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Road Engineering Journal
Copyright © 1997 by TranSafety, Inc.
October 1, 1997
TranSafety, Inc.
(360) 683-6276
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Study Compares Older and Younger Pedestrian Walking Speeds
Designing Traffic Signals to Accommodate Pedestrian Travel
California Study Addresses Issues of Roadway Noise Control for the Benefit of Endangered Songbirds
Factors that Determine the Reduction in Property Values Caused by Traffic Noise

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California Study Addresses Issues of Roadway Noise Control for the Benefit of Endangered Songbirds

A highway reconstruction project on California's State Route 83 resulted in a study to learn the project's impact on the least Bell's vireo, an endangered songbird. The project itself was small, encompassing about 600 meters (2,000 feet) of a two-lane highway. However, the study addressed issues with broad implications. In "Traffic-Noise Impact Study for Least Bell's Vireo Habitat Along California State Route 83" (Transportation Research Record 1559), Douglas E. Barrett reviewed the study and its implications.


The least Bell's vireo is a small, migratory songbird whose population was once abundant. Development and degradation of its California habitat led to a marked reduction in its numbers. As a result, the California Fish and Game Commission listed the bird as endangered in 1980. In 1985, the U.S. Fish and Wildlife Service (FWS) proposed listing it as endangered, and in 1986 it was placed on the federal endangered species list under the United States Endangered Species Act (ESA) of 1973. The ESA prohibits any activities, including degradation of habitat, which would "jeopardize" or "adversely affect" an endangered species.

Most research on how noise affects wildlife has involved high-level but short-duration noise--like that produced by low-flying military aircraft or helicopters. In contrast, the noise produced by highway traffic is quieter and more constant. This type of noise can interfere with the auditory signals birds rely on by making those signals inaudible, changing their perceived location, or reducing the distance over which the signal can be heard or interpreted. Birds depend on these signals for a variety of activities--contacting mates, warning of danger, monitoring their young, and detecting predators. The background noise produced by traffic, with its effect on birds' communication abilities, can render an otherwise suitable nesting area unsuitable. Under the ESA, this qualifies as degradation of habitat.


The construction project on California State Route 83 (CA-83) involved no physical changes. Instead it was to be used as a detour while nearby CA-71 was under construction. As a result, traffic volumes on CA-83 would increase for the 6-to-12- month duration of the construction project. With CA-71 traffic diverted onto CA-83, the latter's afternoon peak-hour traffic volumes were expected to climb from about 580 vehicles per hour in both directions to about 1,460 vehicles per hour. The ESA required clearance for the detour route from the FWS, since CA-83 crossed a habitat area of the least Bell's vireo.

Because of the increased traffic on CA-83, the traffic noise level during the loudest hour was expected to increase by 3-to-4 decibels (dB). For this study, the FWS established a noise level of 60 dB during the loudest hour as the level at which the noise would have an impact on the least Bell's vireo. The construction could not avoid the least Bell's vireo's nesting season, since the project could last up to 12 months.

Because the expected noise impact would be temporary, restrictions on traffic speed were enlisted to lessen the noise. The speed limit was reduced from 50 miles per hour (mph) to 35 mph, which would abate the projected sound level by 2-to-3 dB. This solution resulted in a 1-dB remaining increase in loudest-hour noise level. The project's sponsoring agency felt the FWS might delay the project if they did not take all possible steps to mitigate the noise impact. As a result, the agency suggested temporary noise barriers for approximately 600 m (2,000 ft) on each side of CA-83.

Permanent noise barriers often have surfaces that absorb noise, but this approach was not practical or cost-effective for a temporary noise barrier. Instead, the proposed noise barriers were simple plywood tilted away from the road at a ten-degree angle to lessen noise reverberation between the barriers. Both New Jersey and Nevada had successfully employed this strategy with permanent noise barriers. The FWS rejected the proposed noise barriers but offered a compromise that allowed the detour project to continue. The highway agency agreed to fund a project to control the arundo plant within the expected noise-impact area. Also known as giant reed, the arundo plant invades and destroys the native willow riparian habitat of the least Bell's vireo. This solution, although unrelated to noise impact, was intended to provide more long-term benefit to the vireo than a temporary plywood barrier.


The CA-83 study raised significant issues relating to the impact of noise on endangered species. For example, the study revealed that neither the California Department of Transportation (Caltrans) nor the FWS had a centralized list of noise-mitigation projects for endangered species. Nonetheless, the issue of noise mitigation for endangered species had been considered on at least one temporary and seven permanent noise-mitigation highway projects in California.

The CA-83 study also brought into question the validity of the FWS's loudest-hour noise-impact criterion of 60 dB. Biologist John Rieger developed the criterion for a California highway project in 1987-88. Rieger assumed that if he found an area where least Bell's vireo nests existed near the highway, the noise level on that stretch of roadway must be acceptable to the bird. Finding ten least Bell's vireo nests along Route 76, he calculated the loudest-hour sound level at the location of each nest. The highest and lowest numbers were discarded, and the remainder were averaged-- yielding a result of 61 dB. Rieger never intended this number to set a precedent or become a standard for noise-impact mitigation for endangered species, yet both resulted. In fact, with each noise-impact study that has used it, a 60-dB criterion has become more firmly established as the standard of use.

This noise analysis relies on sound-level and loudest-hour computations, both of which were developed in relation to human hearing. Their use for wildlife "may be an inappropriate, albeit convenient, sound metric when referring to animals because they hear and respond to sound differently than do humans." Current noise analysis procedures and criterion may overstate the impact of noise on the least Bell's vireo and other songbirds.

In addition, the CA-83 study raised the issue of money--how much should be spent on noise mitigation projects? Rieger, currently a manager at Caltrans, has estimated that $9 million has either been spent on or committed to noise mitigation projects for endangered birds in Caltrans District 11. This commitment exists "despite the lack of any policies or clear guidelines to determine cost-effectiveness for noise mitigation." The Federal Highway Administration (FHWA) has developed guidelines that stipulate that projects must incorporate "noise abatement measures which are reasonable and feasible" to lessen or eliminate noise impact. However, guidelines exist only for areas used by humans. The author pointed out, "To date, there has been no evidence that the concept of 'reasonable and feasible' applies to endangered species. . . ."

Finally, no guidelines suggest how much noise reduction is adequate. California, Virginia, Texas, and Massachusetts have minimum noise reduction standards of 5 dB for residential areas; if noise-mitigation efforts cannot reasonably reduce noise levels by 5 dB, these states will not propose such mitigation. For the benefit of endangered species, however, since no guidelines suggest how much mitigation to provide, no guidelines define how much money is "reasonable."


The CA-83 study focused interest on the issue of noise mitigation and endangered species, and interest will probably continue to grow. As a result, more money will likely be spent on noise mitigation for birds and other endangered species. Further research is needed to provide the guidelines and criteria that are currently lacking or not well-defined. With each study like the CA-83 project, "de facto policy precedents are established more firmly." The author concluded, "The alternative is to create uniform and appropriate spending guidelines through regulation."

Copyright © 1997 by TranSafety, Inc.

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