Part 2: Noise Barrier Design Principles

Other parts of the report

Part 1: Table of Contents, Introduction and Background

Part 3: Design Prototypes

Part 4: Conclusions, References and Appencicies

Final Report, July, 1990
Prepared by Julie Farnham and Edward Beimborn
Center for Urban Transportation Studies
University of Wisconsin--Milwaukee
Milwaukee, Wisconsin 53201

Sound good/look good.

Design Guide

Basic Design Principles

This section is intended to provide a basic introduction to design principles that can be utilized in barrier and landscape design. It will also include discussion on how and what people see when traveling on highways. Issues to be addressed include distance and motion, line, form, scale, balance, rhythm and sequence and orientation. Each of these factors must be considered and carefully chosen in order to create a design that is compatible with its surroundings.

Distance and motion affect what motorists see when driving through the freeway corridor. Speed alters the peripheral cone of vision and the distance to the motorist's point of focus. Figure 1 illustrates the vision cone for three vehicular speeds. In general, as speed increases, the cone of vision narrows and the focal distance increases. Likewise, at slower speeds, peripheral vision is expanded and the focal distance is closer to the observer. Vision cones delineate the area within which objects are generally in focus. Objects outside these cones become blurred.

Figure 1: Distance and Motion

Lines are created by joining two points. Lines are fundamental to three dimensional forms. The character of forms is expressed by lines. Smooth, flowing, horizontal lines may suggest calm and serenity, while bold, vertical, angular lines may suggest strength and tension. The most prominent line created by a noise barrier is the top profile (Figure 2). Lines can be created on the surface of the barrier wall using various materials and texture patterns. Lines may also be implied. For example, a row of street trees may appear to form a solid line.

Form depicts volume in three dimensions --length, width, and depth. A barrier wall itself is a three dimensional form. This is commonly referred to as positive form. The horizontal wall configuration can be designed to create spaces which are commonly referred to as negative spaces or forms (Figure 2).

Scale establishes a frame of reference. It is a relative measurement. People commonly relate to their environment using the human body or other elements of familiar dimension as the reference measurement. Noise barrier walls can be 15' tall or more. Such a wall would appear massive and overpowering located adjacent to a back lot line since residential fences are more commonly 4 to 8' tall (Figure 2). The perceived scale of barrier walls can be reduced through the use of landscaping and/or in the design of the wall itself (e.g. materials, configuration, etc.).

Balance creates order and unity by attaining a sense of equilibrium. Two basic types of balance include: symmetry and asymmetry (Figures 3a and 3b). Symmetry results when elements are arranged equally around a central axis, creating a mirror-image effect. Symmetry is considered to be a formal type of balance. Asymmetry is more informal, lacking a central axis. Elements are juxtaposed in such a way that they counterbalance each other without creating a mirror image.

Figure 2: Fundamental Elements of Design

Figure 3a: Symmetrical Balance

Figure 3b: Asymmetrical Balance

Rhythm and sequence establish consistent, recognizable patterns (Figures 4 and 5). Repeated patterns create a sense of familiarity and comfort. They also provide a sense of progression, unless continued indefinitely. Rhythm and sequence can be created using both the barrier wall and/or landscaping.

Figure 4: Rhythm

Figure 5: Sequence

Orientation refers to the dominant visual direction established through design (Figure 6). Horizontal orientation is associated with relatively flat and expansive landforms. This type of orientation tends to direct the eye forward, reducing the apparent height of a barrier wall by emphasizing its relationship to the horizon. Vertical orientation is associated with upward reaching forms such as skyscrapers. This type of orientation tends to direct the eye upward, increasing the apparent height of a barrier wall.

Figure 6: Visual Orientation

Barrier Design

This section is divided into subsections pertaining to design issues specific to barrier structures, landscape treatment, and integrating barriers and landscape.

Parts of the Barrier Wall

Walls consist of three basic parts: the cap, the body, and the base (Figure 9). The cap provides a top edge to the wall -- giving it a definite finished appearance. The body is the dominant part of the wall. The base refers to where the body meets the ground. It provides a sense of connection with the landscape.

