4. THE REMAINING GAPS

During the presentations of the state-of-the-art, participants and speakers were asked to identify gaps and issues that either continued from the previous workshop or had emerged since the previous workshop. These gaps/issues were captured and displayed in real time to the participants. This listing was then sorted into categories similar to the categories used in the first Roadmap. The unedited, but sorted gaps/issues are attached in Appendix C. Each of the gap areas will be described in general terms.

Clearinghouse: The clearinghouse has significant potential value to the community. The workshop participants made a number of suggestions about materials that should be added to the clearinghouse. For example, the participants suggested that the clearinghouse should become an archive for data to be shared in common format and as a repository for educational materials for the states and consultants.

ETG: The participants expressed the continuing need for measurement recommended practice and provisional standards. They urged the completion of the OBSI standard currently under development. They also urged more frequent meetings and a faster response from the ETG.

Quieter Current Pavement Technology: The participants acknowledged the significant and interesting ongoing worldwide effort to develop quieter pavement and encouraged an international synthesis of quieter pavement technology. They also identified two technologies, double layer porous asphalt and pervious concrete as technologies that should be investigated for their practicality for U.S. application.

Technology-based Education: Based on the success of Tire-Pavement Noise 101, it was suggested that more advanced short courses be developed to meet the needs of the pavement community as both general knowledge advances and as new technology evolves.

Measurements: The participants identified a number of gaps in the measurements area. The most crucial are the completion of the OBSI Provisional Standard, the development of a reference test tire for tire/pavement source testing, recommended practices for validation and storage of reference tires, development of additional measurement methodologies for the U.S., and a document to identify the applications for and advantages/disadvantages of the various measurement methods. There was also strong sentiment that we needed to develop an objective, reproducible, time-stable scale of noise generation potential based upon texture profile. This texture based noise index would weight the spectral components of texture based upon their contribution to overall tire-pavement noise generation in much the same way the International Roughness Index (IRI) weights different wavelengths of roughness based upon the their contribution to vehicle response. Currently there is an understanding that texture and noise are related, however only simple empirical relationships have been developed to date. It is appropriate to pursue a more robust investigation of the texture -noise relationship with the availability of more advanced texture measurement tools such as the line laser technology.

Research Noise/Safety/Durability/Cost: The participants identified a number of areas of research that should be addressed. The primary concern about current quieter pavement technology is the durability of the reduced noise effect. This remains a significant open issue. Even for a relatively low truck mix, trucks tend to dominate noise emissions from highways. Yet trucks have not been studied extensively. Thus a group of gaps/issues was developed that focused on truck noise sources, the behavior of truck tires, and the effect of trucks in a traffic mix.

Cost/Benefit: A recurring theme at the Workshops has been whether the benefits of quieter pavement could be monetized and compared against other noise mitigation alternatives to help decision making. Furthermore, the cost of investment in noise mitigation for traffic noise versus the benefits that accrue is a desirable aspect of environmental issues in general.

Policy and Guidelines: The participants raised a significant number of issues primarily around whether the benefit of quieter pavement could be used to reduce impact and noise mitigation. This might be implemented by allowing pavement type to be used in TNM or by allowing pavement specific REMEL data to be used. Another series of policy needs/issues revolved around the national sharing of data for the purpose of justifying either QPPP or pavement specific REMELs data.

Construction, Acceptance and Monitoring: The participants noted at various times that construction affects noise and that many pavements are observed to change noise characteristics over time. The construction, monitoring and maintenance of pavement are important to achieve expected results and to keep noise exposure levels in communities at expected levels.

Accelerated Testing: Current techniques to build and observe innovative pavement throughout their service life require a significant amount of time. In order to move the technology of quieter pavement ahead with the assurance that the reduced noise effect will be durable, it is necessary to find test methods whereby the life of the pavement is accelerated and the effect on noise emission can be monitored. One of the alternatives under consideration would be to couple noise testing with existing accelerated pavement testing for durability.

Public education and involvement: Many of the practitioners that deal with communities find it a challenge to explain traffic noise to lay groups. Much of this challenge is due to the complicated characteristics of human hearing and perceptions of annoyance and pleasantness. Educational tools to assist in these public education efforts would be helpful. These challenges are compounded by the expectations the public has of quieter pavement. The benefits and limitations of the technology are not well described in news announcements. And lastly, many practitioners note that complaints are rising from communities far outside of the region defined as impacted by federal guidelines. A relatively long list of gaps/issues was developed identifying various elements of this problem. This issue had considerably more emphasis at the second workshop than at the first workshop.

