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LAeq1h as well as L10. The NAC for residential uses is an LAeq1h of 67 dBA or an L10 (hours) of 70 dBA. These levels are based on speech interference effects in areas of frequent outdoor hu- man use.
SHAs must develop their highway traffic noise policies in accordance with the FHWA Noise Regulation. SHAs must establish a level to be used when determining a traffic noise impact. That level must be at least 1 dB less than the NAC to meet the “approach or exceed” requirement. Most SHAs define approach as 1 dB (i.e., the limit would be 67 − 1 = 66 dBA for LAeq1h). SHAs must also define what constitutes a “substantial increase” over existing noise, something that is particularly important for areas with no existing highway. The FHWA Noise Regulation permits values between 5 and 15 dB. The most commonly used value for substantial in- crease is 10 dB.
Typical highway studies examine noise to distances ad- equate to identify all impacted noise-sensitive land uses. This distance can vary significantly from project to project. Some SHAs evaluate to 500 ft, which incorporates the first few rows of homes or other uses, and some distance beyond.
At a minimum, SHAs must evaluate noise barriers for im- pacted land uses. The SHA may also consider the alternative abatement measures listed in the FHWA Noise Regulation that are discussed in Reducing Highway Traffic Noise.
The FHWA requires that noise abatement measures that meet both the feasibility and reasonableness criteria in a SHA’s noise policy be incorporated into the project plans. Feasibility means that (1) the construction of an abatement measure would not be anticipated to pose any major design, construction, maintenance, or safety problems and (2) the measure will provide a minimum noise reduction of 5 dB for the majority of the impacted first-row receptors (properties in the row closest to the project roadway).
The determination of reasonableness for an abatement measure is a three-step process. (1) The noise reduction design goal in the SHA’s noise policy must be achieved, (2) the abatement measure must be cost effective per the SHA’s noise policy, and (3) the benefited residents and/or property owners must support the measure.
SHAs have flexibility in determining the noise reduction design goal and the cost-effective criteria as well as estab- lishing procedures for gauging the support of the benefited residents and property owners.
Highway traffic noise can also be an important source for multimodal projects that involve both transit and highway components. The FHWA has stated that the FHWA Noise Regulation applies to multimodal projects even though the term "multimodal" is not defined in the regulation. A pro- posed transit project that would share an existing highway right-of-way is not necessarily a multimodal project. The FHWA has established a procedure for determining if the project is multimodal in accordance with the regulation that considers the lead agency, project purpose, and funding source (available at http://acousticstoday.org/multimodal).
Highway traffic noise can also be an important source to consider when conducting noise studies for development projects that use funding from the HUD. Federal Regulation 24 CFR Part 51 Subpart B (HUD Noise Regulations) and the HUD Noise Guidebook outline the noise study process for such projects (HUD, 2009).
Predicting Highway Traffic Noise
Noise studies for federal projects conducted in accordance with the FHWA Noise Regulation must use the current ver- sion of the FHWA Traffic Noise Model (FHWA TNM) com- puter program (Anderson et al., 1998; Menge et al., 1998). The program calculates worst hour equivalent sound levels for locations representing noise-sensitive land uses in a proj- ect area. The FHWA TNM contains the following components: • Modeling five standard vehicle types, including automo-
biles, medium trucks, heavy trucks, buses, and motorcy-
cles as well as user-defined vehicles
• Modeling both constant-flow and interrupted-flow traffic
using a 1994/1995 field-measured database (database in-
cludes over 6,000 isolated vehicle pass-by events!)
• Modeling the effects of different pavement types as well as
the effects of graded roadways
• Sound-level computations based on a one-third octave-
band database and algorithms
• Graphically interactive noise barrier design and optimization
• Attenuation over/through the rows of buildings and dense
vegetation
• Multiple diffraction analysis
• Parallel barrier analysis
• Contour analysis, including sound level contours, barrier
insertion loss contours, and sound level difference contours Figure 8 shows a screen capture for a TNM project. The im- age shows a plan view with a skew view (cross section) and table of sound level results overlaying the plan view.
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