Guidelines On Design of Noise Barriers

Guidelines On Design of Noise Barriers


  1. Introduction
  2. Design considerations
  3. Aesthetic aspects
    3.1 Overview
    3.2 Elements to be Considered
    3.3 Approach
    3.4 Experience of Adopting Different Forms of Mitigation Measures in Local Context
  4. Maintenance
  5. Checklist
  6. Bibliography / References

3. Aesthetical Aspect

According to the Environmental Impact Assessment Ordinance (EIAO), direct technical remedy should be given wherever practicable to remedy or compensate for adverse noise conditions brought about by a new road scheme. The form of direct technical remedy represents any form of direct screening, which includes e.g. earth mounding, barriers and enclosures, that can be incorporated into the road design.

This Guidelines intend to provide guidance on how the aesthetical impact brought by the roadside barriers could be minimised by the appropriate choice of the form and materials used.

3.1 Overview

There is no dispute that the roadside barriers erected would protect residents living next to roads from excessive traffic noise. However, the roadside barriers itself could also affect the aesthetical perception of both road users and residents. In a broad sense, a new road scheme changes the visual quality of the area through which it runs as perceived by the people who live and visit the area. This is partly due to presence of the road and its structures and mainly because the road is man-made, and its alignments, materials, signs, lighting and traffic can be out of character with the rest of the landscape. Thus the amount of visual intrusion of a road is dependent on the quality and type of landscape through which it runs.

The visual impact of roadside barriers on adjoining communities, as well as on the motorists is a major consideration in the design of roadside barriers. A tall roadside barrier placed close to the low rises could have severe visual effect as a tall barrier creates unwanted shadows and blocks panoramic views. On the motorist side of the barriers the emphasis should be on the overall form of the barrier, its colour and texture. Small details will not be noticed at normal highway speeds. However, the emphasis should be on avoiding a tunnel effect through various forms, and visual treatment. Landscaping can be used effectively to accomplish this. It is always the challenge to design an aesthetically pleasant roadside barrier that can protect residents in the vicinity.


3.2 Elements to be Considered

3.2.1 Architectural

The appearance of barriers would be governed by the choice of the "form" , which can be regarded as "the broadly perceived shape of an object" . In view of the linear nature of the noise barrier, simple plan vertical shape appears to be monotonous and creates a wall effect. The visual quality can be enriched through manipulation of the linear form, such as segmentation, curving and articulation of the surface texture and colour.

The overall appearance of barriers could be further articulated through applying of architectural concepts such as rhythm, proportion, order, harmony and contrast (not in any priority order). Such considerations are particularly useful where tall or extensive lengths of barriers are required in urban areas and where it may be desirable to break down the scale of an otherwise monolithic feature by using combination of contrasting materials. In laymen term, the five concepts could generally be interpreted as: -

Figure 3.2.1a Rhythm: To repeat the forms in a sequential manner


Figure 3.2.1b Proportion: To compare in size or number of 2 or more components in the vicinity


Figure 3.2.1c Order: To arrange the components in a systematic, logical or controlled manner


Figure 3.2.1d Harmony: To put the components in an agreeably proportional or ordered composition


Figure 3.2.1e Contrast: To put in adjacency the strikingly different forms, colours or textures


To reduce the visual impacts of barriers, it is often useful to design the solution appropriate to their locality. The linear barriers could either be broken down, for examples, by using alternative solid and transparent panels, by using colour variations or plantation to soften the sharp edges of barriers. Therefore, designed solutions are preferable than mass produced barrier systems.

3.2.2 Visual Impact

Barriers would no doubt affect the aesthetic perception of road users and people living there which to certain degree termed as visual impact. The fundamental is to design the barriers with appropriate scale and character compatible and matches with the local environment. If it is not possible to design a barrier that blends into the local environment, the aim should be to reflect some of its features such as materials, colours, textures and shapes, in a form of barriers which has aesthetic appeal, without being dominant in the field of view. Sometimes, transparent panels may be used to lighten the overall impact, either to create "windows" which partially restore views, or along the top section of a barriers to reduce its apparent height.

Figure 3.2.2 Barriers which have Aesthetic Appeal

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3.2.3 Compatibility with Local Features

To some extent, local residents would tend to accept the barriers which have relationship with its surroundings and are compatible with the appearance of the adjacent neighbourhood.

As a general rule, the character of the neighbourhood should be looked into to provide a checklist of its distinguish elements. For example in the urban context, the design of a barrier needs to capture something of the neighbourhood, such as the prevalence of a particular material or style in buildings; for a leafy suburb a barrier incorporating planting might blend in more readily. Alternatively, the design of a barrier in the vicinity of a local point such a group of high rise blocks might best echo the visual dominance of that image. For the rural and new town situation, it is preferable to have a 'natural' form to harmonise the local vicinity. The use of earthworks and planting should be developed to create a visual impression which seems to preserve the rural.