Figure 7: Parts of a Wall

Top Profile

The profile line created by the top of the wall defines the general character and form of the wall. The top profile can be designed to reflect, contrast, or remain neutral to the surroundings. Figure 8 illustrates and example of a peaked top profile imitating a steepled skyline, and a horizontal, wavy top profile contrasting the vertical forms of the urban downtown skyline.

Wall Configuration

The configuration of a barrier wall can be altered by changing its horizontal alignment. Figures 9 and 10 illustrate two common wall configurations: serpentine and castellated. Both of these break up the line of the wall creating a more three-dimensional form. This results in a more visually interesting wall. In addition, the "negative" space created by the undulations can function as planting pockets. While these configurations potentially increase the structural strength of the wall, they also increase the cost. How much the cost would increase depends on specific details such as materials and the degree and quantity of undulations.

Figure 8: Top Profile of Wall

Figure 9: Serpentine Wall Configuration

Figure 10: Castellated Wall Configuration

Materials and Textures

The most common materials for constructing noise barriers are precast concrete, metal, and wood. Brick is also used, but only to a limited extent because of higher cost (Figure 11). Several surface finishes and textures are available to provide a large variety of options for barrier wall design. Appendix B lists the common barrier wall materials and various surface finishes and textures currently available.

The character of the barrier wall is directly related to the materials and textures used in the design. In general, wood appears inherently more suburban and rural than concrete or metal, which appear harder and more urban. However, in spite of the material used, the character of the wall can be significantly modified by the type of surface treatment used on the wall body.

Surface texture and patterns can be applied to lend any desired character. How texture and patterns are perceived depends on the speed of the observer (Figure 12a, b, c). At high speeds, textures become blurred and patterns may not be discernable. Coarse textures and simple, bold patterns should be used in high speed situations. On the other hand, the residential side of noise barriers is commonly experienced by slower moving observers. People walking or driving at slow speeds are able to distinguish finer textures and more intricate and complex patterns.

Figure 11: Common Barrier Wall Materials

Figure 12: Wall Texture

a. High Speed (freeway side)

b. Slow Speed (residential side)

c. Wall Texture

Variety can also be achieved through the use of contrast on the wall surface. Smooth and rough textures can be juxtaposed and pattern orientations may be varied to create contrast (Figure 13). Relief on wall surfaces can create shadow patterns. A variety of colors can be used to create contrast.

Other important considerations when selecting barrier materials include cost and maintenance. On a cost per linear foot basis, metal is the least expensive material (see Appendix B). Obviously, standardized units are cheaper than custom pieces. Likewise, unornamented panels cost less than decorative or textured panels. The challenge to designers is to create variety within the constraints posed by standardized units, and to use customized panels in strategic locations to provide visual impact without being prohibitively expensive.

Figure 13: Surface Contrast

Landscape Plant Materials

Plant materials are grouped into general categories relative to their size and habitat (Figure 14a). These categories include:

Overstory -- tall plants (typically trees) that form overhead canopies

Understory -- shorter plants (shrubs and small trees)

Ground cover -- plants that grow close to the ground (typically less than 12" tall). May be used to stabilize soil or slopes.

Vines -- plants that attach themselves to other objects for support.

Plants may also be grouped into categories based on their texture -- fine, medium, or coarse (Figure 14b).

Fine textured plants are characterized by small leaves and twigs, smooth bark, and slender, graceful branching habits.

Coarse textured plants are characterized by large leaves, thick and/or corky twigs and sturdy or stiff branching habits.

Medium textured plants are those not distinctly fine or coarse.

For design purposes descriptive plant form categories have been developed because certain forms lend themselves to specific functions and portray particular characters or moods. Figures 15 and 16 illustrate common form descriptions for trees and shrubs.

Figure 14: Plant Categories

a. Size

b. Texture

Figure 15: Common Tree Forms

Figure 16: Common Shrub Forms

Natural Landscaping

Using plants native to a region makes aesthetic and economic sense. Urban development, agriculture and road construction destroys much of the original plant communities that existed prior to settlement. Restoration of the original plant community helps blend the roadway into the surrounding landscape and provides an interesting and aesthetic view of the road. Wild flowers and prairie grasses contribute a wide variety of textures and colors to the roadside landscape. Flowers bloom at different times, from spring to fall, offering a continuous display of changing colors. In addition, many prairie grasses change color throughout their growing season.

