceptance of maintenance responsibility of back slopes or other areas outside the noise barrier should be obtained. Subsequent approval by the state DOT and FHWA is then sought.

Final Design. Information on soil conditions at the final noise barrier location should be obtained from the soils engineer. The required depth of the investigation should correspond to the depth of post embedment or depth of spread footings. For construction in new embankment areas, care must be taken to avoid excessive differ­ential settlement, because of concern for wall tilting, rotation, or cracking (of rigid systems). If a combination wall and berm is to be constructed, consider specifying an embankment material that will result in an economical wall design. It may be desirable to use a cohesive material of uniform thickness that does not move when saturated with water for the upper portion of the berm.

Wall alignment can be modified slightly when necessary to make adjustments for standard panel sizes or material sizes; to fit with existing features such as trees, signs, lights, or utilities; or to better meet safety or drainage requirements.

Often, wall designs are based on standard agency plans. Special designs may be required where a wall ties into a bridge abutment or retaining wall, where the wall height exceeds the standards, where lights or signs are constructed integrally with the wall, where the wall must also serve as a retaining wall, or where soil properties are outside the range of those anticipated in the design standards.

State and local government agencies sometimes mandate that noise wall corridors be developed. As part of roadway improvement, they anticipate a need by local residents that will help approve the roadway system.

It is wise to keep a stock of compatible replacement materials on hand to repair damage from impact or vandalism. Consideration should be given to keeping replacement materials where they can weather to match installed barriers, such as for pressure-treated timber components. Also, if color is added to concrete panels during manufacture, it is desirable to make future replacement panels in the same operation.

The control of graffiti remains a problem in some urban areas. There are some anti­graffiti surface treatments available, but they are generally costly. Power washing and repainting are current options.

Plantings should be tolerant of roadside environments and require little or no maintenance. Access must be provided to both sides of the barrier for mowing, general maintenance, etc. Sometimes this may require backside access from city streets, or over­lap openings along the length of the barrier. In some cases, arrangements can be made with abutting property owners to maintain the area behind the barrier. If the noise barrier is over 5 ft (1.5 m) high, the right-of-way fence can usually be eliminated.

Some block masonry noise walls and retaining wall combinations made of 2000- to 3000-lb/in[13] [14] [15] [16] [17] [18] [19] [20] (14- to 21-MPa) dry cast units have exhibited extensive disintegration after 3 to 5 years. This disintegration is caused by salt spray from winter driving traffic during removal of snow and ice from the roadways. Testing of blocks removed from barriers showed similar disintegration and chloride ion content on the front and back or fill side. Application of treatments to seal only the front or exposed surfaces will not be effective for the back surfaces. Sealing the surfaces must be repeated and becomes a costly maintenance item. Work is underway to evaluate high – strength (5000-lb/in2) (35 MPa) dry cast blocks that should reduce susceptibility to chloride contaminants.

Care must be taken not to install a noise barrier in such a way that it will be a safety haz­ard. The general considerations presented in Chap. 6, Safety Systems, apply here. Noise barrier design should incorporate all of the safety design techniques used in the basic roadway design. Examples of features that should be considered include trans­verse location to provide required clear zone, slopes of berms, sight distances, wall ends, plantings, and transitions.

Ideally, noise barriers should be located beyond the clear zone. If not, a traffic barrier may be warranted. It is usually best to design the traffic barrier as part of the noise barrier. If a wall is located at or near the edge of the shoulder, the portion of the wall above the traffic barrier should be capable of withstanding the force of an occa­sional vehicle that may ride up above the top of the barrier. Concrete or masonry construction would often be used in this case. However, laminated wood construction may also be used.

At locations such as ramps, intersections, and merge areas, care must be taken to avoid blocking the line of sight between vehicles. The AASHTO Guide on Evaluation and Abatement of Traffic Noise gives the following suggestions for placement of noise barriers:

For on and off ramps, the minimum set back of a noise barrier is based upon the stopping sight distance, which is a function of the design speed and radius of curvature of the ramp. For ramp intersections, proper barrier location is set by the sight distance corresponding to the time required for a stopped vehicle to execute a left-turn maneuver (approximately 7.5 s). For intersecting roadways, barrier placement is determined from stopping sight distance, which depends on driver reaction time and deceleration rate.