Figure 3.2.3 Barriers Compatible with their Surroundings


3.2.4 Coordination with Road Furniture

In general, priority of design should be given to the protected side since the purpose of a barrier is to protect the environment enjoyed by the people. However, the design of barriers must take into account the visual effects of the traffic sides, recognising their role as a backdrop to the motorists' view of the road.

Efforts would always be spent in the design of roads and bridges to ensure that their visual impact is acceptable. However, the visual unity is often spoiled by uncoordinated elements such as road signs, lighting columns, gantries, safety fences and parapets. The design of a roadside barrier should complement the engineering design of the road and therefore needs to be developed as part of an overall concept. Consideration of visual impact early on in the design process will help designers to avoid unnecessary conflicts. The designer should also take note of the compatibility of the rhythm of various elements along the road to determine the suitable module for the barrier.

There are several advantages to be gained from identifying a suitable module for a barrier that will help to coordinate it with other elements. As well as being cost effective in terms of installation and maintenance, the repetition of units can create a sense of order and harmony which is conducive to road safety.

Figure 3.2.4 Coordination with Street Furniture


3.2.5 The Protected Side

A barrier can drastically change the outlook for residents, who in addition to a loss of view, may also suffer loss of daylight. A barrier is experienced by the residents as a feature which perhaps dominates the space, and such impact would remains unchanged unlike the impact of variable traffic volumes. A designer can provide a barrier which minimises this potential intrusion by using attractive materials which display in plan and elevation. Planting incorporated within the barrier design will soften its overall impact by imparting a more natural character and relieving the monotony of a horizontal skyline.

3.2.6 The Road Users' Side

The road user experiences a length of barrier for a very short space of time and will nearly always view the design at an oblique angle. The road user in general will perceive only a broad impression of the design, its pattern of colour and its contrast with the surroundings. The driver in particular will absorb a very limited amount of visual information because of vehicle speed and concern for other traffic on the road.

Barriers over 3 metres high substantially conceal the view of existing landmarks from the road, but they can also conceal visual clutter which might otherwise distract the attention of drivers. Where barriers are needed over considerable lengths in urban and semi-urban areas, their appearance should be designed to avoid monotony. Features which create a monotonous appearance are the unrelieved face of a barrier constructed from a single material, and a stark and unvaried horizontal top. Surveys of drivers in Holland have indicated that a view which is unchanging for 30 seconds is monotonous; this suggests that changes in design every half mile, or approximately 800 meters, are desirable for long barriers adjacent to a high speed road.

Variation in the type of barrier, changes in its longitudinal profile, and transparent panels over structures, will all act as visual signposts helping drivers to recognise where they are along the route. Changes should be introduced at natural "break points" and care should be taken to ensure that barriers complement or even enhance the road users' broad picture of the road.

3.2.7 The Impact of Tall Barriers

In urban areas in particular the Hong Kong situation, a straight barrier is often called to protect the high rises next to roads. However, tall barriers tend to be out of scale and proportion to its surrounding and associated structure. The resulting vertical surface may in fact be visually more incompatible with an urban environment. A careful study of the areas requiring protection should be carried out to determine whether the barriers would be acceptable as a dominant feature in the protected area, or whether they should be subordinate to the existing townscape elements. It is always useful to include breaking down the scale of the barrier structures to fit the scale and character of the surroundings, as evidenced by the size and the appearance of the adjoining buildings and their component parts.

The scale of the barrier can be reduced by providing alternative solid and transparent panels and together with the introduction of set back or recessed panels, or by the arrangement of elements on the facade of the barrier, so that these component parts (such as the structural frame and the infill panels) would harmonise with the pattern of the surroundings. The sensitive choice of colours will also help to integrate the barrier with its setting. In some areas the barrier could take the form of a facade, as a new feature designed to enhance the character of townscape.

In some cases, cantilever barrier is built instead of a very tall barrier. The cantilever barrier is one which cantilevers out towards and above the roads. Visually, it could minimise the impact as it would reduce the overall barrier height. However, a substantial section of materials must be avoiding to protrude over the carriageway of the road. From residents' point of view, the cantilever barrier could diminish the impact on the viewer from outside because the top part curves away from the viewers and hence appears lighter. The top section should also be avoided to be seen as too substantive. Some good design could blend the cantilever with the scenic surroundings.

Figure 3.2.7 Examples of Curved Barriers

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3.2.8 Use of Transparent Barriers

Where a barrier is required to provide noise protection to properties in close proximity to the highway there are likely to be adverse effects due to the loss of view, loss of daylight, and enclosure effects. The loss in the quality of the view and the need for light will need to be assessed for each property affected by a tall barrier alongside the road, and the design of the barrier should be adjusted to mitigate these adverse effects. Measures to be considered include the incorporation of transparent panels coordinated with the windows of properties behind the barrier.

Transparent barriers can also be used as a more general means of reducing the prominence of the barrier as perceived both from the protected side and from the new road. A reduction in impact can be achieved by incorporating transparent panels at regular intervals along the barrier, or by glazing the top part of the barrier (typically one third of the height to reduce its apparent height and dominance. For some cases, specific pattern may be added onto the transparent panels to avoid blindly birds' collision.

