Planning To Minimize Highway Noise Impacts
James P. Cowan	© & Author Info

Abstract

Highway noise affects more people in this country than any other environmental stressor. Its description, travel, effects, and control are also widely misunderstood. This paper provides a discussion of the practical options available to minimize the impacts of highway noise on communities. It begins by briefly explaining the most misunderstood principles of highway noise terminology, travel, and control. With this foundation laid, traffic noise policy and planning issues are summarized. Case studies are then provided from different areas of the country to give practical examples of positive experiences in the use of these principles.

Introduction

It was estimated more than 10 years ago that highway noise affects more than 18 million people in the United States and more than 100 million people worldwide¹. These numbers have undoubtedly increased since that time with the expansion of highway systems throughout the country and the world. Highway noise has been associated with reduced real estate value and degrading the quality of life for millions of American families. With proper planning of communities near busy highways, these values can be significantly reduced. This paper has been written to educate the planning community about the practical options available to minimize highway noise impacts to communities. With this in mind, we will begin by explaining the principles of outdoor sound and noise control, along with regulatory background for traffic noise. Case studies will then be discussed to enforce the explanations of the principles.

Highway Noise Control Fundamentals

The principles of highway noise sources, terminology, outdoor sound travel, and outdoor sound control will now be explained to form the foundation of this discussion.

The principal noise sources of highway vehicles are engines, exhaust systems, and tires. Brakes on trucks can also generate significant noise levels. The contribution of each of the principal noise sources to the noise environment depends on the speed of each vehicle. At speeds greater than 30 miles per hour, tire noise tends to dominate the noise signature of most vehicles while exhaust noise tends to dominate noise signatures for speeds below 30 miles per hour.

The sound that we hear is typically described in terms of sound pressure levels. Sound pressure levels rate the pressure fluctuations that stimulate our sense of hearing. Because of the nature of the human hearing mechanism and its reaction to these pressure fluctuations, it is most practical to describe sound pressure fluctuations in terms of decibels. Decibels are based on a logarithmic scale, similar to that used to rate earthquakes (the Richter scale), and therefore do not follow the same mathematical rules as most of our other rating systems. Our ears respond differently to changes in pitch or frequency. This variation in response has been programmed into a filtering system that is used on most sound measuring equipment. Decibels that have been filtered in this manner (to take account for human frequency sensitivity) are known as A-weighted decibels and are denoted as dBA or dB(A). Typical dBA levels that we encounter range from 0 dBA, at which sounds can just be detected by a person without hearing damage, to 120 dBA, at which most people will feel pain from the sound exposure. Figure 1 shows common sound sources with their approximate dBA sound levels for reference.

Figure 1 Sound Pressure Levels for Various Sound Sources in dB(A)

An addition of 10 dBA would sound as if a sound source doubled in loudness. For example, a 70 dBA sound source would sound twice as loud as a 60 dBA source. Alternately, a 60 dBA sound source would sound half as loud as a 70 dBA source.

Sound levels are usually rated not only by their volume but also by their time period of exposure. The most common ratings of sound levels over extended periods of time are Leq and Ldn. Leq is the equivalent sound level which is an energy average of sound over a specified period of time. Since sound levels are typically changing continually, the Leq provides a single number rating for the sound over specified time periods. The Federal Highway Administration requires the use of a 1-hour Leq as the basis for state Departments of Transportation to evaluate the potential impact of a new or expanded highway. Other agencies use the Ldn as a yardstick for land use compatibility with sound sources. The Ldn is the day-night equivalent level, which is a 24-hour Leq value with the stipulation that all levels occurring between 10:00 pm and 7:00 am have 10 dBA added to them to compensate for the extra sensitivity of sounds occurring during normal sleeping hours.

Sound travel outdoors, especially over distances greater than 200 to 300 feet from a sound source, is highly dependent on weather conditions. The atmospheric conditions that affect sound travel the most are temperature variations, wind currents, and humidity. In terms of temperature variations, sound waves generally bend toward cooler temperatures. For example, with all other weather conditions remaining unchanged, on a typical summer’s afternoon temperature decreases with increasing altitude. In this case, sound waves tend to bend upward and generate what is known as a shadow zone. If you are in a shadow zone, you may be able to see a sound source at a distance but not hear it since shadow zones can decrease sound levels by up to 20 dBA at distances greater than 500 feet from a sound source. Shadow zones can also be set up when sound is travel against the wind.

These conditions are reversed when temperatures are lower closer to the ground than higher up, as would be the case early in the morning or over a calm body of water. In these cases, sound waves tend to bend toward the ground, bounce off of the ground, and travel farther than expected. This is why sound is said to "carry" well over water. Sound also tends to travel farther than expected when it is travelling with the wind. Figures 2 and 3 illustrate the points discussed above.

Figure 2 Sound Travel in Temperature Variations

Figure 3 Sound Travel in Wind Currents

Sound can be controlled at its source, in the path between the source and the listener, or at the listener. In most outdoor noise situations, it is only practical to control the sound in the path between the source and the listener. Although there are many options to control noise sources indoors, our options are much more limited outdoors. In this case, we are limited to erecting some type of barrier between the source and listener or creating some type of buffer zone in the same area. Barriers must be solid; fences and/or vegetation provide minimal sound reduction. They must be either some type of wall or earth berm. Bear in mind, however, that even the most effective barriers only provide 15 to 20 dBA of noise reduction because a significant amount of sound energy travels over and around barriers. Another limitation of outdoor barriers is that they are only effective within distances of roughly 200 to 300 feet. If the distance between a noise source and a listener is greater than 300 feet, a barrier will typically be ineffective.

