Noise may be defined as unwanted or excessive sound that is bothersome to human beings and wildlife. The 1998 International Labor Office encyclopedia lists the construction industry as the fourth noisiest industry sector. [N]

State and federal agencies and the transportation industry as a whole have employed a wide variety of methods to minimize noise from construction and highway operations. Though some noise regulation exists, many of the most innovative and comprehensive noise control practices have evolved outside of the regulatory context.

Roadway or traffic noise is generated by the vehicle engine and emission of exhaust, aerodynamic sources, and tire/pavement interactions. For vehicle speeds over 50 miles per hour, the tire pavement interaction dominates this mix and the source level is dependent on the vehicle type, tire type, and speed.

Sound walls are the only solution currently approved by FHWA for addressing noise impacts; however; a sound wall attenuates noise only within the acoustical shadow of the wall and benefits only those directly behind it. Caltrans has built more than 600 miles of sound walls at an average annual cost of $60 million. [N] As of 2004, sound walls cost more than $1,300,000 per mile. [N] Noise walls also tend to be very expensive per residence. For example, the 1990 construction cost for a noise wall on I-40 in Knoxville, TN was estimated at $25,000/affected home, as walls would have needed to be over 20 ft. high to be effective. [N] More recently on I-285 in Atlanta the criteria for a noise wall was $50,000 or less per affected home and a noise level of 69 dB(A) or more. [N] For U.S. 441 in West Boca, FL, the requirements to construct a noise wall included a noise level of 67 dB(A) or more, a cost of less than $30,000/affected home and a noise reduction of at least 5 dBA. [N]

The only component of traffic-related noise under the control of DOTs is the acoustical property of the pavement. Quieter highways have the potential to reduce noise levels at the source, reducing the need for expensive sound walls, and benefiting a larger percentage of the community; however, more scientifically based criteria for designing quiet pavements are still needed. A 2005 NCHRP study will undertake the development of adequate quiet pavement design criteria by performing a nationwide survey of both asphalt concrete (AC) and Portland cement concrete (PCC) pavements, using innovative sound measurement technology to develop a nationwide index that ranks various pavement acoustical properties from the quietest to the loudest. [N] Colorado DOT will also be conducting research (2005-2010) on evaluating tire/pavement and environmental traffic noise, starting in 2005, with completion scheduled by 2010. [N] NCHRP Synthesis 268: Relationship Between Pavement Surface Texture and Highway Traffic Noise presented a comprehensive synopsis of pavement/tire noise as it relates to roadways along with detailed information on acoustical definitions and concepts, the theory of tire/pavement noise generation and current mitigation practice, measurement techniques, interior vehicle noise, reported noise emission results for pavement type and texture, effects of pavement wear, surface friction, and maintenance and safety considerations. [N] The study concluded that, "In general, when dense-graded asphalt and PCC pavements are compared, the dense-graded is quieter by 2 to 3 dB(A)," a reduction that corresponds to doubling the distance from the noise source or reducing the traffic speed by 25 percent. [N] In particular the report found that, "open-graded asphalt show(ed) the greatest potential for noise reduction for passby noise. Reduction when compared to dense-graded asphalt ranged from 1 to 9 dB(A)." A 9dB(A) reduction corresponds to a reduction in traffic noise by almost 50 percent. Even dense graded hot mix asphalt surfaces (DGAC) have been found to be quieter than PCC pavements. [N] Stone-matrix asphalt (SMA) has also been found to be a relatively quiet surface. [N] England is moving forward with a 10-year plan to install quieter surfaces (SMA or OGFC) on 60 percent of main trunk roads.[N]

The Colorado Department of Transportation developed a report synthesizing information on road noise issues and mitigation practices in Colorado, to assist in the selection of acceptable pavement/tire noise abatement methods.[N]

Caltrans and TxDOT are actively involved in quiet pavement studies focusing on open graded asphalt concrete. TxDOT refers to these as Porous or Permeable Friction Course (PFC). TxDOT's first PFC was placed in 1999, and, since that time, approximately 25 PFC projects have been constructed in Texas. PFC mixtures are gaining popularity due to their ability to reduce the risk of hydroplaning, reduce the amount of splash and spray, reduce pavement noise, improve visibility of traffic striping in wet weather, and improve ride quality.