Figure 3.2.8 Barriers with Transparent Panels

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3.2.9 Use of Colour

Many barrier systems comprise acoustic panels which can be produced in a range of colours. It is of general consensus that the appearance of a barrier can be toned down to help it merge with its surroundings, or made to stand out as a striking and highly visible addition to the environment by the use of colour. In general, cooler blue / grey shades at the top of a barrier and warmer brown green earth colours near to the ground would help to reduce the monotonous looking. This variation in colour tends to reduce the apparent height of a tall barrier at the roadside. Colour graduation may be less effective at some distance, where the barrier appears in silhouette.

The local setting for the barrier should determine whether it is appropriate to add a splash of colour to an otherwise drab scene. The use of bright colours to create a feature should be careful. They are most effective when restricted to key parts of the barrier, for example, to emphasise its structural form. Large areas of strong colour on a barrier can result in an unpleasantly bright rather than attractive appearance.

Figure 3.2.9 Colour Graduation

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3.2.10 Use of Vegetation

Planting can often be used to soften and enhance the appearance of a barrier, providing variation from season and in different daylight conditions. Vegetation which overtops a barrier will relieve the stark horizontal line which otherwise draws attention to it, so reducing the intrusion on its surroundings, but care must be taken to make use of species which will blend into the natural landscape.

Figure 3.2.10 Examples of Vegetated Barriers

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3.2.11 Modifications to Barrier Designs

Small variations in the alignment of the barrier, such as stepping or zig-zags, may have only a marginal effect on noise attenuation, and so they can be used to create a more attractive design, particularly on the protected side. They can also assist the establishment of planting to soften the appearance of the barrier. Figure 3.2.11 Zig-zags Barrier


3.3 Approach

The following principles should form the basis of the first considerations for barrier designs: -

a) Barrier appearance should be considered initially from the view point of those living alongside the road. Barriers should as far as possible reflect the characters of the local neighbourhood and should preserve or even enhance the quality of the environment for local residents.
b) As far as possible, barriers should be designed so that it is not apparent to the road users or to those who live alongside road that there is actually a barrier there.
c) Barriers from the motorists' view point should reflect the character of the locality through which the road passes in order to provide a sense of place. However, if extensive lengthy barriers are necessary, the designer should apply appropriate design concepts to add visual interest in order to avoid a monotonous appearance.

In general, the size of barriers will largely be determined by requirements for noise attenuation. Considerations of structural stability, safety and maintenance will also influence their appearance. However, this still leave a considerable amount of freedom to vary the form and finish to reflect the character of the neighbourhood through which the road passes. The use of materials and structural forms appropriate to the adjacent landscape and the application of architectural principles to the design of barriers will reduce their visual impacts.


3.4 Experience of Adopting Different Forms of Mitigation Measures in Local Context

3.4.1 Straight Solid Barriers

Concrete or other solid materials could be used for short barriers. To reduce the visual impact, features or patterns could be added on the surface of barriers.

Figure 3.4.1 Examples of Straight Solid Barriers

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3.4.2 Straight Barriers with Transparent Panel

For very tall barriers, it is useful to have transparent panels at top to reduce the visual impact.

Figure 3.4.2 Straight Barriers with Transparent Panels

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3.4.3 Barriers with Combination of Transparent Panels and Solid Panels

A combination of transparent and solid panel would lighten the visual impact and at the same time maintain the attractiveness by using colourful panels. Figure 3.4.3 Transparent and Solid Panels

3.4.4 Semi-enclosure

To minimise visual impact, transparent panel should be considered on both sides. Figure 3.4.4 Transparent Panel in Semi-enclosure

3.4.5 Earth Mound

An earth mound is an obvious solution to noise pollution in country side because it can be made to fit in with the landscape more naturally than any vertical structure, especially as it can support planting which greatly improves its appearance in most rural contexts. The amount of space which an earth mound requires is a major constraint as it requires more land than vertical barriers. Figure 3.4.5 Example of Barrier Sitting on top of an Earth Mound

3.4.6 Vegetated Barriers

A number of 'green barrier' systems have been developed which use living plant material in conjunction with soil-filled supporting structures up to 4m high. In most cases, these need careful maintenance including irrigation in dry weather. If planting fails through lack of water or disease, the barriers lose their visual appeal and may not be easily restored. In the longer term, well-established living barriers may need to be rebuilt if the planted material causes the supporting structure to deteriorate. Any consideration of this type of barrier should take into account of the appropriateness of the planted species to the locality and to their maintenance requirements.

Particular attention should be paid on the safety issue for carrying out maintenance works to vegetation adjacent to an expressway. Designers should consult and agree with Transport Department, Police, Highways Department and the landscape maintenance party early at the design stage on a particular arrangement for future maintenance. Figure 3.4.6 Vegetated Barrier



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Friday, 28 April, 2006