Determining the Influence of Road Surfaces on Vehicle Noise at Locations Adjacent to a Roadway: Precision and Bias Statements
Community & Cultural Concerns, Environmental Process
Research Idea Scope
Tire-pavement noise has become an important consideration for highway agencies, as the public increasingly demands that highway traffic noise be mitigated. Title 23, Part 772 of the Code of Federal Regulations (CFR), titled “Procedures for Abatement of Highway Traffic Noise and Construction Noise,” outlines procedures for noise studies that are required for approval of Federal-aid highway projects. However, as pavement selection is not currently included as an abatement measure that may receive Federal funding under 23 CFR 772, there are no procedures in place to specifically measure the effects of pavement type on traffic noise level. While sound walls and other abatement measures provide a means for addressing highway traffic noise, improved pavement surfaces may allow for a competitive alternative for noise mitigation or for avoiding noise impacts.
Draft standard test procedures for three methods of determining the influence of road surface on vehicle noise are currently under development for the American Association of State Highway Agencies (AASHTO). These include the On-Board Sound Intensity Method (OBSI), the Statistical Isolated Pass-by Method (SIP), and the Continuous-Flow Traffic Time Integrated Method (CTIM). The OBSI method provides procedures for measuring exclusively tire-pavement noise at locations very near to the tire-pavement interface. The SIP and CTIM procedures describe methods of determining the influence of road surface on vehicle noise at locations adjacent to a roadway (e.g., “wayside” locations) under various in-situ highway traffic conditions. More specifically, the SIP procedure describes methods for measuring the influence of road surface on highway traffic noise through the measurement of isolated vehicles in existing traffic and allows for the comparison of vehicle noise on roadways of varying surfaces across studies. The CTIM procedure describes a method of measuring sound generated by existing traffic for all vehicles on all roadway lanes for test sites where measuring single vehicle pass-by events would be difficult due to continuously flowing and dense traffic volumes. With the procedures for these test methods underway, questions remain as to the measurement uncertainty, repeatability (precision), and bias of each technique. The precision and bias statements for the OBSI measurement method are currently being developed through the NCHRP 1-44-1 project. “Determining the Influence of Road Surfaces on Vehicle Noise at Locations Adjacent to a Roadway: Precision and Bias Statements” proposes to address issues related to precision and bias for the SIP and CTIM wayside test methods.
The SIP and CTIM procedures include issues specific to each individual test method, as well as issues that overlap between the two methods. This project would review existing historical information as it relates to the SIP and CTIM measurement methods and conduct additional analysis and field measurements to assess remaining issues. The sensitivity of modeling and calculation techniques would also be addressed. The purpose of this research would be to assess precision and bias issues to be included in the CTIM and SIP specifications currently under development for AASHTO.
The following are objectives of the proposed research:
1. Conduct and evaluate historical information regarding precision and bias for wayside noise measurement methods.
2. Assess precision and bias with respect to modeling and calculation techniques.
3. Plan and conduct test studies to address precision and bias issues for both CTIM and SIP test methods.
4. Refine the CTIM and SIP guidance documents, as appropriate, to reflect the results of this study.
5. Develop precision and bias statements to be included in the CTIM and SIP guidance documents.
These objectives can be met via the following tasks divided into two phases:
1. Research and synthesize sources of information regarding precision and bias for wayside noise measurement methods.
2. Plan and conduct initial field measurements to begin to address some of the precision and bias repeatability issues.
3. Address the precision and bias issues related to modeling and calculation methods.
4. Document and deliver an interim report.
5. Plan and conduct further field measurements to address precision and bias.
6. Analyze the results of the phase II testing, with reference to the results from phase I testing and analysis, to develop limits to be implemented in the proposed procedures.
7. Develop precision and bias statements to be included in the CTIM and SIP specifications and refine the specifications as needed to accommodate the lessons learned from the field measurements.
8. Document and submit final guidance and a final report on the project.
Urgency and Payoff
Tire-pavement noise is the most significant noise source contributing to the generation of highway traffic noise. The range of noise levels between the quietest and loudest pavements measured is on the order of, and sometimes greater than, reductions typically achieved by sound barriers. As such, the control of tire-pavement noise through the selection of quieter pavement surfaces is a logical strategy for highway noise mitigation. In many situations, including both those in which barriers may or may not be able to feasibly achieve the same noise reductions, pavement selection could be a simpler and lower cost alternative method of reducing noise levels.
Before quieter pavement strategies can be implemented, a database of pavement noise characteristics must be developed to allow DOTs the ability to easily select a pavement that meets their criteria. Additionally, testing methods must be available to assess the success of the applied mitigation strategy and to determine when rehabilitation of the pavement for noise purposes must occur. In order to accumulate data for these purposes, standardized testing methods are necessary. With the current development of the three draft AASHTO test procedures, an understanding of the measurement uncertainty, repeatability (precision), and bias of each technique is needed.
The results of this study would be quickly assimilated into the draft SIP and CTIM test procedures, resulting in more precise testing methods and a more complete understanding of the variation expected through the use of these techniques. With the optimization of the wayside highway noise measurement techniques, researchers and DOTs may more quickly go forward with the measurements of various pavement surfaces, which would begin to develop a database of pavement noise characteristics and eventually lead to the ability to use pavement selection as a noise mitigation strategy.
Bruce Rymer, CALTRANS