The noise control methods described above deal with reducing the sound level. Another noise control measure that can be used for outdoor sound is masking. Masking adds a pleasing sound to the environment to cover up the unwanted sound. This is usually effective only at specific locations where the affected people are willing to accept this addition to the sound environment. Natural sounds, such as the running of water in a fountain, are the most common of outdoor masking sources.

Highway Noise Regulations

This discussion has been divided into the categories of federal, state, and local levels to establish the responsibilities of the different governmental levels.

The Federal Highway Administration (FHWA) has promulgated noise regulations, in Title 23 Code of Federal Regulations Part 772 (Procedures for Abatement of Highway Traffic Noise and Construction Noise), that establish limits for the consideration of noise mitigation for new or expanded highway projects. These limits, known as noise abatement criteria, are in terms of land use and 1- hour Leq values that cannot be "approached or exceeded." The definition of "approach or exceed" is deferred to each State’s Department of Transportation. The most common of these noise abatement criteria is 67 dBA, used for residential communities. In addition to the absolute limit of 67 dBA, the FHWA regulations also have a provision that noise levels generated by a new highway project do not "substantially increase" noise levels to a noise-sensitive area (such as a residential community). The definition of this term is also deferred to each State’s Department of Transportation.

The Federal Highway Administration offers literature to the planning community to assist them in dealing effectively with noise issues. Guidelines for Considering Noise in Land Use Planning and Control is an interagency report published in 1980 as a joint effort of the U.S. Department of Transportation, Department of Defense, Environmental Protection Agency, Veterans Administration, and Department of Housing and Urban Development. Another valuable document available through the FHWA is The Audible Landscape: A Manual for Highway Noise and Land Use, published by the FHWA in 1976. The former document deals with planning for all transportation noise sources while the latter deals with only highway noise. Although these documents have not been written recently, the principles and examples listed in the documents are as valuable today as they were when they were written.

As is mentioned above, each State’s Department of Transportation is required to interpret the "approach or exceed" and "substantial increase" phrases in the FHWA regulations. Depending on the state, "approach or exceed" has been defined as from 1 to 3 dBA of the 67 limit. Therefore, depending on the state, the limit for noise abatement consideration ranges from 64 to 66 dBA. A "substantial increase" in noise levels ranges from 6 to 15 dBA, depending on the state. Some states have noise codes and guidelines through branches other than their Departments of Transportation; however, only the Departments of Transportation deal with highway-generated noise.

Local municipalities, through their zoning and administrative codes, can restrict the development of residential and other noise-sensitive land uses in the vicinity of highways, although most municipalities do not.

Planning Issues

As is mentioned above, noise control options for residential communities are generally limited to the construction of noise barriers and providing buffer zones between highways and residential communities (to have as large a distance as possible between the highway and the affected communities). If outdoor activities would not be at issue, sound control designs can be incorporated into the residential buildings facing the highway. These designs include installing windows and doors that would be more efficient in sound reduction than typical designs and designing building facades without windows or doors facing highways.

The FHWA assumes no responsibility in enforcing noise issues. State Departments of Transportation evaluate the potential noise impacts of new or expanded highways on noise-sensitive areas. If the FHWA criteria are predicted to be approached or exceeded, or if a substantial noise increase is predicted at an existing noise-sensitive location as a result of a proposed highway construction project, the State Department of Transportation has the responsibility to consider noise abatement (based on its established procedure for approving the construction of noise abatement measures). This only deals, however, with state and interstate highways. Local roads are the responsibility of each local municipality.

New State highway construction is funded by each State’s Department of Transportation. New interstate highway construction is usually funded by a combination of federal and state money. It is this money that is also used for the construction of noise abatement measures, if approved by the agencies involved. Funding for local roads is the responsibility of each local municipality. Noise abatement designs can be funded by the municipality or the developers of communities, depending on the local administrative codes.

The public is the ultimate client of developers, planners, and municipal officials. An educated public provides the greatest opportunity for cooperation and resolution to noise-related disputes in the most effective ways. Noise problems are also most effectively controlled when they are recognized and resolved in the design process of any community. It is the responsibility of the planning community to be educated about noise issues as they relate to planning. It is then the planning community’s responsibility to educate the public about these noise issues before problems arise. In that way, all parties will understand the issues at hand and will be able to make informed, effective decisions in the planning process.

Case studies will now be presented that will demonstrate the value of planning properly for highway noise issues in residential communities.

Case Studies

In 1979, the FHWA published 5 case histories dealing with highway noise and compatible land use. Although now 20 years old, the principles demonstrated through these case histories are current. Three of these studies were in California, one was in Minnesota, and one was in Michigan. Mitigation measures in these studies include berms; barriers; zoning ordinances precluding residential construction and/or requiring soundproofing designs for residential construction adjacent to highways; facing windows, doors, and balconies away from highways; and providing buffer zones between highways and residential construction.

The Florida Department of Transportation provided technical assistance to the Palm Beach Planning and Zoning Department to minimize noise impacts on a planned upscale residential community adjacent to the Florida Turnpike in Boynton Beach. In this case, the land developer funded a noise study to determine the potential impacts and methods to mitigate such impacts before the community was constructed. The study resulted in the placement of an aesthetically pleasing earth berm between the development and the Turnpike. Most importantly, the study helped to develop a pro-active working environment with the local planning and zoning department, the state highway department, and the local residents.

Notes

1. Nelson, P.M. Transportation Noise Reference Book. London: Butterworths, 1987.

James P. Cowan
Acentech, Inc.