The participants rank ordered the unedited list of gaps/issues. The number of votes received by each gap/issue is listed in parentheses behind the gap in Appendix C. This ranking was highly constrained and should be taken with some level of caution since the participants used a diverse set of voting strategies. The top ranked gaps/needs are:

1. Establish construction guidelines and best practices (14) - a large group of participants, regardless of area of practice, recognize from the data collected to date that construction plays a significant role in both the early life performance of quieter pavement and the longevity of the reduced noise effect. However, there is very little guidance for construction of quieter pavements. Currently, construction is specified in relatively general terms. Significant variations are found in nominally identical pavement. In fact in almost every pavement type using current methods, some sites are found that could be considered relatively quiet pavement. Careful correlation studies should be done to identify which pavement parameters cause this variation and what construction specifications control such parameters. A significant effort should be devoted to develop construction guidelines and QA/QC practices to reduce variation and make it possible for agencies to specify pavement that is consistently quieter than current technology.
2. Standard tire for tire/pavement source noise testing (e.g., the SRTT) (11) - tire design and fabrication changes with time and when vehicle models change or vehicle performance specifications change. In addition, a single model of tire (e.g., the Goodyear Aquatread) of a particular size has many variations depending on the customer/vehicle. Thus, tire manufacturers do not manufacture the same tire for the time period of interest for studies of tire/pavement noise from pavement test sites. To further compound this problem, tires age even when they are not used. Tire rubber compounds change when exposed to heat and light. To preserve the characteristic of a tire over a period longer than several years, it is necessary to store the tire in a special environment. In view of these situations, we do not have a source of tires that we can trust to be the same. To address this problem it will be necessary to specify a standard tire that is easy to construct repeatably and to find a source that is willing to manufacture this same tire over a long period of time.
3. Synthesis of global practice (11) - significant effort is funded in Japan and Europe to identify quieter pavement alternatives and explore how to make such innovations practical for typical traffic mix and loads. The European Scanning Tour for Quiet Pavement and the recent comprehensive book by Ulf Sandberg and Jerzy Ejsmont provide a summary of those results a couple of years ago. There is also significant effort to develop measurement standards through ISO TC43/SC1/WG 33. Many, but not all, of these advances in technology and standards are applicable in the U.S. Rather than repeat these efforts, the U.S. should have a dedicated effort to monitor these activities, establish a liaison with international leadership in the area of quieter pavements, and attempt to synthesize the appropriate technology into application. A continuing synthesis of global practice would be of significant value to the U.S. efforts to develop quieter highways.
4. Understanding the durability of the noise reduction effect (9) - there is evidence that some noise reducing pavements lose their acoustical benefit over time as the pavement ages. There are two studies, one on I80 near Sacramento, and one on Highway 138 in California where the noise reduction benefit has been monitored for part of the service life. There have been other studies where pavement, particularly the Arizona ARFC, was placed at many sites before it was recognized as a noise reducing pavement. Thus, an attempt has been made to make noise measurements of pavements that are believed to be of similar construction with different age to determine whether there is a systematic change in the noise properties. Additional systematic studies are required to monitor the effect of pavement age and condition on the noise reduction effect. Such studies should be done for all of the various alternatives that will be considered for noise reduction benefit with significant enough samples that the typical variations that are known to occur in pavement noise emissions are accounted for.
5. Relative advantages/disadvantages of various measurement methods for different applications, specifically wayside and OBSI. (8) - the various wayside and source measurement methods have different intent. This is confusing to engineers new to the quieter pavement field. In addition, an incorrect technique is sometimes used for a certain measurement. A document that would describe the methods and their application would be useful to standardize practice in the U.S. and ensure that the correct measurements are taken as the quieter pavement community attempts to advance this technology.
6. Durable pavements with acoustic longevity (8) - some quieter pavements, particularly the poro-elastic concepts attempted in Sweden and Japan, have very short service life. Porous asphalt pavements have also been found in some cases to have a short service life. Other porous pavements with reasonable service life sometimes lose their acoustical benefit. Thus, we still seek more pavement options that have comparable service life to current technology and maintain an acoustical benefit over that service life.
7. Consistent terminology (7) - in both the pavement industry and on the acoustical side, terminology required for communication of the pavement type and the acoustical benefit have evolved using local standards. An agreed list of terminology would be helpful to allow practitioners to communicate and to advance the technology.
