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The sound of traffic

The sound of traffic

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    Above: A device monitors noise levels in a residential neighborhood adjacent to a Quiet Pavement Pilot Program freeway segment. Photo: Arizona DOT. Below: This trailer—built by the National Center for Asphalt Technology—is used to directly measure tire/pavement noise levels using the close-proximity method. Photo: NCAT

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    The NCAT trailer uses microphones inside a sound-baffled chamber in the trailer to measure noise levels without outside interference. Photo: NCAT

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    Asphalt pavements constructed with an open-graded friction course have been proven to generate less traffic-related noise than traditional hot-mix asphalt pavements. Photo: NCAT

Highway design was never easy. Increasing scrutiny and critical public opinion only have added to the challenges facing transportation engineers. With each project, there always seems to be someone upset with the proposed location, the number of lanes, and the type of pavement. Environmental issues have taken a front seat in roadway planning and design, and now there is a growing concern about the noise levels generated by traffic.

The push to reduce noise produced by the traveling public has resulted in the search for new abatement technologies and alternatives. Research is proving that specific types of pavement surfaces can reduce noise levels significantly on roadways. State DOTs are investigating quiet pavements as an alternative to more traditional methods of noise abatement, such as noise walls and vegetative screening.

Noise emanating from roadways has become more of an issue with changing population density and an increasing number of noise sources. A rapidly growing population with a tendency to settle in urban areas creates a situation in which a larger number of people are affected by passing vehicles.

The U.S. government first attempted to address noise concerns by passing the Noise Control Act in 1972. The act, last updated in 1978, has proven to lack the effectiveness hoped for by supporters and is not strictly enforced. But road noise is annoying, and the residents living near highways are demanding that the government reduce noise levels.

According to the report, Reducing the Impact of Environmental Noise on Quality of Life Requires an Effective National Noise Policy (available at www.volpe.dot.gov/acoustics), the time is now for a new national noise policy. The authors suggest that establishing such a policy will be challenging because many noise producers also have a beneficial side and actually improve quality of life. Cars provide mobility and convenience, and few people would be willing to sacrifice several trips in order to decrease their contribution to traffic noise Instead, policy makers will have to find a balance between the demand for noise abatement and the need to take advantage of benefits provided by noise producers.

Measuring Noise

Noise can be considered as any unwanted sound that adversely affects quality of life by interfering with speech, sleep, learning, leisure, and property values. But noise can be subjective—one person may enjoy loud music while the person next door may be annoyed by the sound. To translate noise levels into a definitive measurement, sound pressure levels are measured along a logarithmic scale in decibels (dB). An A-rating network, or correlation, is used to equate this value to one that is more representative of that perceived by the human ear. When sound levels have been corrected in this manner, the unit is designated as dB(A).

Noise measurements range from a level 0 dB(A) at the threshold of human hearing to 140 dB(A)—the point where serious hearing damage can occur. Noise levels on a quiet night may reach 30 dB(A) and a rock concert typically can produce a sound level of 120 dB(A). Noise from highways can range from 65 to 85 dB(A).

Studies have shown that doubling the distance from a noise source will result in a decrease of 3 dB(A). For example, if the sound level measured at 16 feet from a source is 85 dB(A), then moving to 32 feet from the source will result in a decrease in sound levels of 3 dB(A) to a reading of 82 dB(A). In areas along highways with “soft” ground, this decrease can amount to 4.5 dB(A). Also, a change in sound levels of 10 dB(A) will be perceived by humans as a doubling or halving of the loudness of a specific sound.

Generating Traffic Noise

The level of traffic-related noise is dependent on traffic volume, traffic speed, and the type of vehicle. A vehicle produces noise from the engine, the exhaust system, and the tires. When a vehicle is traveling more than 50 mph, 75% to 90% of all the noise it generates is produced at the contact point between the tires and the pavement.