The establishment of natural landscaping along a highway requires careful selection of plants and some patience. Prairie grasses and wild flowers will take two to three years to establish themselves and may require a nurse crop when planted. During the first several years the roadside will be dominated by annual weeds. These will eventually be replaced by the perennial grasses and wild flowers of the prairie environment. Plants should be selected to provide for a diversity of color, height and flowering time. This will result in a more naturalistic landscape with a variety of textures and colors throughout the year. Prairie plants change over the season with short growing flowers appearing early. These are gradually replaced with taller flowers and grasses later in the season.

Prairie habitats are divided into five types: wet, wet mesic, mesic, dry mesic and dry. Wet prairie areas have saturated soils, typically deep clay silt loam and peat and are dominated by sedges rather than grasses. Mesic prairie is a medium condition with medium deep silt or sandy loam soils. Dry prairie tends to have shallow sand or limestone soils. Each species of native grass or flowers has a set of habitats which it prefers. Roadsides tend to have a variety of conditions ranging from wet areas along ditches to drier zones along embankments.

In addition to aesthetics, natural landscaping can reduce roadside maintenance costs. Fertilization and mowing activities can be significantly reduced. Once established, prairie grasses and wild flowers provide an excellent means of soil stabilization. Native plants can be especially resistant to drought and will continue to thrive even when other types of plants dry up. Ideally prairies should be burned once every four to five years during early spring to reduce build-up of dead grasses and to eliminate woody plants. This adds nutrients to the soil and accelerates growth in the subsequent seasons. Controlled burning along a roadside may require special efforts to maintain safety. An alternative may be to use mowing or to forgo burning for longer time intervals.

The use of prairie plants along roadsides is becoming increasingly popular. The Wisconsin Department of Transportation has initiated a major project in central Wisconsin along U.S. Highway 51 to establish prairie grasses and wild flowers along the roadside. In addition many states have active programs to plant wild flowers along roadsides and have "adopt a roadway" programs to facilitate maintenance and landscaping. Currently one-fourth of one percent of federal funds for landscaping are to be used for native plantings along highways. Appendix B lists some grasses and wild flowers currently used by WDOT.

Uses of Landscaping for Aesthetics

Landscaping can be used in several ways to improve the aesthetics of freeway corridors, and noise barriers in particular. Figures 17 through 21 illustrate several functions of landscaping relative to noise barriers.

Noise barrier walls can be softened through the use of plants that camouflage their hard edges (e.g. cap, base, and ends). Vines cascading over the top of walls and base plantings can be used effectively as softeners.

The scale of barrier walls can be reduced by using plants to break up the expanse of wall surfaces and to reduce the relative height of the wall. Mature overstory trees are tall relative to a 15' barrier wall. Planting overstory trees in front or behind a wall can effectively reduce the apparent wall height. Shrubs and vines can be used to break up the expanse of the wall body.

Visual direction can be added through the use of plantings that accentuate horizontal or vertical lines. Creeping vines and low spreading ground covers emphasize horizontally while pyramidal, conical and columnar plants provide vertical elements, drawing the eye upward.

Plants can be used to create points of focal interest or accent. Plantings that provide contrast in color, form and/or size will be visually prominent. Accent plantings can provide aesthetic stimulation for motorists. They may also be used to subtly direct views and provide locational cues.

Plants can be strategically placed to frame views and objects.

Figure 17: Soften

Figure 18: Reduce Scale

Figure 19: Horizontal and Vertical Emphasis

Figure 20: Create Accent

Figure 21: Framing Element

Integration of Wall, Landform, and Plants

A primary goal in designing noise barriers is to integrate them into the landscape. Problems develop when barrier walls are placed on the landscape with little attempt to integrate them with the surrounding landforms or existing built elements such as bridge abutments, endwalls, and guardrails. As a result, barrier walls can appear as obtrusive objects in the environment. Barrier walls can be integrated with the landscape in two basic ways. The structure itself can be designed to appear to "grow" out of the landscape, or the landscape (plants and earth) can become part of the barrier structure.

