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Public Roads - March/April 2002

FHWA Model Predicts Noise Impacts

by Cynthia Lee and Judith Rochat

Highway traffic noise has been a federal, state, and local problem, even before the first noise barrier was built in 1963. Over the years, community and motorist concerns have fueled the push to improve noise measurement and modeling tools that aid state transportation agencies in addressing the highway traffic noise problem.

One such tool is the Federal Highway Administration Traffic Noise Model® (FHWA TNM®). The FHWA TNM is a new state-of-the-art computerized model used for predicting noise impacts in the vicinity of highways. It uses advances in acoustics and computer technology to improve the accuracy and ease of modeling highway traffic noise, including the design of efficient, cost-effective highway noise barriers.

The FHWA TNM contains the following modeling components:

  • Five standard vehicle types, as well as user-defined vehicles.
  • Constant-flow and interrupted-flow traffic.
  • Effects of different pavement types and graded roadways.
  • Sound-level computations based on a one-third octave-band database and one-third octave-band algorithms.
  • Graphically interactive noise barrier design optimization.
  • Attenuation over/through rows of buildings and dense vegetation.
  • Multiple diffraction analyses.
  • Parallel barrier analyses.
  • Contour analyses.
This screen capture of TNM shows the menu bar, tool bar, a plan view of an example case, an input dialog, and an output table.

These components are supported by a scientifically founded and experimentally calibrated acoustic computation methodology and a new flexible database. The database is made up of more than 6,000 individual vehicle pass-by events, measured at 40 sites across the country. It is the primary building block around which the acoustic algorithms are structured.

FHWA TNM Version 1.0 was initially released in March 1998. Since then, FHWA made two additional minor releases and one additional major release, Version 1.1 in September 2000. Although developed for use in the United States, there are FHWA TNM users in 20 countries.

Version 2.0 is currently undergoing final testing and will be released in 2002. This version will be the last major release of the model, prior to mandatory phase-in on Dec. 31, 2002. At that time, the FHWA TNM will become the official FHWA noise model for highway traffic noise prediction and barrier design.

This is a typical acoustical and meteorological instrumentation setup at a measurement site behind a noise barrier from the TNM validation study.

 To ensure computational accuracy, a comprehensive nationwide validation study is being performed on all aspects of the FHWA TNM. The accuracy of FHWA TNM Version 1.0 was originally assessed by comparing its computations to measurements made by multiple organizations. However, these measurements, as a whole, do not represent a structured study performed with consistent data collection and data reduction and analysis techniques. Furthermore, not all aspects of FHWA TNM computations were investigated. FHWA has assigned the validation study to the Volpe National Transportation Systems Center Acoustics Facility to more accurately examine FHWA TNM predictions.

This video camera setup is recording the noise of the traffic that passes by the measurement site for later input into the TNM model.

A comprehensive study of the accuracy of the FHWA TNM entails performing sound-level measurements and doing modeling at many sites around the country, then analyzing all of the data. This large task is divided into multiple phases and interim results will be published to serve as a reference for the FHWA TNM.

Currently, FHWA TNM validation measurements have been performed at 22 sites in four states. At each site, acoustical, meteorological, traffic, and site survey data were collected. For Phase 1 of the study, the data were analyzed for site geometry and wind conditions, as well as other variables. Results indicate that, on average, the FHWA TNM is predicting sound levels within one to two decibels of the measured levels.

Phase 1 findings will be published in 2002. Findings for future phases will be published as additional analyses are completed.

Cynthia Lee is an acoustics engineer at the Volpe National Transportation Systems Center in Cambridge, Mass. Her work covers many aspects of transportation-noise research, including the measurement, analysis, and modeling of aircraft noise, noise in the National Parks, highway noise, and locomotive-horn characteristics for the Federal Aviation Administration, Federal Highway Administration, and Federal Rail Administration. In support of FHWA, she is part of the FHWA TNM's development team and is currently responsible for TNM testing and technical support. She has a bachelor's degree in electrical engineering from Northeastern University.

Judith Rochat is a physical scientist at Volpe National Transportation Systems Center in Cambridge, Mass. Her work includes the support of the FHWA TNM and the FAA Integrated Noise Model, a computer program used to predict noise in the vicinity of airports. She has a doctorate degree in acoustics from Pennsylvania State University and a bachelor's degree in applied mathematics from the University of California, San Diego. She is a member of the Acoustical Society of America (ASA) and the American Institute of Aeronautics and Astronautics (AIAA).

For additional information, visit the FHWA TNM Web site (http://, the Volpe Center Acoustics Facility Web site (, and the FHWA Highway Traffic Noise Web site (

Information about purchasing the latest TNM version can be found on the McTrans Web site (; by contacting the McTrans Center, University of Florida, 2088 Northeast Waldo Road, Gainesville, FL 32609; by telephone (352) 392-0378; or by fax (352) 392-3224.