In research supported by the U.S. DOT Volpe Research Center Acoustics Facility (VCAF), near-field measurements (at the tire) and wayside measurements are each being used to evaluate AC and PCC pavements on California State Route 138. This study, unique in scale and scope, has placed commonly utilized AC pavements (30mm DGAC, 30mm OGAC, 75mm OGAC, 30mm Rubberized OGAC, and 30mm Bonded Wearing Course) in one location, exposing them to the same environmental and traffic conditions. All tested pavement courses will be placed over a 30 mm DGAC leveling course. VCAF is measuring noise pressure levels at 25 ft, 50 ft, and 200 ft from the edge of the pavement for 5 years, with existing traffic and a controlled test vehicle and applying a modified Statistical Pass-By (SPB) to evaluate the different pavements and account for multiple vehicle types, tires, and speeds. So far, Volpe has found that the quietest pavements are OGAC and RAC type O, though noise suppression effectiveness of pavements is vehicle dependent, with a lower effect shown for heavy trucks. [N]

Caltrans recently completed a three-year study to determine if the noise attenuation benefits of open graded asphalt concrete (OGAC) decreased over time. A 9-kilometer portion of pavement on Interstate-80 near Davis California was rehabilitated in June 1998. The new pavement cross section consisted of a 60 mm dense graded asphalt concrete (DGAC) leveling course that was overlaid with 25 mm of OGAC. Noise measurements a month prior to the pavement rehabilitation established the baseline condition. Additional measurements were made immediately after placement of the DGAC leveling course, and after the completion of the OGAC overlay. Immediately after application of the DGAC base roadside noise levels declined by 3 to 4 dBA from the baseline condition. After application of the OGAC, roadside noise levels declined by about 5 dBA over the baseline condition. Noise levels continued to be 4 to 6 dBA lower than the baseline condition over the entire period of the study. [N]

NCHRP has research planned for FY 2005 on cold weather performance of new generation open-graded friction courses (NGOGFC). [N] While there are numerous reported benefits of NGOFC or PFC mixtures, safety and winter maintenance concerns are often cited as the primary objections to increased use. The research will examine whether attributes of new generation OGFC (NGOGRC) will translate into better performance in winter conditions. In addition to the safety issues, concerns have also been raised about the increased maintenance cost of these mixtures due to the need for additional salt and/or sand treatment. Many agencies, particularly the European ones, have adopted innovative methods of maintaining NGOGFCs to ensure free drainage to surface water. It is also known that several agencies are revising their design criteria to improve the performance of NGOGFC. The use of modified binders and additives has improved the durability of NGOGFCs, but has not solved the potential icing problem. Research is needed to determine the liability versus benefit of using NGOGFC in geographic regions that are susceptible to numerous freeze/thaw cycles. Although no performance problems such as raveling have been reported with NGOGFC, there are still concerns that these mixes could experience the performance problems associated with the old OGFC mixes if the NGOGFC mixes are used in climatic regions susceptible to numerous freeze/thaw cycles. The concerns are the most likely reason that NGOGFC mixes are predominately used in warmer, more arid climates such as the southern and western regions of the United States. [N]

There are numerous differences between NGOGFC (or PFC) and first generation OGFC. NGOGFC contains approximately 20 percent more asphalt (by volume) than conventional OGFC. NGOGFC is designed to have a minimum of 18-percent air voids, whereas conventional OGFC was not designed based on air voids. Conventional OGFC mixture typically contained between 10- and 15-percent air voids. At the lower air void range, moisture could get trapped within the void matrix of the conventional OGFC. The void structure of NGOGFC allows the mix to be more permeable and less likely to trap water, which could potentially freeze. NGOGFC contain fibers and is heavily modified with polymers unlike conventional OGFC mixes. In addition, NGOGFC mixtures are more open graded than the conventional OGFC mixtures. The open texture allows NGOGFC to get flushed out by high-speed traffic, therefore reducing the potential to get clogged over time. NGOGFC mixtures are typically placed more thickly than conventional OGFC (1.5 to 2.0 inches as opposed to 1.0 inch). The thicker, more open matrix allows the NGOGFC to drain more water off the roadway more quickly than conventional OGFC. Research on NGOGFC indicates that the mixes typically last between 10 to 14 years, which is significantly longer than the first generation OGFC mixtures, which typically lasted between 5 and 7 years. The NCHRP project will provide recommendations for DOTs on how to maintain NGOGFC in different environmental zones, including the issue of how to avoid clogging or unclog voids due to sanding operations, provide recommendations on design requirements for NGOGFC, and identify topics that should be studied further. [N]