Wall endings can be designed to integrate barriers with the landscape (Figures 22a and 22b). Gradually tapering or stepping them down to the ground level will give the appearance that the wall is literally growing out of the ground. Earth berms and plantings may also be used at wall ends to tie the barrier into the landscape. In situations where existing structures such as bridges and guardrails are present, an attempt should be made to connect the wall end with the structure. This will provide visual continuity between highway structures.

Figure 22: Integration of Wall, Landform, and Plants

a. Wall Endings

b. Tie Wall to Landscape and Existing Highway Structures

Plants and wall structures can be integrated in a variety of ways. The wall itself can be designed using earth and plants as the primary construction materials. Living barriers, which are used in Western Europe, are such an example (Figure 23). These are essentially vertical earth walls which function as the growth medium for willow plants. The earth is contained in a frame constructed of white willow posts interwoven with basket willow twigs. The twigs sprout leaves, covering the structure and giving the appearance of a wall-like hedge. To protect against dry periods, irrigation systems are installed within the wall. Other maintenance is quite minimal, consisting of trimming excessive growth every two years and performing weed and disease control as necessary. In Germany these walls have life expectancies of twenty to thirty years.

Living barriers provide an attractive alternative to the common barrier constructed of hard materials, both in terms of appearance and maintenance. However, willows are deciduous plants and therefore lose their leaves in the fall. The attractiveness of these walls during the winter months may be a point of concern. In addition, the basket willow does not thrive in Southeastern Wisconsin. Until an acceptable substitute is found, this limitation might make living barriers difficult to implement here. As used in West Germany these barriers are in the range of 12 feet high. It is not known if higher barriers such as would be used in the Milwaukee area can be built and maintained.

Figure 23: Living Barrier

A similar approach would be to design planter troughs into the wall structure (Figure 24). Irrigation systems could be incorporated into the troughs. The level of the planters could be varied to produce a cascading effect. Because of the more elaborate, multilayered design, this type of wall could be expensive. In addition, the types of plants that can grow and survive in planters are limited. Above ground planters are susceptible to freezing temperatures unless they are quite large and adequately insulated. Annuals are commonly used in planters in urban areas and would be appropriate and very attractive in wall planters. However, they must be planted annually and are therefore too maintenance intensive for extensive use. They might, however, be appropriate on a limited basis.

Figure 24: Planting Troughs Nested in the Wall

Plants can be integrated with walls by attaching them to the surface or by providing holes through the wall which they can grow through (Figures 25a and 25b). These approaches are especially appropriate in situations where planting space is very limited. Vines are the most appropriate plant type for either of these approaches. Vines can attach themselves to the surface of concrete and wooden wall materials. Wire supports can be used to attach plants to metal surfaces. A system of wires and pins can be attached to the wall surface which vines can use for growth support. Vines planted on the residential side of the noise barrier will eventually cascade over the top. In addition, small holes can be drilled through the wall surface to allow the vines to grow through and spread on the freeway side.

Figure 25: Integrate Wall and Plants

a. Attaching Plants to Walls

b. Utilize Plants on Opposite Side of Wall


Freeway lighting is typically limited to tall overhead fixtures required to light the roadway. Decorative lighting can be used to provide nighttime variety and focal interest (Figure 26). Light fixtures can be placed to cast shadows on the surface of barrier walls. Shadows may be created by plants and/or surface relief on the barrier itself. This may be especially desirable in the winter when landscape color is subdued. Light fixtures can also be attached to the wall itself to provide a "wash" of light over the surface. Segments of barrier walls that have been specifically ornamented may be highlighted with spot lighting.

Figure 26: Lighting to Create Shadow Patterns and Spotlight


Maintenance contributes greatly to the attractiveness of noise barriers and landscaping and is a primary economic concern in the design of barriers and landscaping. This section will discuss the major issues relevant to maintenance and how they can be addressed through design and/or management and maintenance practices.

