How effective are noise barriers?

As you drive along interstate highways that traverse bustling cities and suburbs, the landscape has likely changed dramatically due to the installation of enormous sound barriers. Typically constructed from concrete and averaging 14 feet in height, these noise barriers transform the backyards of neighborhoods into isolated spaces, and in some narrower sections, they create tunnel-like experiences for drivers. Since their introduction, approximately three thousand linear miles of these barriers have been built across the United States. Notably, California boasts 760 miles, while Florida comes in with 252 miles of sound walls.

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For the most part, residents express a desire for these sound walls. In California, there’s already a waiting list for their construction. During a recent public meeting with representatives from the Florida Department of Transportation, which I had the privilege to attend, many residents of Boca Raton were anxious to learn if sound walls would soon be erected to shield their homes from proposed turnpike expansions.

However, one particular individual's concern struck a chord at this meeting. He shared his experience of living near a highway noise barrier that, he claimed, turned his previously quiet home into a source of amplified noise. This phenomenon isn’t rare. Residents several streets away from barriers, or those situated uphill, along with individuals exposed to certain weather conditions, may find these structures insufficient. On the contrary, sound walls can sometimes inadvertently enhance noise levels. Engineers and acousticians have long understood that the sound barriers lining America’s highways have limited effectiveness and that a range of improved technologies could be explored.

The dilemma remains: There’s little motivation to innovate. Robert Bernhard, a noise control expert and vice president for research at the University of Notre Dame, summarizes it well: “Walls are not a very effective solution.” He highlights that the federal government funds only the installation of noise walls during highway expansions, which diminishes the incentive for continuous research and the development of alternative solutions.

The path that sound travels is less than mystical, leading to the conclusion that standard sound barriers have effectiveness constraints. Notably, the Noise Control Act of 1972 mandates that noise pollution affecting communities be considered for mitigation, viewing noise as an environmental concern. Typically, highway agencies assess whether sound mitigation is necessary when constructing new interstates or widening existing ones, and sometimes even during smaller projects.

The noise that provokes community concern stems from three primary sources: the roar of vehicle engines and exhaust, aerodynamics around trucks and cars, and tire-road interaction. While tire-pavement noise dominates at highway speeds for cars, the sounds from trucks also contribute significantly.

Noise Mitigation Analysis

State authorities utilize specified noise models to forecast future auditory disruptions when highways are expanded. This complex calculation incorporates a melange of factors, including anticipated traffic mix, nearby structures and vegetation that could obstruct noise, the geographical layout of the land, and acoustic diffraction around barriers, among other elements.

Only if noise is projected to exceed the federal limit of 67 decibels (dB) during peak hours, and it is deemed reasonable and feasible to mitigate at least 5 dB for some residences, does the installation of sound walls become obligatory, contingent upon local community interest. However, the interpretation of what constitutes ‘reasonable’ can vary widely across states, resulting in significant differences in the presence of noise walls nationally.

The Real Impact of Noise Barriers

Despite the sound reduction provided, residents living beside highways may still find their backyards less serene. For context, a dishwasher operating in the next room generates about 50 dB, similar to the ambient noise of a relaxed city. The established criteria aim to merely allow conversations at a picnic table or shouting across a few feet, which may not achieve a truly pleasant environment. Bruce Rymer, a senior engineer at the California Department of Transportation, explains that our perception of noise is sometimes psychological: merely seeing the tops of trucks can lead individuals to perceive the environment as noisier, a phenomenon referred to as psychoacoustics. Consequently, barriers engineered to obstruct lines of sight can lead to a reduction of approximately 5 dB.

However, sound travels in waves rather than straight lines, allowing it to circumvent the barriers. Homes situated several blocks away can still perceive highway noises, even with barriers reaching 16 feet. Robert Berrios, an environmental programs coordinator at FDOT, articulates that noise can be both absorbed and reflected, but much of the sound energy can simply pass overhead.

Weather conditions further complicate the efficacy of barriers. In a study conducted by Bowlby & Associates, it was shown that residents living hundreds of feet from a highway might hear sounds 5 dB louder if the wind is blowing towards them, as explained by Darlene D. Reiter, the firm’s president. Unfortunately, current regulations do not account for such weather-related factors and assume neutral conditions to determine noise levels. Variations in temperature effects can also lead to abnormal sound propagation, as demonstrated during early morning hours when cooler ground and warmer air can refract sound waves downward toward homes that would otherwise experience minimal noise interference.