8. Develop double layer porous asphalt (DLPA) (6) - the DLPA pavement is a technology that showed good performance in Europe but is not currently being explored in the U.S. To implement DLPA in the U.S. a number of questions must be answered about mix design using local materials, construction practices, and long term performance. While some Europeans claim relatively long service life (up to 10 years) and minor deterioration of the noise reduction benefit, none of their installations have been in place long enough to verify these claims. A significant number of questions must be resolved in order to bring a DLPA solution to practical application. However, from the data gathered in Europe, the benefit could be substantial.
9. Develop an International Noise Index (INI) (6) - for passby noise, the ISO has established a statistical passby index (SPBI) which accounts for a statistical variation of vehicles at a typical variation of speed. Other technological areas, such as pavement smoothness, have found it useful to develop a single number index metric to characterize the properties of pavement in order to compare various options. An INI would be useful for helping us to determine the relative performance of pavement. An INI might also be useful for construction acceptance.
10. Measurement Methods (6) - measurements of pavement and tire/pavement noise will be used for many purposes, including research to understand tire/pavement noise, construction acceptance, network monitoring, quantifying tire/pavement source levels, understanding noise propagation characteristics, quantifying traffic noise propagation to the wayside, and community noise due to traffic. Only a few measurement methods have been standardized to accomplish these objectives. A relatively long list of standard measurement methods should be developed including, time-averaged wayside measurements, community noise measurements, sound propagation measurements, pavement acoustical properties measurement, and controlled passby or coastby.
11. Pavement type (surface texture) in TNM (6) - TNM currently has a database that distinguishes between concrete pavement, asphalt pavement, and open graded asphalt pavement. However, FHWA policy requires the use of average pavement, which is the average of concrete and asphalt pavement, for noise prediction regardless of the pavement used in the application. In addition, current FHWA policy does not permit the use of the noise reduction benefit of quieter pavement. These policies have been formulated based on the documented uncertainty about tire/pavement noise emissions due to variations and possible loss of noise benefit with pavement age. The issue was raised whether sufficient data have been collected to change this policy and what type of data would be required in order to allow the noise reduction benefit of quieter pavement to be used in TNM prediction.
12. Traffic Mix Effects (5) - while less data is available about tire/pavement noise emissions from truck tires, the data that have been collected indicate that the noise reduction benefits of quieter pavements are different for truck tires than for car tires. In most cases measured thus far, the benefit is less for truck tires. In addition, the noise sources on trucks are different than the noise sources in automobiles. For example, the stack noise is radiated from high above the pavement. In addition, trucks are relatively long and the noise history at passby sites at 25' and 50' is somewhat different from automobiles. The relationship between tire/pavement noise measured at the source and traffic noise from a traffic stream consisting of a significant percentage of trucks may not behave in the same fashion as the studies done thus far which have focused primarily on traffic streams that consist primarily of light vehicles.
13. Construction and construction acceptance (5) - in the future there is some expectation that pavement acceptance may be based on tire/pavement noise characteristics. Acceptance testing using noise characteristics may not only be for noise considerations but as a check on other performance properties as well. The challenge of these approaches includes the break-in period for new pavement. Noise characteristics have been observed to change from early traffic and due to winter maintenance. The benefit of this approach is the simplicity of making a source
14. Cost/Benefits of Quieter Pavement (5) - the potential benefits of quieter pavement include the potential to avoid impact and the reduction of the degree of noise mitigation required. In addition, for the region outside the area of potential impact, quieter highways may reduce community annoyance, and subsequent resistance to highway projects. Quieter highways also potentially increase the quality of life of residents near enough to the highway to hear traffic noise. All of these benefits must be weighed against possible increased costs to construct new pavements and possible increased costs to maintain or replace quieter pavements. In order to evaluate quieter pavement alternatives, it would be valuable to monetize, or otherwise quantify, both the benefits and costs of quieter pavement. Cost/benefit analysis would be helpful not only for engineering decision-making but an understandable metrics for us to establish policy and to communicate with the public
15. "We need to be able to answer the public's questions about quiet pavement and annoyance" (5) - with increasing publicity about quieter highways in various parts of the U.S., citizens and community groups are increasingly requesting the placement of quieter pavement in their neighborhood. The terminology of noise and the indirect relationship between our typical metrics and the public's perceptions create additional confusion. The state and local transportation agencies that interface directly with the public and community groups would find it very helpful to have materials that would help to explain traffic noise and to describe the benefits and limitations of current quieter pavement technology.

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