Robert J. Bernhard, director of the Institute for Safe, Quiet, and Durable Highways at Purdue University, and Roger L. Wayson, P.E., associate professor of the Civil & Environmental Engineering Department at the University of Central Florida, authored—along with other contributors— a research report titled An Introduction to Tire/Pavement Noise (SQDH 2005-1), which describes in detail this tire/pavement interaction. The report indicates sound is produced at this interface through several mechanisms referred to as tread vibration, air pumping, slip-stick, and stick-snap (adhesion). The sound produced is then enhanced by other factors such as horn and organ-pipe effects related to the tire geometry and acoustical resonance related to excitation of air in the tire.

A thorough understanding of this sound-producing mechanism can be used to better determine the noise-reducing capabilities of a particular pavement type. The SQDH 2005-1 report lists pavement porosity as one influencing factor. Porosity allows for absorption of sound and decreases the strength of air pumping and the effects caused by the horn and resonance mechanisms. Gap-graded, thin overlays with small aggregates also help to lessen noise production at the tire/pavement interface. These pavements are similar to stone-matrix asphalt and small aggregate Superpave mixtures.

The report also lists pavement texturing, specifically negative texturing, as a pavement characteristic influencing noise production. Conclusions from the report call for a better understanding of the problems surrounding these issues in order to obtain more accurate predictions of noise, optimal quiet pavement designs, and better measurements of actual noise reductions.

Regulating Traffic Noise

The Federal-Aid Highway Act of 1970 required the Federal Highway Administration (FHWA) to develop standards for mitigating highway traffic noise. In implementing this mandate, the FHWA developed a manual, the Highway Traffic Noise Analysis and Abatement: Policy and Guidance. This report, accessible online at www.fhwa.dot.gov/environment, aids designers in meeting the Federal Regulation 23 CFR Part 772, “Procedures for Abatement of Highway Traffic Noise and Construction Noise.” This regulation requires a determination of traffic noise impacts for any type of road project designated as a Type 1 project. These involve new roadways, addition of new lanes to existing roadways, or significant horizontal or vertical realignments.

When sound levels are predicted to approach or exceed the Noise Abatement Criteria (NAC) set forth by the FHWA, noise mitigation must be considered. The FHWA has established a table for the NAC, listing a specific decibel level for certain activity categories, such as residences and churches. For highways near residences, this level is set at 67 dB(A). The FHWA stresses that “the NAC should not be viewed as federal standards or desirable noise levels; they should not be used as design goals for noise barrier construction.” Instead “the NAC should only be used as absolute values which, when approached or exceeded, require the consideration of traffic noise abatement measures.”

As of May 2005, sound level predictions necessitated by the regulation must be established through the use of the FHWA Traffic Noise Model (TNM) software, a computer program used for predicting noise impacts near highways. The Acoustics Facility of the Volpe National Transportation Systems Center, a federal organization within the U.S. DOT, provided technical oversight for the development of the FHWA TNM. The program is available for purchase through the McTrans Center at the University of Florida. More information about the TNM program is at www.trafficnoisemodel.org.

If a noise impact approaches or exceeds that established by the NAC or a substantial noise increase is predicted—in the range of 5 to 15 dB(A), a level specifically established by each state—then a noise abatement strategy is chosen based on whether it is reasonable and feasible. This is determined by the implementation characteristics and cost as it relates to the amount of noise reduction.

Currently, the FHWA does not recognize quiet pavements as a noise abatement strategy on federally funded projects. The use of quiet pavements is allowed on Type 1 projects but will not be considered as a noise reduction benefit.

Developing A Pilot Program

The cost of noise abatement can be high. One typical strategy is the construction of sound-absorbing barrier walls, the average cost of which can range from $1 million to $2 million per mile. There are also some limitations with barrier walls. Noise reduction only occurs within sight-lines, so little to no benefit is realized in some topographical areas.

The use of vegetation sometimes is considered an abatement strategy, but due to the expanse of trees and shrubs required to mitigate noise, this can result in the need for large areas of right of way.

These factors have further added to the interest in quiet pavements. The FHWA has developed a Quiet Pavement Pilot Program (QPPP) to offer states an opportunity to investigate the use of quiet pavements as a noise mitigation strategy. A QPPP is implemented according to specific requirements established by the FHWA, and guidance for this program is at www.fhwa.dot.gov/environment/noise.