In fact, some residents living on elevated terrains or near open freeway exits may discover that the noise escalates despite the installation of barriers. Harry Molaison, an attorney based in suburban New Orleans, noted that the sound level in his backyard increased due to a break in the sound barrier intended to accommodate a highway exit. “We no longer enjoy the tranquility we once had,” he lamented.

These challenges have spurred research, such as a recent study funded by the University of Pittsburgh examining whether noise barriers could be constructed from materials that absorb sound, rather than reflecting it. Even with advances in barrier materials beyond traditional concrete, the question persists: is this the most effective allocation of taxpayer funds?

Cost Implications

Highway walls are an expensive undertaking, averaging over $2 million per linear mile for one side of the roadway, according to Rymer. Cumulatively, the expenditure on sound barriers from 1990 to 2020 has surpassed $6 billion. States maintain differing thresholds to determine what qualifies for reasonable intervention. For example, in California, walls are deemed justified if the federal cost to taxpayers reaches $92,000 per affected home. Such expenditures detract from funding allocated to mass transit, deteriorating tunnels, bridges, and other transportation necessities.

As Jim Cameron, founder of a commuter advocacy organization in Connecticut, pointed out in a newspaper column, “Three miles of sound barriers on both sides of an interstate could purchase another M8 railcar for Metro-North, removing 100 riders from the state’s highways.”

Furthermore, massive walls can hinder wildlife, obstructing frogs, turtles, and snakes from migrating between habitats, as pointed out by Elizabeth Deakin, a professor emerita of city and regional planning at the University of California, Berkeley. This impasse may disrupt wildlife communication, migration patterns, and even reproductive cycles.

It’s entirely rational for neighbors living adjacent to highways to seek solutions to mitigate noise pollution. The disruptive sound of heavy traffic can significantly impair enjoyment of outdoor spaces and is even linked to health implications. A report from the World Health Organization indicates that environmental noise can contribute to heart disease, hypertension, and cognitive impairments in children. For residents, addressing the source of noise, particularly tire-road interaction, or exploring absorption solutions could prove to be a more efficacious and cost-effective approach.

While some tire manufacturers have begun researching quieter tire designs, their efforts have predominantly focused on minimizing noise within vehicles rather than on the exterior. Though electric vehicles, such as those produced by Tesla, produce less noise than traditional combustion engines, they maintain similar sound profiles at highway speeds.

Innovative projects are emerging in some European countries to explore unconventional noise reduction tactics. One example includes the use of luminescent solar concentrators: colorful, translucent panels that generate electricity for homes while not obstructing views. Others are assessing the potential for dense bamboo or other plant species to organically form noise-dampening vegetative barriers.

Altering Pavements as a Source of Noise Reduction

Yet, the greatest promise for noise reduction seems to lie in modifying roadway pavements. Several states, especially Arizona, California, and Florida, have begun implementing quieter pavement technologies designed to enhance the asphalt's porosity or alter the texture of concrete. In these instances, improving the road surface can lead to a noise reduction of up to 9 dB. Dana M. Lodico, a senior consultant, states that extensive studies have shown this method to be effective, particularly in warmer states where road conditions are conducive to such innovation. Anecdotes illustrate that quieter pavements present a more cost-efficient alternative compared to standard sound barriers as revealed in comprehensive reports and decades of research.

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Despite the potential of these innovations, the existing framework of federal highway funding is likely to hinder their widespread application. Currently, the Federal Highway Administration has yet to endorse pavement alterations as an accepted method of noise reduction. According to agency representative Doug Hecox, there remains uncertainty regarding the durability and consistency of noise reduction achieved through pavement changes.

This challenge means that states will continue to install barriers as part of highway projects, even when modifications to the pavement provide a viable solution. Given that states bear the long-term maintenance costs of quieter pavement, many officials may opt for the more permanent (yet costly) solution of sound walls funded primarily by federal resources.

In summary, as Bernhard succinctly puts it, “The predominant culture is cost avoidance.”

Meryl Davids Landau is a Florida-based journalist whose work has appeared in a variety of publications, including U.S. News & World Report, Glamour, Vice Media, Parents, Reader’s Digest, Good Housekeeping, and Prevention, among others.

Mammoth barriers also block small animals — frogs, turtles, snakes — from getting from one habitat to another, said Elizabeth Deakin, professor emerita of city and regional planning at the University of California, Berkeley. This may affect wildlife communication, migration, and even reproduction.