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Highway Traffic Noise
The Ldn and Lden metrics are also
averages of sound energy, where
the period of time is 24 h. For the
day-night sound level, there is a +10
dB nighttime sensitivity penalty ap-
plied between the hours of 10 p.m.
and 7 a.m. For the Lden metric, the
same nighttime penalty is applied
and an additional penalty of +5 dB
is applied to evening hours (7–10
p.m.). Some agencies require the
use of these metrics, including the
FTA for residential land uses (A-
weighted Ldn) and the Department of Housing and Urban Development (HUD; also A-weighted Ldn). California laws require the use of A-weighted Lden.
There are several applicable measurement standards and guidance documents for conducting highway traffic noise and related measurements. The guidance documents in- clude:
•Measurement of Highway-Related Noise. Report No. FHWA-PD-046, 1996, Federal Highway Administration (FHWA) and Volpe National Transportation Systems Cen- ter, US Department of Transportation (DOT); available at http://acousticstoday.org/measure.
• Methods for Determining the Insertion Loss of Outdoor Noise Barriers. S12.8, 2013, American National Standards Institute, Acoustical Society of America; available at http:// acousticstoday.org/noise.
• Method for Determining the Acoustic Impedance of Ground Surfaces. S1.18, 2010, American National Standards In- stitute, Acoustical Society of America; available at http:// acousticstoday.org/impedance.
• Methods for determining the effects of pavements include - Onboard sound intensity (OBSI), American Association of State Highway and Transportation Officials (AASHTO)
T 360; available at http://acousticstoday.org/highway.
- Statistical isolated pass-by (SIP), AASHTO TP 98; avail-
able at http://acousticstoday.org/pass.
- Continuous-flow traffic time-integrated (CTIM) method,
AASHTO TP 99; available at http://acousticstoday.org/traffic.
- Statistical pass-by (SPB) method, International Organiza- tion for Standardization (ISO) 11819-1; available at http://
acousticstoday.org/passby.
- Close proximity (CPX) method, ISO 11819-2; available at
http://acousticstoday.org/proximity.
Examples of highway noise measurements are shown in Figure 7.
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Figure 7. Left: onboard sound intensity (OBSI) measurements. From Pavement Effects Imple- mentation Study (Rochat et al., 2012). Right: wayside noise measurements. Continuous-flow traf- fic time-integrated measurements from Arizona Quiet Pavement Pilot Program. Available at http://acousticstoday.org/jrochat/.
For typical highway projects, measurement methods found in the FHWA’s Measurement of Highway-Related Noise are applied. This FHWA guidance document contains methods for determining existing noise levels (e.g., for prediction model validation purposes or evaluating the need for a noise barrier), determining barrier insertion loss (e.g., for purpos- es of determining the effectiveness of a noise barrier), and collecting vehicle noise emission levels (e.g., for inclusion in highway traffic noise prediction models) among other things. This document is currently being updated and will include pavement-related measurement methods as well as a new section providing practitioners with a clear path to applying project-appropriate measurement methodologies.
Regulatory Process
Highway traffic noise regulations vary by country. In the United States, the FHWA Noise Regulation contained in 23 Part Code of Federal Regulations (CFR) 772 (available at http://acousticstoday.org/regs) applies. This regulation requires that state highway agencies (SHAs) conduct noise studies for “Type I” projects during the environmental process. Type I projects generally involve increasing road- way capacity that includes roadway widening or new align- ment. The noise studies must identify noise-sensitive land uses that will be impacted by the project and evaluate noise abatement for those impacted uses. Noise studies are also executed for “Type II” projects that involve constructing ret- rofit noise barriers for existing development with no change to the adjacent highway.
The FHWA Noise Regulation contains noise abatement cri- teria (NAC) for various land use activity categories. Noise impacts occur when predicted sound levels associated with the project in the design year approach or exceed the NAC for a particular land use category or if the project causes a substantial increase in existing sound levels. The NAC are in terms of the hourly A-weighted equivalent sound levels,