The noise pollution generated by traffic is a significant and growing problem for transportation agencies. Generally, traffic noise is addressed in one of three ways: controlling the noise at its source, directing the path of sound, or using regulatory and receiver controls or policies. Transportation agencies typically use the second alternative (directing the path of sound), because it falls within their jurisdictions. Moreover, noise barriers, particularly in urban areas, have been the method of choice because they are effective and comparatively inexpensive. However, noise barriers often significantly alter and dominate the visual highway environment; and, without careful planning and construction, can become a form of pollution themselves.

In an effort to refine its guidelines for noise mitigation, the Texas Department of Transportation (DOT), in cooperation with the Texas Transportation Institute, conducted a national survey of highway-noise-barrier practices. Beverly B. Storey and Sally H. Godfrey discussed the results of the survey in "Highway Noise Barriers: 1994 Survey of Practice" (Transportation Research Record 1523). They concluded that transportation agencies could improve the perception and acceptance of noise control efforts if the public were given both more knowledge about noise barriers and a more active role in their design.

METHODS

The survey was sent to all state transportation agencies and similar agencies in Puerto Rico and Ontario, Canada. It dealt with a number of noise-barrier issues, including barrier materials, aesthetics, and community involvement. An extensive literature review provided background on highway-sound physics, current research and development, technological advances, and existing Federal Highway Administration (FHWA) noise policies and aesthetic design criteria.

RESULTS

Ninety percent of the transportation agencies that received the survey responded to at least part of it.

Barrier Material Selection

Noise barriers are generally constructed of one of the following: concrete, earth, wood, brick or masonry units, metal, vegetation, mineral aggregates, plastic, glass, or composites of these materials. These materials can produce barriers in a variety of shapes and sizes, and each material has specific characteristics that make it suitable for a particular noise situation.

The survey revealed that the free-standing thin wall is the most commonly used noise barrier, and agencies cited a number of factors they use to select barrier materials, including site geometry and compatibility, durability and integrity, cost, acoustic properties, and community preference. Some agencies reported that their only criterion in barrier selection was low cost or low bid price; "however, lowest-cost criteria virtually eliminated public participation and aesthetic considerations from the decision-making process." Even when they did consider other criteria, 33 percent of the agencies indicated that low cost was the most important one.

Concrete was the most commonly used noise-barrier material. Both Florida and Pennsylvania thought concrete the most durable material; nearly all barriers in these states (96 percent and 86 percent, respectively) are made of concrete. In addition to being durable, concrete is relatively inexpensive and easy to maintain, and its transmission loss (a minimum 32 decibels(dB)) is high compared with other materials. Though public acceptance of earth berms was equal to concrete, earth-berm barriers were second on the lists of transportation agencies. Soil made available from highway construction is one clear advantage of earth berms, as is their "excellent attenuation by absorbing sound." Barriers and berms are often used together to "reduce the total structure necessary to achieve the specified attenuation levels."

Metal barriers (particularly steel) fared well in the durability portion of the survey; wooden barriers, however, did not. While its low cost and availability made wood the material of choice in Alaska and Virginia, some states reported ongoing problems with repairing and replacing wooden barriers. Wood's acoustic properties are poorer than those of more solid materials (a transmission loss of between 18 and 23 dB), and it has a tendency to shrink, warp, deteriorate, and discolor. In Colorado, more than 50 percent of the barriers are made of wood, and 55 percent of its annual transportation budget is used to maintain these barriers. Other states reported vandalism problems, and, in a few cases, arson.

Brick or masonry units, on the other hand, offer "an excellent transmission loss of 33 dB," but they cost more to construct and need to be replaced when damaged. Information from the survey about plastic and glass barriers was limited, since Maryland was the only state to report using these materials. The literature review revealed that lexan, glass, and fiberglass are the three main plastic or glass materials used in noise barriers. In particular, fiberglass is weather-resistant and durable, and it offers effective sound absorption and design flexibility.

Several agencies reported using proprietary noise-barrier systems and absorptive treatments, a practice generally on the increase. While the majority of agencies that have used absorptive treatments have had success with them, 74 percent of responding agencies have not used them because of perceived cost and concerns about durability and effectiveness. Most of the agencies (about 60 percent) placed barriers on existing structures, such as bridges, retaining walls, and center medians.

Less than half the agencies reported using "sound masking or psychological barriers for mitigation of noise." Landscape enhancements for noise mitigation are usually aesthetic rather than acoustical. Studies have suggested that thick vegetation can absorb and scatter about 5 dB per 30.5 meters (100 feet) of distance; however, the density required for effective results often exceeds most available urban rights-of-way. Among the agencies that reported using landscaping for noise-barrier purposes, some perceived that "vegetation provided greater psychological benefits than acoustic attenuation." In those states where the public has been involved in barrier selection, citizens and property owners have tended to choose landscaping over a structural barrier.

Aesthetics

The visual quality of noise barriers is a critical factor, since "they become a major line element [in the highway corridor], second only to the roadway itself." Design considerations such as color, texture, scale, line, proportion, and form must be carefully evaluated. This survey, like a 1981 predecessor, found that transportation agencies do not have enough data on the public's perception of aesthetically pleasing barriers, nor do they have enough money to satisfy perceived preferences. Transportation agencies are faced with the ongoing challenge of providing the public with a transportation system that is safe, cost-effective, and environmentally aware and responsive.

Both transportation agencies and the public are becoming increasingly aware of the significance of aesthetics in relation to noise barriers. However, the states responding to the survey varied in the amount of public input considered in barrier design. One-fourth "considered few or no responses from the public for barrier-design approval," generally because of design restrictions and the need to select the lowest bid. Sixty-five percent of the states "required a majority approval by the residents to construct a noise barrier." Some states included renters and/or property owners in the barrier decision-making process.

The literature review revealed that in Europe public involvement has been a part of noise-barrier design for some time. In addition, the European experience "with designing acoustically effective and visually appealing barriers exceeds efforts made by most U.S. transportation agencies." In fact, many European barriers are near "works of art." The use of color, creative design, and a variety of materials makes European noise barriers artistic and "aesthetically exciting components of their transportation systems."

Perceived Effectiveness and Community Involvement

A noise barrier can be effective on both an actual (acoustic-performance) and a perceived (community-opinion) level. Fifteen of 34 agencies believed there is a relationship between aesthetics and perceived effectiveness. Utah found that if the public felt a noise barrier was appealing or acceptable, they were less likely to find fault with its noise-reduction capabilities. Transportation agencies generally measured this perceived effectiveness through public involvement, including public hearings, special meetings on noise issues, informational mailings, surveys, and questionnaires. The present survey revealed that while the public definitely wants reduced noise levels, they "do not necessarily want noise barriers as a permanent feature of their backyards." Despite this qualified endorsement, most agencies reported "that active public involvement is vital for successful noise mitigation." The survey also revealed that significant effort is needed to communicate a proposed barrier design to the residents affected by it. However, the visual information given to affected residents is generally poor, and aesthetic designs are not consistent on highway and residential sides of barriers.

CONCLUSIONS / RECOMMENDATIONS