Federal-aid highway projects that require a traffic noise analysis must use the latest version of the Federal Highway Administration Traffic Noise Model (FHWA TNM), according to 23 CFR 772.9(a). While the FHWA TNM provides for the accurate prediction of traffic noise levels along the wayside of a highway, accurate results are not necessarily guaranteed. Accurate results depend upon the quality of the input data and the care with which the user replicates objects in the physical world with objects in the virtual world of the FHWA TNM. This study provides TNM users with the best sources for information and input data that are critical to the development of an accurate model of highway traffic noise. This report provides best practices and guidance related to:
Sources of Quality Geospatial and Elevation Data
There is a wide range of geospatial data available to TNM users in a variety of clearinghouses, catalogs, and portals, and hosted by a broad range of partners/stakeholders, including various Federal, state, local, and tribal governments through their agencies, as well as academia and the private sector. While this study does not include a catalog of all sources for geospatial and elevation data, the study team identified The National Map as a good source for orthographic images, elevation, geographic names, hydrography, boundaries, transportation, structures, and land cover. The National Map is a collaborative effort among the United States Geological Survey and other Federal, state, and local partners to improve and deliver topographic information for the nation. Research conducted in support of this study shows that a number of state highway agencies (SHAs) and their consultants rely on The National Map for geospatial and elevation data for highway noise studies.
Traffic Distributions across a Multiple-lane Highway
The effects of a non-uniform traffic distribution across a multiple-lane highway are generally less than 1 dBA at typical distances from the near lane of travel. Because other factors affecting sound propagation (e.g., rows of buildings, noise barriers, tree zones, etc.) have larger effects on calculated sound levels, traffic distributions across the lanes of multiple-lane highways may be ignored for environmental noise studies prepared in support of the permitting process under the National Environmental Protection Act (NEPA). Although the effect of a non-uniform traffic distribution is small, the study team believes the effect should be considered for certain roadway geometries and receiver locations, especially during the final design stage of a highway project, when final decisions about noise barriers are made.
The recommended Best Practices for modeling non-uniform traffic distributions on a multiple-lane highway are summarized as follows:
Noise Barrier Optimization
While every state highway agency has established a policy to identify what constitutes a feasible and reasonable noise barrier design, few have established the methods and procedures to identify the optimum noise barrier design. Consequently, the noise barrier design optimization process is one of the primary challenges for SHAs. TNM users and analysts recognize that the acoustical benefits provided by a noise barrier generally increase with increased barrier height, but only up to a point. A point of diminishing returns is met when further increases in barrier height yield little or no increase in acoustical benefit. The optimum noise barrier design is that design that provides the best balance between barrier cost and acoustical benefit. The process used to identify the optimum design is called noise barrier optimization.
This report reviews the current practices in use by SHAs for noise barrier optimization and presents a spreadsheet-based tool that helps TNM users determine the appropriate balance between a low-cost noise barrier design that meets the minimum acoustical requirements and a barrier design that provides the most benefits within the state's cost-effectiveness limit.
Quality Assurance
Quality assurance / quality control (QA/QC) of a product or service, such as an environmental noise assessment or a noise abatement design report, depends upon the processes and procedures that the responsible organization (SHA, engineering firm, acoustical consulting firm, etc.) has set in place. That is, quality assurance depends upon the organization's Quality Management System. This report provides guidance to those organizations looking to incorporate QA practices into highway noise studies or to enhance policies already in place, and provides examples of simple processes and/or tools that may be used for QA. The recommended processes and tools include the use of spreadsheets and special views within the FHWA TNM to verify the accuracy of vertical geometry, as well as the use of checklists to document not only the development and review of TNM object input, but also the development and review of noise study reports and noise abatement design reports.
Report Structure
This report provides Best Practices and guidance related to a range of subjects of interest to TNM users. Chapter I provides sources of quality elevation data and includes advice for TNM users when faced with conducting a search for geospatial and elevation data. In addition, it identifies best practices for distributing traffic volumes across the lanes of multiple-lane roadways and provides guidance related to the use of design hourly speeds, level-of-service speeds, and posted speed limits. Chapter II reviews the current practices for noise barrier design optimization and provides an overview of a Noise Barrier Optimization Tool to help users identify the optimum noise barrier design for a given scenario. The final chapter - Chapter III - provides best practices for quality assurance as it pertains to the development of TNM models and noise study reports.
The appendices provide supporting details including, but not limited to, a small sample of additional sources for geospatial data, compiled traffic data for the development of non-uniform traffic distributions for multiple-lane highways, tabulated sound-level results for the sensitivity analyses that were performed in support of the best practices identified in Chapter I, a sample quality assurance plan, and sample checklists for the development of noise study reports.