The AASHTO Guide Specifications for Structural Design of Sound Barriers indi­cate that, when locating a sound barrier near a gore area, the wall should begin or end at least 200 ft (60 m) from the theoretical curb nose location.

Protrusions that could constitute a hazard must be avoided near traffic lanes, as well as facings that could become missiles in the event of a crash. Also, surfaces must not create excessive glare.

Sometimes it is necessary to store plowed snow between the roadway and the barriers over a width of 6 to 10 ft (1.8 to 3.0 m). In such cases, it should be removed as soon as practical to avoid blowing on the roadway and freezing. Also, there has been some occasional damage to wall panels from the pressure created by snowplows, and this should be avoided as well. Aside from snow storage, highway engineers should consider the potential for roadway icing problems resulting from deep shadows cast by walls.

The end of a noise barrier or earth berm can be a hazard to approaching traffic. When exposed to approaching traffic within the clear zone area, it should be treated with protection similar to that for other fixed objects. Barrier rails or crash cushions may be appropriate. End slopes for earth berms should be 6:1 or flatter, with 10:1 or 15:1 desirable.

Often, a detailed study is required to address the question of aesthetics. Alternative systems can be compared, with sketches, renderings, plan drawings, and other visual aids prepared to assist in the process. A multidiscipline team approach is desirable, including design engineers, planners, landscape architects, and environmental personnel. Public input to the selection system helps achieve acceptance of the final system. Designers should be concerned with the visual impact from both the driver’s side and the land user’s side of the wall.

Some of the important aspects of aesthetics include scale relationship, relationship to environmental setting, line form, color, and texture. A high barrier alongside a row of single-story houses is not desirable, nor is one placed so close to the residences that unwanted shadows are created. A rule of thumb is to locate the barrier at a distance of atleast 4 times its height from the residences. Barriers higher than 16 ft (5 m) should be critically evaluated for potential unsightly impact.

Evergreens and other plantings are often used with noise barriers to enhance appearance. Vines, encouraged to grow up the posts and across the top, have been appreciated by the public. Most agree that walls with extensive landscaping are the most visually appealing.

When the elevation changes along the length of the wall, it is generally considered more pleasing to step the wall rather than to taper it. Ordinarily, the wall will be con­structed vertically. There has been some use of walls that have the top tilted away from the roadway in an effort to reduce echo, but such walls tend to give the appear­ance of instability when viewed from the back side.

On concrete panel walls, etc., it is necessary to place steel brackets or similar devices at the top of the joints between panels to hold the panels in alignment. A slight horizontal difference of 1 or 2 in (25 or 50 mm) between the tops of adjacent panels may give the illusion that some panels are in distress. This illusion is greatly enhanced by sun shadow lines that, under certain conditions, cast increasing shadows as one looks along the panels. For walls already in place, maintenance forces can tilt panels back in place with a backhoe or similar equipment and add the brackets.

If a barrier is located in an area with dominant architectural features, this should be considered in the selection of barrier material, texture, and color. On the other hand, if located near dominant roadside features such as bridges, there should be an effort to create a strong visual relationship to such features.

In most cases, there should be some consistency in color and surface treatment. For example, some agencies use color scheme and architectural treatment to distinguish between particular corridors.

In general, barriers with darker colors are preferred to lighter ones because they tend to blend better with the background. Although it is usually desirable to avoid visual dominance, murals painted on noise barriers have been well received in some urban regions. The murals tend to discourage graffiti, and in some cases, youth groups have been active in restoring murals defaced by graffiti.

With concrete barriers, a textured appearance can give the effect of shadows and is often considered desirable. Deep textures are more effective than shallow ones. Such treatments can be achieved by a raking technique on the surface of the newly placed concrete. Colors can be obtained with additions to the mix, or by applying a pigmented sealer after the barrier is constructed. The latter technique helps take care of small color variations between panels and minor field problems. Also, coatings can aid in removing graffiti and restoring the intended surface.

For a pleasing visual effect, as well as for safety and acoustic considerations, barriers should not begin or end abruptly. To achieve this, they may be stepped down, flared, or tied into an earth berm, a hillside, a bridge abutment, or another feature. Tapering or stepping is particularly desirable where the height of the barrier exceeds 6 ft (1.8 mm).