Acoustics tests on Asphalt-Rubber open graded mixes or Porous Friction Course (PFC)are occurring in Arizona, California, and Texas. As described above, Asphalt-Rubber OGFC is part of Caltrans five year study of noise reduction from various pavement types. In Arizona, resurfacing of the old concrete US 60 with AR OGFC during a major design/build widening project generated a 9.5 dBs reduction and much public feedback, including requests for further resurfacing efforts. [N] ADOT has committed to undertake a $100 million AR OGFC resurfacing effort if the public extends a special freeway tax. ADOT is currently running noise studies on many of its older AR pavements to determine the reduction capabilities of the material over time.

As part of the state's "smoothness" campaign for the state's roadways, TxDOT undertook a noise study as part of a resurfacing of the I-35 in San Antonio, where a 1.5 inch Porous Friction Course that was placed over the existing concrete surface. PaveTex Engineering conducted noise measurements prior to and after the new surface was applied and documented an average reading on the new PFC surface of 10dB quieter than an adjacent section of the old concrete pavement. [N]

A joint study prepared for the Sacramento County Public Works Agency, Transportation Division by the Sacramento County Department of Environmental Review and Assessment and consultants in acoustics and noise control engineering found an average four decibel reduction in traffic noise levels as compared to the conventional asphalt overlay used elsewhere. This noise reduction continued to occur six years after the paving with rubberized asphalt, at which time the study was concluded. The sponsors found this degree of noise attenuation to be significant, as it represented a 60 percent reduction in traffic noise energy, and a clearly perceptible decrease in traffic noise. This degree of traffic noise attenuation from rubberized paving has similar to the result documented in several non-related studies conducted in recent years at various other locations, both nationally and internationally. [N]

The Netherlands has five years of experience with second generation porous asphalt surface courses with rubberized asphalt binders, ranging from test sections to large scale use. The new concept consists of a double-layered porous asphalt construction, made up of a bottom layer of coarse porous asphalt (single-grained gradation, aggregate size 11 - 16 mm) and a top layer of fine-graded porous asphalt (aggregate size 4 - 8 mm). The binder in both layers consists of rubberized asphalt. The fine texture of the top layer causes a reduction of traffic noise, from 3 to 4 dB(A) at 50 km/h up to 5,5 dB(A) at 100 km/h (and 7 to 12 dBA quieter than PCC pavements). The bottom layer has a higher discharge capacity compared to conventional porous asphalt, which makes the sideways drainage of water, even on wide roads, considerably better. Pollution, dirt and silt on the road surface are kept from entering into the construction due to the "sieve" behavior of the top layer. In the Netherlands , a vacuum cleaning method consisting of water under pressure (up to 120 bar) is sprayed onto the surface to remove nearly all of the accumulation. The rotating movement of the spray nozzles makes sure that water enters the top layer from all directions. Directly behind the spray bar the water, containing dirt, is sucked up and recycled before again entering the circuit. Cleaning the two-layered porous asphalt in this way is much more effective compared to conventional porous asphalt, because the dirt is concentrated in the upper part of the top layer. On the older road sections with Twinlay, the bottom layer appears to be clean after being in use for several years, which assures the horizontal drainage of water through this layer. Depending on the severity of pollution, cleaning is required once or twice a year. [N] Like conventional porous asphalt the two- layered construction requires adjusted salting operations, as salt can more easily be carried off with meltwater. [N]

Further information on recycling rubber into pavement technologies, and the performance thereof, is reviewed in Chapter 5, Pavement, Materials, and Recycling.

