Environmental Technical Assistance Program Newsletter

State departments of transportation across of the country are looking at new and better ways to predict traffic noise levels, as well new materials for highway sound barriers, in order to reduce the overall cost of noise abatement efforts.

[Photo courtesy of the Missouri Department of Transportation.

This second of a two-part series on state DOT noise abatement strategies examines some of the research state DOTs are conducting to find more cost-effective and innovative designs and materials to mitigate the high levels of roadway noise: everything from using less expensive materials or smaller barriers to designing duel use barriers such as vegetation walls and solar panels.

Traditionally, sound wall barriers are pre-cast or cast-in-place concrete sections. However, several different types of materials have been in use since the beginning of sound barrier construction in the 1960s. The latest noise barrier inventory – maintained by the Federal Highway Administration –indicates barriers constructed with a single material make up 84% percent of all noise barriers. Furthermore: 

• Concrete comprises 55 percent of those barriers.
• Block represents 18 percent of those barriers.
• Wood comprises 5 percent of those barriers.
• Metal, berm, and brick together account for another 5 percent of single material barriers.
• “Other” materials comprise the final 1 percent of those barriers, including acrylic, composite, fiberglass, glass, opaque plastic, and transparent plastic.

There are noise walls designed with absorptive material to absorb sound, but the majority are built to deflect sound. Barrier analysis and design must meet the requirements of federal regulation 23 CFR 772 for all federally funded projects.

The Ohio Department of Transportation is in the start-up phase of a research project expected to begin January 2021 to look at the effectiveness of vinyl fence as a noise barrier as opposed to the now-standard concrete barriers.  The project involves constructing two 8-foot tall vinyl fences at two different locations, testing them for noise reductions, and comparing the reductions to nearby concrete noise walls.

The Ohio DOT plans to use readily available vinyl fence materials that already have specifications available – indeed, one type of fence earmarked for testing is available at the local Home Depot.

Noel Alcala, the noise and air quality coordinator at the Ohio Department of Transportation and noise workgroup coordinator on behalf of the American Association of State Highway and Transportation Officials, explained in an interview that while vinyl is a lot less durable overall than concrete, it is “a lot less expensive and we have an indication that the noise reduction will be very noticeable.”

The Ohio DOT is also studying the feasibility of using vegetation growing on noise walls as both a sound absorber and an air quality improvement and plans to complete a report on that effort by the end of 2020. That effort – based on research conducted in 2010 – looked at a 400-foot long section of “green noise wall.” That wall consisted of a 12-foot-high section of stacked, 70-pound bags sprouting plants and grass as a way to muffle highway sound.

Trees and shrubs can decrease highway-traffic noise levels if high enough, wide enough, and dense enough, but it would take at least 100 feet of dense vegetation to provide the same benefit as the smallest feasible noise wall. Because of this, the FHWA has not approved using vegetation for noise abatement. 

However, future studies could show more innovative choices will result in higher levels of noise absorption. For instance, environmentally friendly and sustainable bamboo growth may provide the needed noise level abatement to meet FHWA requirements. One feasibility study presented at the 2016 International Congress and Exposition on Noise Control Engineering indicated that the noise-reducing effect of a bamboo barrier with a height of 5 meters (roughly 16 feet) and a thickness of 6 meters (nearly 20-feet) is roughly comparable to a 3-meter (nearly 10-foot) high solid noise wall.

Some states are looking to try new noise barriers with a dual purpose. In 2018, for example, the Minnesota Department of Transportation completed a literature review, Harnessing Solar Energy through Noise Barriers and Structural Snow Fencing, on the topic.

That study found that sound barriers equipped with solar panels are used successfully in Europe, though there are no current in-place solar panel noise barriers in the United States. However, two states, Massachusetts and Georgia, are currently working with partners to pursue pilot studies using solar panels built into sound walls so they can produce renewable energy without compromising their abilities to reduce noise – and do so safely. Given the number of miles of sound barrier currently constructed in the United States – over 3,000 miles total – the study found the potential for energy production could be at least 400 Gigawatt hours annually, roughly equivalent to the annual electricity use of 37,000 homes.

The California Department of Transportation is also looking at innovative designs to save money on noise abatement construction.

Currently, Caltrans is exploring the feasibility and effectiveness of lower height barriers and berms. Its latest research suggests that at freeway speed, most all light-vehicle noise is at the tire/pavement interface — literally at the zero-foot level. Meanwhile, on most heavy trucks — even with tall exhaust stacks – most noise is below 3.3-foot level.

That means shorter less-expensive noise barriers would help attenuate noise. A low vehicle noise source also means a short (4-foot) solid-concrete safety-barrier could provide noticeable noise mitigation if positioned correctly in the road cross-section. Shorter barriers mean lower overall costs. State transportation agencies across the United States are making strides to protect the public from high levels of traffic noise. Click here for a list of current state DOT noise reduction projects.

State departments of transportation across of the country are looking at new and better ways to predict traffic noise levels, as well new materials for highway sound barriers, in order to reduce the overall cost of noise abatement efforts.