Views of several noise walls are shown in Figs. 9.2 through 9.5 to illustrate some of the effects that can be achieved. Figure 9.2 shows concrete-block construction and deep texturing with vertical grooves. The wall is stepped rather than tapered in height. Figure 9.3 shows timber tongue-in-groove construction, with a natural finish and a stepped height. In Fig. 9.4, the alignment of the timber barrier has been changed to a buttress configuration, and extensive plantings have been added. A much different effect has been obtained with concrete post construction, in Fig. 9.5, where the light posts make a distinct contrast with the darker timber.

Making use of variant sun shadow lines on tops of concrete posts yields a changing view of the posts and wall as the sun direction changes during the day. Morning and afternoon shadow lines are greater and thus tend to make aesthetically pleasing wall tops. Also, early morning and late evening sun glare is reduced by north-south noise walls.

Presuming acoustical requirements are met, selection is usually based on cost and aesthetics. Costs that must be considered include site preparation, the barrier material itself, foundations, fabrication, erection, and maintenance. Aesthetics should be judged with the involvement of personnel with diverse backgrounds, and public participation should be encouraged. However, there are numerous factors that go into the final selection. Some factors that should be considered in wall selection are summarized in Table 9.1.

The reasonableness of constructing a noise barrier can be judged from a cost-benefit analysis. For example, Minnesota uses the following procedure. The benefit is based on the summed insertion loss (noise reduction) for each residence in the first two rows

TABLE 9.1 Factors to be Considered in Noise Wall Selection

Site

Site geometry Right-of-way width Relation to source height Configuration, single or parallel

Noise source

Traffic type and volume Noise frequencies Extraneous noise sources Material

Structural integrity Durability and maintenance Susceptibility to vandalism Acoustical properties

Cost

Site preparation Wall material Foundations Fabrication Erection Maintenance Aesthetics

Scale relationship

Environmental relationship

Line form

Color

Texture

Community preferences

of homes nearest the noise wall where the insertion loss is greater than 5 dBA. The ratio of this sum in dBA to the cost of the barrier in thousands of dollars must be greater than 0.4 for the benefit to be considered reasonable.

Except for berms and brick or masonry construction, most noise barriers are of post-and – panel construction, that is, vertical posts spaced a distance apart with horizontal or verti­cal panels running in between. Rails or girts may also run between the posts to support the panels. Posts are embedded in the foundation soil to design depth, which depends on wind loading, soil properties, and frost depth. Brick and masonry walls generally require spread footings, underlain with uniform layers of soil.

According to a 2006 FHWA survey, the main materials that have been used for noise wall construction, in order of usage, are the following:

• Concrete

• Block and brick

• Wood

• Metal

• Earth berms

Other materials sometimes used include plastic, glass, composites, and gabions (rock- filled wire baskets). Glass and clear plastic are alternatives where it is desirable to not block scenic views.

Concrete. Users indicate that selection has been based on cost, durability, low main­tenance, surface treatments available, and acoustical properties. Concrete walls can be precast, cast in place, or of post-and-panel construction. Precast concrete panels may be of either prestressed or reinforced construction. Various surface finishes such as texturing are available and are relatively inexpensive. A 4-in-thick (100 mm) wall pro­vides a relatively high transmission reduction of 32 dBA.

Block and Brick. Brick and masonry construction is also popular, mainly because of its pleasing appearance and acoustical properties. However, initial cost is likely to be higher, depending upon the geographic location, as well as repair cost if damaged. Slump block, cinder block, stone, and brick have all been used. Units can be arranged to produce various patterns. The typical transmission loss is 33 dBA, and this can be improved by the addition of mineral wool or fiberglass to the wall interior.

Wood. Attributes that favor selection include favorable cost, ease of construction, aesthetic appeal, and availability. Disadvantages include shrinkage, warpage, deteriora­tion, difficulty of quality control, discoloration around fasteners, and low resistance to vandalism. Wooden walls have been constructed from timbers, planks, plywood, and laminated products. Often, these materials are used for the panels or facing and con­crete or steel is used for the posts. Tongue-and-groove construction should be used for panels running between posts to eliminate gaps. The durability of wooden walls can be enhanced by using materials that have received a pressure preservative treatment. Wood provides a transmission loss of 18 to 23 dBA/in (0.72 to 0.92 dBA/mm) of thickness.