The United States has experience with tined ( Arizona, California, Colorado, Iowa, Michigan, Minnesota, New Jersey, North Dakota, Virginia, and Wisconsin) and textured ( Michigan) surfaces of PCC pavements to address roadway noise. For PCC pavements, Caltrans is partnering with the Western States-American Concrete Pavement Association on the Interstate-280 pavement rehabilitation project in San Mateo County. In this project the noise production from the old longitudinally tined pavement will be compared to noise production from a PCC pavement with diamond grinding, a PCC pavement with texture grinding, and a PCC pavement overlain with 30 mm of open graded rubberized AC. Noise measurements will be made for three to five years to assess the longevity of noise reduction. As of June 2002, several AC pavements had been placed on a test section of the roadway; plan sheets of the study location are available here. In a comparison study, Volpe has helped Caltrans compare 3 PCC test sections (longitudinal tining, burlap dragged, and broomed tining) and helped Arizona DOT compare three as well (uniform longitudinal tining, uniform transverse tining, and randomly spaced transverse tining) finding that the quietest surface treatments are CA burlap dragged, CA broomed, and AZ uniform longitudinal tining. Again, the percentage of heavy vehicles should be considered in determining overall effectiveness of surface treatments. [N]

New noise research areas being considered include: developing a better understanding of the attributes of pavement that reduce noise generation for different types of vehicles, evaluating pavement performance with age, and developing maintenance techniques that preserve the noise reducing characteristics of the pavement and developing quieter tires without compromising safety.

To date, little data have been developed on how the transmission of pavement noise will be influenced by the porosity and/or rigidity of the internal pavement structure, though the Recycled Materials Resource Center is supporting research in this area. [N] Several recent European research projects examining the issue of the high pitched whine and/or low-pitched rumble commonly associated with PCC pavements have indicated that the construction of porous pavements may provide one method for absorbing noise. While many of these pavements have shown significant initial noise reductions when installed, the propensity of these pores to ‘clog' with debris over time is a cause for concern as this may lead to a reduction in performance with time. Preventative anti-clogging measures may be required. Two approaches for increasing the porosity of PCC and its noise reduction properties exist. The first approach (used in some applications in Europe) involves increasing the porosity of the hydrated cement paste component of the material (typical techniques include the use of air-entraining/entrapping agents, gap-graded aggregates, or mixtures with low sand content while the second involves the use of "aggregate" with a higher than typical porosity. In addition to dissipating the noise that is generated by tire-road interaction by increasing the energy dissipated by moving air (friction), it is anticipated that the use of a low stiffness ‘aggregate/fiber' inclusions may provide an effective means to reduce the stiffness of the pavement and increase the viscous-dampening capacity of the concrete. This is similar to the methodology that is used in machinery vibration isolation pads. By increasing the impedance incompatibility between the concrete components, the sound transmission path can be interrupted which could possibly increase the dampening capacity of the pavement. While little has been reported on the use of inclusions to absorb sound, some work has been performed to investigate the influence of lightweight aggregates and rubber particles on the elastic modulus. With around 280 million tires being dumped annually in the U.S. , scrap tires may be a potential source of flexible inclusions for PCC. This project has been funded by the Recycled Materials Resource Center and research results are due in the next year or two. [N]

The substantial variation found with pavement type/treatment has also prompted FHWA initiation of the Quiet Pavement Pilot Program (QPPP), which will evaluate quiet pavements in terms of noise reduction benefits and longevity, while ensuring safety, and identify pavement specifications and maintenance requirements necessary to maintain the noise reduction benefits. The program will also help introduce quiet pavements as a feature in highway noise prediction models. [N] States with preliminary quantification of quiet pavement benefits qualify for the program. ADOT has already completed an agreement with FHWA and enrolled in the program. Caltrans is working on an agreement. QPPP will collect data over the life of quiet pavement applications included in the program, including pavement parameters and specifications, pavement control parameters, noise data near roads and in communities, and proper noise reductions to include in a noise prediction model. The program will also determine the need for FHWA policy change and key factors that would be included.

