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.](images/07img01.gif) |
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.
![Photo of an acoustical and meteorological instrumentation setup at a measurement site behind a noise barrier from the TNM validation study.](images/07img02.jpg) |
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.
![Photo of a video camera setup on a highway](images/07img03.jpg) |
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 TNMs
development team and is currently responsible for TNM testing and
technical support. She has a bachelors 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 bachelors
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:// www.tiac.net/users/a1f04/tnm/),
the Volpe Center Acoustics Facility Web site (http://www.volpe.dot.gov/acoustics/index.html),
and the FHWA Highway Traffic Noise Web site (http://www.fhwa.dot.gov/environment/noise/index.htm).
Information
about purchasing the latest TNM version can be found on the McTrans
Web site (www-mctrans.ce.ufl.edu); 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.
Other
Articles in this issue:
"Stone-Walling"
in Arkansas
Arkansas
Combines Best Practices for an Innovative Insterstate Rehabilitation
Program
Small
Investment, Dramatic Dividends — Saving Lives in "Blood
Alley"
National
Review of the Highway Safety Improvement Program
Weather:
A Research Agenda for Surface Transportation Program
Highway
Quality Awards
FHWA
Model Predicts Noise Impacts
Synergy
in Action: FHWA's Transportation Pooled-Fund Program