Maintenance Goals and Objectives

Maintenance begins as soon as the barrier and landscaping are installed and continues throughout their life span. Consequently, it constitutes a substantial ongoing cost expenditure. A major goal of maintenance is to minimize the cost. This is best achieved by using quality materials, appropriate to the given situation, and proper construction techniques which will also increase the life of the wall.

A consistent schedule of maintenance must be followed to ensure that little problems are taken care of before they turn into large and potentially expensive catastrophes. Therefore maintenance capabilities must be considered during the design phase. It is better to use simpler designs that can be adequately maintained than to use more elaborate schemes which demand a disproportionate amount of maintenance to remain attractive and effective.

Maintenance Issues

Material selection is an initial concern to the designer. The chosen material must be durable enough to withstand local environmental conditions. Of the common barrier materials, wood is the least durable material, being more susceptible to weathering damage. However, all materials designed for use in barrier wall construction should be manufactured to be adequately durable. Again, using quality materials, even if the initial cost is higher, will pay off in the long run.

Another concern relative to material selection is vandalism. Vandalism is a major maintenance concern regardless of the construction material used. Vandalism can range from physical destruction to surface defacement. The best way to deal with vandalism is to minimize the opportunities for its occurrence. Noise barriers present a blank canvas for "graffiti artists". Using plants, particularly vines, to obscure the wall surface reduces the potential canvas area. Rough surface textures can also be effective in discouraging the defacement of barrier walls with spray paint or other surface applied substances since the texture obscures legibility of the graffiti. Physical destruction can be held in check by minimizing access to the wall. Prickly plants can be used as armor to dissuade potential vandals from getting too close to the wall.

On the other hand, access to walls is necessary for maintenance purposes. Likewise, access must be provided for landscape maintenance. Typically access is gained on the side of the wall that will receive maintenance (freeway or residential). While some barriers are designed with maintenance doors, this is discouraged because of the potential for persons other than maintenance personnel to use them.

Identify Actors and Duties

Wisconsin state statutes indicate that maintenance within the freeway corridor is the responsibility of the state (WDOT). The state can contract with counties or municipalities to provide maintenance activities. The Milwaukee County Department of Public Works currently maintains all of the freeway corridors within its jurisdiction. Specific maintenance tasks are delegated to the County by WDOT area maintenance supervisors. Roadside maintenance generally includes care and protection of trees and other vegetation, and planting to prevent or minimize soil erosion.

Economic Considerations

Retrofitting urban freeway corridors with noise barriers is an expensive endeavor. For example, costs can be over $2 million per mile for a single side of a freeway. The state may use federal funds to cover the cost of barrier construction. Noise barrier projects compete for available funds with other projects such as bridge replacement, freeway modernization, safety improvements, resurfacing and capacity improvement projects.

Several factors affect the cost of barriers and landscaping. Materials are probably the most fundamental cost factor. Type of material used affects cost as does the quantity of material used. Experience in Milwaukee County thus far has shown that metal barriers have been the least expensive. Wood barrier use has had a unit cost about 50% higher than metal while concrete has had a cost 65% higher than metal (Appendix A). The elaborateness of the design has a direct bearing on the quantity of materials used. Barrier designs with undulating configurations require more materials and therefore will cost more. Landscaping will add cost to a barrier system, but it is not as critical as the selection of a material. A limited landscaping may add 5% to the cost, an average landscaping 8 to 10%, while an elaborate system could add 15 to 20% (Appendix C). Actual costs will depend upon the extent of work, site conditions, and maintenance considerations.

Design trade-offs are inevitable. Priorities will vary from location to location. Decisions must be made as to how much should be spent on the barrier versus the landscape treatment. An elaborate landscaping scheme coupled with a lower cost barrier material may be cheaper than using simple landscaping with more expensive material. Such tradeoffs should be made clear to local citizens and elected officials so they can select the option which best fits with their community. In some contexts dense landscaping may be required to blend the barrier with its environment. In such a situation, a simple barrier could be used, retaining more of the funding allotment for landscaping. In other, especially more urban locations, the form of the wall may be the most important design feature, with a simpler landscape scheme used to accentuate the wall form. For more complete cost comparison information refer to Appendix A and Appendix C.

Part 3: Design Prototypes