[Photo courtesy of the Ohio Department of Transportation.]

The first article of this two-part series will examine some of the new tools state DOTs are developing to better gauge the impact of noise on communities near transportation infrastructure such as highways. 

First, some background: According to the noise barrier inventory maintained by the Federal Highway Administration, more than 3,000 linear miles of noise wall barriers have been built since the 1970s across the United States.

Such sound barriers remain an essential part of highway design and construction as the World Health Organization determined that prolonged exposure to high levels of noise “interferes with people’s daily activities … disturbs sleep, causes cardiovascular and psychophysiological effects, reduces performance and provokes annoyance responses and changes in social behavior.”

The Noise Pollution and Abatement Act of 1972 requires state departments of transportation to monitor and manage highway traffic noise generated by federally funded projects on existing and planned roadways. When that sound exceeds certain thresholds, 23 CFR 772 specifies mitigation efforts must be undertaken to reduce highway traffic noise.

Yet the cost of meeting those regulations and protecting the public against the roar of the vehicles — the predominant sound for both cars and trucks is that of tire-pavement interaction – takes considerable funding.

For example, between 2014 and 2016, FHWA found that total construction costs for noise barriers topped $671 million in just a three-year period – an average of $2 million per mile of noise wall.

That’s why many state DOTs are trying to find ways to reduce the cost of noise abatement efforts, noted Noel Alcala.

“The main goal of more accurate noise abatement modeling can result in cost reduction,” explained Alcala, the noise and air quality coordinator at the Ohio Department of Transportation and noise work group coordinator on behalf of the American Association of State Highway and Transportation Officials, in an interview. “Modeling noise levels more accurately can likely reduce costs noise wall in construction.”

States currently use the FHWA Traffic Noise Model 2.5 to predict noise levels that will occur once a road is built or expanded – even if that expansion occurs several decades in the future.

That complex formula includes the mixture of cars and trucks expected on the road; the buildings and vegetation in the area that would block some sound; the configuration and ground quality of the land between the road and the homes; the ways the sound is expected to diffract around the wall; and other key factors.  Other weather factors can also play a role, such as wind direction and ground and air temperatures. Also considered is the height where the noise is actually originating – at roadway level or top of truck stacks. 

The overall goal of some states, such as California – which sports over 750 miles of highway sound walls – is lowering the noise level at the transportation infrastructure source – a far-cheaper effort than blocking the noise with a more expensive sound wall. 

To that end, the California Department of Transportation has worked over the last few years to implement the most recent National Cooperative Highway Research Program or NCHRP findings into newer modeling techniques.

Caltrans relied on documents such as NCHRP Report 635: Acoustic Beamforming: Mapping Sources of Truck Noise, which experimentally validated a practical acoustic “beamforming” measurement technique.

“Beamforming” uses a type of camera that takes pictures (or video) of soundwaves and illustrates that very little acoustic energy comes from tall truck exhaust stacks. Meanwhile, it shows that most highway noise is tire/pavement related and usually occurs very close to the pavement.

NCHRP Project 1-44, reported in NCHRP Report 630, also proved valuable to Caltrans efforts as it proposing a test method for measuring tire-pavement noise at the source using the on-board sound intensity (OBSI) method.

The agency also is implementing AASHTO T 360-16 (2020) – the Standard Method of Test for Measurement of Tire/Pavement Noise Using the On-Board Sound Intensity (OBSI) Method – to reevaluate noise modeling efforts and improve modeling to provide more accurate results. Caltrans has also moved to use more precise noise measurement processes, which it believes will lead to lower community noise levels.

“The AASHTO Standard which precisely measures pavement acoustics, allowed a lot of this work to be accomplished, explained Bruce Rymer, senior engineer in the Hazardous Waste, Air, Noise, & Paleontology Office within Caltrans.

“Using this new measurement technology, we found pavement acoustics had a much larger influence on roadside noise levels than previously acknowledged,” he said, “This is important for state DOTs because noise impacts of transportation projects on roadside communities have to be quantified and analyzed in order to receive federal funding.”

Other states are looking at ways to more accurately predict the costs of sound mitigation as well – now and in the future.

Iowa, for example, is looking at developing a new noise wall cost GIS-based prediction tool that incorporates residential lot information while ensuring that projects requiring noise abatement are properly planned and funded.

Charles Bernhard, Iowa DOT’s Traffic Noise Engineer, said those new processes include five distinct efforts:

Mapping out five-year highway project plans.

• Eliminating highway projects that do not have an annual average daily traffic or AADT of greater than 10,000.
• Eliminating projects by “work type” that do not meet the definition of a Type I project for noise
• Creating a “parcel layer” that includes all residential parcels within 200 feet of the “edge of pavement” of the remaining highway segments.
• Visually evaluate each remaining highway segment to determine if the segment has a “cluster” of seven or more residential parcels within the 200-foot “buffer.” 

Part 2 of this noise abatement series will examine some of the new types of sound wall materials some state DOTs are studying.