Noise emanates directly from primary noise sources such as exhausts and encased engines and from tires, where noise emissions depend upon the pavement type. Secondary noise sources arise due to reflections from pavement and vertical surfaces such as highway noise barriers. Noise barriers can be quite effective in reducing noise for receptors within 120 feet of a highway and are still effective in providing noise reduction beyond that distance. Ten decibel sound reductions are considered attainable, but noise barriers must be high enough and long enough to block the view of a road. Noise barriers do little residences on hillsides extending above a barrier. Also, openings in noise walls for driveway connections or intersecting streets greatly reduce the effectiveness of barriers. [N]

Drawbacks of noise walls include cost, impacts to viewsheds, shading, and the ability of noise barriers to reflect sound energy from an elevated location and spread the highway noise over a wider area. Absorptive sound barriers offset this effect. A number of new sound barriers made with polycarbonate or molded or molded crumb rubber panels have been developed to increase absorption. While such materials exhibit a much better performance than concrete with respect to sound absorption and transmission loss, at this point polycarbonate or rubber sound barriers are performing less well in terms of other criteria including cost-effectiveness, technology maturity, durability, cost and convenience in installation, cost and convenience in maintenance and repair, and aesthetics. Crumb rubber coating retrofit options are discussed below under retrofit practices. In the meantime, some states dealing with strong public criticism regarding noise increase at a distance from new wall installations has led to temporary suspension of Type II programs.

Sound refraction is influenced by the effective sound speed as a function of height above the ground surface. Current highway prediction methods assume a neutral, homogeneous atmosphere; however, prevailing atmospheric conditions can cause receivers beyond those adjacent to a highway to be exposed to highway noise otherwise considered inaudible using standard prediction methods. This effect may not only increase audibility of highway noise but can produce noise levels that exceed the applicable noise impact criteria. When the effective sound speed increases as a function of height, as is the case for downwind and temperature inversion conditions, sound refracts downwards. When the effective sound speed decreases as a function of height, as is the case for upwind and temperature lapse conditions, sound refracts upwards.

Arizona DOT is conducting research, due in 2005, to determine the extent of variations in highway noise propagation and the impact on noise exposure attributable to atmospheric conditions, and recommend procedures to ensure that state agencies base their respective noise mitigation studies and decisions on the best noise measurements possible.[N]

State and local governments spend more than $100 million each year on noise walls and other methods to avoid or mitigate the noise impacts of highways. California has built more than 600 miles of sound walls costing more than $60 million. In July 2000, $226 million was allocated to deliver 63 sound wall projects located throughout the state. FHWA has developed increasingly accurate models that enable States to reduce their costs substantially through better modeling and prediction of noise impacts and better design of noise walls and other mitigation. A detailed listing of noise barrier data may be found in FHWA's " Summary of Noise Barriers Constructed by December 31, 2001." [N]

Most highway sound barriers are built with pre-cast concrete or concrete blocks and have very high acoustic reflectivity (90 percent and above) and low sound absorption for the frequency band of highway noise between 125 Hertz and 400 Hertz. Consequently, the effort to develop new materials for building better noise-reduction sound barriers has increased in recent years, though progress has been limited.

Arizona DOT is exploring crumb rubber based coating, a porous mix of multi-sized crumb rubber particles "glued" with certain polymers/paints, which can be sprayed on to new or existing concrete sound barriers. With near zero porosity, molded rubber has good acoustic absorption capacity. Due to the frequency content of highway traffic noise, changing the size distribution of rubber particles may provide a mechanism to better achieve the noise reduction effect. Crumb rubber is durable, and most industrial polymers/paints can have a minimum life of years and above. Crumb rubber is low cost. Spraying provides a quick, inexpensive and easy way to "manufacture" the coating layer. The repair of coating layers should also be simple because of the spray nature. [N]

ADOT is also exploring retrofitting existing noise walls with innovative top treatments, such as angled tops, irregular top edge, T-top treatments, and other applications that can reduce noise levels and eliminate the need for costly wall height increases or wall replacement. ADOT is exploring these strategies to avoid some of the undesirable impacts of noise walls such as blocked views, large shadows, and upward noise refraction. Innovative noise barrier designs and treatments have been successfully utilized in Europe for a number of years. This report is also due in 2005. [N]

In circumstances where source and path noise control measures are not feasible or sufficient, receptor control measures may be necessary.

FHWA and federal agencies have tried to address receiver controls proactively by recommending that local governments use their power to regulate land development in such a way that noise-sensitive land uses are either prohibited from being located adjacent to a highway, or that the developments are planned, designed, and constructed in such a way that noise impacts are minimized. Some State and local governments have enacted legislative statutes for land use planning and control. As an example, the State of California has legislation on highway noise and compatible land use development. This State legislation requires local governments to consider the adverse environmental effects of noise in their land development process. In addition, the law gives local governments broad powers to pass ordinances relating to the use of land, including among other things, the location, size, and use of buildings and open space.