Metal. Metal walls, primarily of cold-formed steel sheet, can be used as stand-alone barriers or in combination with berms. Low cost, maintainability, and ease of con­struction favor use of steel. Disadvantages include vibration problems, denting, and ineffectiveness in the low-frequency range. For steel construction, the panels are flut­ed (have rectangular corrugations) vertically or horizontally, with a channel-shaped cap at the top. Prepainted galvanized sheet and weathering steel have been used, and other durability treatments are available. The transmission reduction is generally between 10 and 22 dBA.

Earth Berms. Earth berms or mounds are preferred by some. Natural appearance, favorable cost, ready availability of the material, low maintenance cost, and acoustical efficiency favor their selection. A disadvantage is the space needed for construction, particularly in view of safety requirements. Sometimes soil is used in combination with a wall where space is limited. For example, if there is not enough space to achieve the full desired height with a berm, a noise barrier can be located on top of a berm of lower height. Berm side slopes of 4:1 or flatter are desirable on the basis of considerations of safety (see Art. 6.2), roadside maintenance, and wall stability. Some states permit up to 3:1, depending on lateral location. Both acoustics and aesthetics can be improved when the berm is combined with a dense planting of vegetation. Vegetation with a minimum depth of 100 ft (30 m) (perpendicular to roadway), height of 15 ft (4.5 m), and density such that there is no clear path between the highway and the adjacent land use areas can result in a noise level reduction of up to 5 or 6 dBA. Existing soils must be capable of supporting the added berm load.

Proprietary Systems. There are a number of proprietary systems on the market. Some products have included recycled materials such as tire rubber, wood processing waste, and plastics. Of course, steel and aluminum products contain a very high level of recycled metal.

Federal Highway Administration (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. Requirements during the planning and design of a highway project include identification of traffic noise impacts, examination of potential mitigation measures, inclusion of reasonable and feasible noise mitigation measures, and coordination with local officials. The regulations contain noise abatement criteria for different types of land uses and human activities. Reasonable and feasible efforts must be made to provide noise mitigation when the criteria are exceeded. Compliance with the regulations is a prerequisite for securing federal-aid highway funds for construction or reconstruction of highways. Further details may be found in the FHWA Noise Standards.

Computer programs based on mathematical models have proven very useful for pre­dicting noise levels and designing noise barriers. The FHWA has released an entirely new, state-of-the-art computer program known as TNM® that provides a traffic noise model for predicting noise impacts in the vicinity of highways. Replacing older models (Stamina and Optima), the new program uses advances in personal computers and soft­ware to improve the accuracy and ease of modeling highway noise, and the design of effective, cost-efficient highway noise barriers. Included are the following components:

• Modeling of five standard vehicle types, including automobiles, medium trucks, heavy trucks, buses, and motorcycles, as well as user-defined vehicles

• Modeling of both constant-flow and interrupted-flow traffic using a 1994/1995 field-measured database

• Modeling of the effects of different pavement types, as well as the effects of graded roadways

• Sound level computations based on one-third octave-band database and algorithms

• Graphically interactive noise barrier design and optimization

• Attenuation over/through rows of buildings and dense vegetation

• Multiple diffraction analysis

• Parallel barrier analysis

• Contour analysis, including sound level contours, barrier insertion loss contours, and sound-level difference contours

Local criteria may be more restrictive than federal criteria. In Minnesota, for example, daytime criteria in residential areas are an hourly L10 of 65 dBA and an hourly L50 of 60 dBA. L10 refers to the sound level that is exceeded 10 percent of the time over the period under consideration (1 h, in this case); L50 refers to the level exceeded 50 percent of the time. Noise abatement projects strive for a minimum reduction of 10 dBA in L10 and 6 dBA in L50 from existing traffic noise levels.

Figure 9.1 illustrates the fundamental function of a noise barrier. The noise source is traffic, particularly large truck traffic, which generates noise by the action of tires on pavement, the drive train, the engine, and the exhaust. The receiver or receptor can be defined as the location where land use results in exposure to highway traffic noise for an hour or more per day. It may typically be set at 5 ft (1.5 m) above ground or at window level. Acoustical design includes controlling noise that passes over the wall and is diffracted to the receiver, noise that is transmitted through the wall, and noise that is reflected from the wall.

Noise levels are expressed in dBA, decibels measured with a frequency weighting network corresponding to the A scale on a standard sound-level meter. The ease of attaining increasing levels of attenuation has been estimated as follows:

5 dBA: simple 10 dBA: attainable 15 dBA: very difficult 20 dBA: nearly impossible

Designs for reductions greater than 15 dBA are usually not considered feasible because of unpredictable and uncontrollable atmospheric and terrain surface effects, scattering from trees and buildings, and other unknowns.