To aid in the consideration of highway traffic noise in land use planning activities, the FHWA has produced the following report: The Audible Landscape: A Manual for Highway Noise and Land Use. Entering the Quiet Zone: Noise Compatible Land Use Planning is a brochure issued by FHWA that can be used by DOTs. It 1) summarizes the general nature of highway traffic noise, 2) provides examples of Noise Compatible Land Use strategies either constructed or planned, and 3) encourages a proactive posture by local decision makers, developers and citizens to share in and actively influence land use next to highways.

Window openings are typically a building's weakest link for noise infiltration. For this reason, acoustical window treatments can significantly reduce the outside-to-inside noise contribution. In some cases (factors include numbers of affected residents, the configuration of work sites, and the proximity of nearby abutters), window treatments may be more cost-effective and viable than noise barriers or curtains. FHWA has published a resource on "Insulation of Buildings against Highway Noise;" the Full Document is available on-line. Effective public outreach and participation are also a best practice for receptor noise control, as it can greatly increase the community's understanding and tolerance of noise.

The following receptor controls were used on the Boston Central Artery project, and are considered best practices in the field: [N]

• Community Participation — open dialog to involve affected residents
• Window Treatments — reinforcing the building's noise reduction ability
• Noise Complaint Process — ability to log and respond to noise complaints
• Temporary Relocation — in extreme otherwise unmitigatable cases

The most extensive example of window treatment controls in the literature comes from Boston's Central Artery Project. [N] [N] In response to the need for more noise mitigation than accomplished with only source and pathway controls, in 1997 the Central Artery Project elected to implement an acoustical window treatment program. The program was initially intended to reactively address continuing nighttime noise complaints for which the Project developed an Off-Site Noise Mitigation Policy establishing eligibility criteria for abutters to receive window treatments. In 1998 however, as a direct result of community suggestions, the Project expanded the acoustical window treatment program to proactively treat bedroom windows in residences that were likely to be adversely affected by nighttime construction noise. Noise models were used to predict which residences would be eligible based on anticipated work schedules and established criteria policies. [N] As a result, some 300-400 bedroom windows were proactively approved and treated, at a cost of about $400,000. This window treatment program continues to this day, and is expected to treat another 200 windows in anticipation of future work at an additional cost of $100,000. [N]

The national pooled-fund study, HP&R 0002-136, Evaluation of Performance of Experimental Highway Noise Barriers summarizes the findings of the multiyear study and presents some additional analyses of previously collected data. The other two reports in the study are FHWA-RD-90-105, Parallel Barrier Effectiveness, Dulles Noise Barrier Project, and FHWA-RD-92-068, Parallel Barrier Effectiveness Under Free-Flowing Traffic Conditions.

AASHTO has produced a Guide on Evaluation and Attenuation of Traffic Noise, containing guidelines for the abatement of traffic-generated noise through highway design procedures and techniques. It discusses 1) the nature of noise, 2) a systems approach for addressing noise, 3) the highway noise study, 4) noise attenuation measures, and 5) noise barrier design considerations. [N]

The Organization for Economic Cooperation and Development (OECD) produced a Roadside Noise Abatement report reviewing current state-of-the-art and national experience with noise abatement techniques for new and existing roads. It presents the regulations and limits prevailing in the different OECD countries and provides the criteria used in measuring, evaluating, and predicting noise. Low-noise road pavements and noise barriers, walls and screens are assessed in detail. The report also describes the impact of road layout -tunnels, cuttings, embankments on noise levels. [N]

To reduce adverse noise impacts on communities, researchers are developing analysis techniques, abatement methods, and land use tools to better evaluate the effects of highway traffic and construction noise. Major issues requiring research are 1) ways in which atmospheric conditions impact traffic noise prediction, 2) relationship of pavement type and texture to noise, and 3) multi-modal transportation noise prediction methodology. [N]

Future research will provide additional model validation and improvements to the model's graphical user interface. Existing traffic noise prediction models, including the FHWA Highway Traffic Noise Model, do not account for atmospheric variations. Information related to the problems of highway construction noise and the consideration of visual quality during noise barrier design will also be updated and enhanced.