Diffracted Noise. The noise that passes over the barrier, which is the most important of the three types of noise, depends on the location and height of the barrier. Attenuation is directly related to the difference between the length of the path from the source to the receiver in the absence of a noise barrier, and the length of the path from the source over the top of the wall to the receiver by diffraction. At a given distance from the roadway, increasing the barrier height increases the attenuation achieved. However, this relation­ship is obviously nonlinear, and as the height of the barrier increases above some reason­able value, the attenuation that can be achieved decreases rapidly. Assuming a barrier height that just breaks the line of sight from the source to the receiver, and assuming that such a barrier provides a 5-dBA attenuation, a rule of thumb is to assume that an attenua­tion of dBA can be achieved with each additional foot of barrier height. But because the relationship is actually nonlinear, this approximation holds for only a limited range. Sometimes it is possible to take advantage of local terrain and locate a noise barrier on a

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SOURCE

FIGURE 9.1 Acoustical concept of noise wall. (From Handbook of Steel Drainage and Highway Construction Products, American Iron and Steel Institute, Washington, D. C., 1994, with permission)
stretch of land at a higher elevation. This reduces the required height and cost. Barrier heights are generally in the range of 6 to 25 ft (2 to 7.5 m). They are generally effective in reducing noise for receptors within approximately 200 ft (60 m) of a highway.

Traffic generates sound waves longitudinally as well as laterally. Thus, care must be taken to extend the length of the barrier sufficiently to achieve the desired end result. A rule of thumb states that the noise barrier should extend, in each direction from the boundaries of the receiver, 4 times the distance from the receiver to the noise wall. This length can be reduced by combining the ends of the barrier with other fea­tures, such as natural knolls, or by flaring the wall toward the land use area to form a barrier to the longitudinal sound waves.

Transmitted Noise. The noise that passes through the barrier depends on its surface characteristics and composition (density). Acoustical performance can be determined by testing in accordance with standards of the American Society for Testing and Materials (Test Designation E90). It is important that the wall not contain gaps or holes. Overlapping sections can be used to accommodate access through the wall for maintenance or other personnel when applicable. In such cases, the overlap should be at least 2.5 to 3 times the width of the opening.

Reflected Noise. There is a possibility that noise barrier effectiveness can be reduced by reflected noise, such as where longitudinal walls are located on either side of the roadway. To avoid this situation, it has been recommended that the width between two parallel barriers be at least 10 times the average height of the barrier above the roadway.

James J. Hill, P. E.

Structural Engineer
Consultant
Anoka, Minnesota

Roger L. Brockenbrough, P. E.

President

R. L. Brockenbrough & Associates, Inc.
Pittsburgh, Pennsylvania

During recent years, there has been increasing concern over noise generated by highway traffic in urban areas. Noise abatement programs have been implemented by many agencies. Source control methods have included the development of quieter pave­ments, quieter tire tread patterns, and speed restrictions. In some regions, noise levels have been reduced by depressing roadways or building tunnels, or by special designs of adjacent buildings. In many cases, however, noise reduction has been achieved through controlling the noise path by the design and construction of noise barriers. Sometimes referred to as sound barriers or noise walls, these longitudinal walls are built specifically to reduce traffic noise. In addition to their primary purpose, noise barriers are sometimes adopted to shield unsightly areas from the public and restore a feeling of visual privacy. A noise barrier project involves many areas including acoustical evaluations, consideration of aesthetics, cost evaluations, roadway safety design, structural design, foundation design, and construction.

This chapter includes information from the following sources: S. H. Godfrey and B. Storey, Highway Noise Barriers: 1994 Survey of Practice, Transportation Research Board, Washington, D. C., 1995; D. Byers, “Noise Wall Aesthetics: New Jersey Case Study,” presentation, Transportation Research Board, Washington, D. C., 1995; Guide on Evaluation and Abatement of Traffic Noise, American Association of State Highway and Transportation Officials (AASHTO), Washington, D. C., 1993; Guide Specifications for Structural Design of Sound Barriers, AASHTO, Washington, D. C., 1989, and Interim Revisions, 1992 and 2002; and Road Design Manual, Minnesota Department of Transportation, 2008.