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adjacent to transportation projects. Properties further from the project often are protected from noise by the first row of properties. Due to the localized nature of noise impacts, it is often acceptable to evaluate the potential for effects to protected populations by assuming a maximum distance at which impacts could occur. This assumed distance can be used to perform a quick buffer analysis in a geographic information system (GIS). If sensitive receptors are located within the area of potential effects, more sophisticated noise receptor or contouring techniques can then be used to characterize the level of effects and the sensitive receptors that would experience them. Regardless of the approach selected, noise analysis results can be overlaid with demographic information in GIS to evaluate effects to protected populations. STATE OF THE PRACTICE Noise modeling analyses frequently are conducted using models developed by the FHWA and the FTA, and transportation projects are evaluated according to the criteria established by each agency. This section describes how noise is evaluated and presents the FHWA and FTA criteria. Table 10-1 lists common transportation factors that affect neighborhood noise levels. See FHWA (1992), FHWA (1995), and FTA (1995) for further information. Reviewing transportation projects with these factors in mind will help to identify projects in which noise should be assessed. Table 10-1. Transportation factors affecting neighborhood noise levels Factor Description Traffic volume Traffic noise increases with traffic volume. Two thousand vehicles per hour sound twice as loud as 200 vehicles per hour. Traffic speed Traffic noise increases with traffic speed. Traffic at 65 miles per hour sounds twice as loud as traffic at 30 miles per hour. Vehicle types Trucks are especially noisy. A single truck sounds as loud as 28 automobiles at 55 miles per hour. Traffic flow Free-flow traffic and stop-and-go traffic create different noise problems. Distance from Sound levels decrease in proportion with the square of distance from the source. roadway Traffic noise is not usually a serious problem more than 150 meters from a heavily traveled road or more than 30 to 60 meters from lightly traveled roads. Barriers Barriers such as buildings and walls are highly effective ways to deflect noise from residential areas or other sensitive receptors. Land use The level of acceptable noise intensity varies by land use. Even moderate noise levels may be unacceptable near churches, hospitals, schools, and other sensitive receptors. Construction Noise from transportation construction projects, although temporary, can cause serious disruptions and should be evaluated as part of noise studies. Source: Derived from Forkenbrock and Weisbrod 2001, p. 130. 232
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Sound level and the noise pattern (continuous, random, or repeated) all are important in characterizing nuisance levels. Absolute noise levels, or the net change in noise levels due to a transportation system change, are thus only part of what must be considered in an effort to understand how a community may respond to altered noise patterns. FHWA. The highway traffic noise prediction requirements, noise analyses, noise abatement criteria, and requirements for informing local officials comprise the noise standards mandated by 23 U.S.C. 109(i). See FHWA (1995) for further information. All highway projects developed in conformance with this noise regulation are considered to be in conformance with the FHWA noise standards. Table 10-2 shows the current FHWA Noise Abatement Criteria (NAC). The NAC are defined in hourly A-weighted decibels expressed as Leq(h) or L10(h). A decibel (dB) is the most common unit of noise measurement. Because the human ear has differing levels of sensitivity to high- pitched and low-pitched sounds, highway traffic noise measurements are adjusted to approximate human hearing. These adjusted measurements are known as A-weighted decibels (dBA). Table 10-2. Noise abatement criteria hourly A-weighted sound level in decibels Activity category Leq(h) L10(h) Description of activity category Lands on which serenity and quiet are of extraordinary 57 60 significance and serve an important public need and A (Exterior) (Exterior) where the preservation of those qualities is essential if the area is to continue to fulfill its intended purpose Picnic areas, recreation areas, playgrounds, active sports 67 70 B areas, parks, residences, motels, hotels, schools, churches, (Exterior) (Exterior) libraries, and hospitals 72 75 Developed lands, properties, or activities not included in C (Exterior) (Exterior) Categories A or B above D None None Undeveloped lands. 52 55 Residences, motels, hotels, public meeting rooms, E (Interior) (Interior) schools, churches, libraries, hospitals, and auditoriums Source: FHWA 1995, p. 7. Although the A-weighted sound level may adequately indicate the level of environmental noise at any instant in time, community noise levels vary continuously. Most environmental noise includes a conglomeration of noise from distant sources that creates a relatively steady background noise in which no particular source is identifiable. A single descriptor called the equivalent sound level (Leq) is used. Leq is the mean A-weighted sound level during a measured time interval. It is the "equivalent" constant sound level that would have to be produced by a given source to equal the measured fluctuating level. The Day-Night Average Sound Level (Ldn) is defined as the A-weighted equivalent sound level for a 24-hour day with a 10 dBA penalty applied to nighttime levels (10 p.m. to 7 a.m.) to compensate for the increased sensitivity to noise during the quieter nighttime hours. 233
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Leq(h) is most commonly used to evaluate project noise impacts. These NAC levels are only to be used to determine impact, and by definition are the absolute levels at which abatement must be considered. Depending on the circumstances of the project being analyzed, it may be necessary to mitigate noise levels that fall either below the NAC or below state-designated criteria. When noise abatement is required, it should be designed to achieve a substantial noise reduction. It is generally not acceptable to merely reduce noise levels to just below the NAC. Figure 10-1 provides an A-weighted decibel scale showing commonly experienced noises for comparison with the FHWA NAC in L10(h). Zero dBA is defined as the faintest sound that can be heard by the human ear. To most people, 60 dBA is perceived as twice as loud as 50 dBA, and 70 dBA is perceived as four times as loud as 50 dBA. 90 dBA is 16 times louder than 50 dBA Modified motorcycle (no muffler) 80 dBA is 8 times louder than 50 dBA Medium truck 70 dBA is 4 times louder than 50 dBA Air-conditioning unit 60 dBA is 2 times louder than 50 dBA Clothes dryer 50 dBA Refrigerator 0 dBA Figure 10-1. Commonly experienced noise levels Source: FHWA 1992, p. 3, reproduced from Forkenbrock and Weisbrod 2001. State highway authorities have the power to determine how to implement the NAC. States can thus develop criteria for abatement levels that approach or exceed the NAC. For example, a state could establish abatement criteria for noise levels that are within 1 to 2 decibels of the NAC, or it could set abatement requirements for noise levels that exceed the NAC. States also have the authority to establish impact criteria for decibel level increases. Under such criteria, the absolute 234
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noise level is not at issue. Rather, the criteria recognize that significant increases in noise levels may have adverse effects in and of themselves. The FHWA model was redeveloped in 2002 and is called the Traffic Noise Model (TNM). This model provides noise levels at discrete receptor locations and also can be used to create noise level contours (Forkenbrock and Weisbrod 2001). FTA. The FTA Transit Noise and Vibration Impact Assessment criteria are shown in Figure 10-2 and Table 10-3. Similar to the FHWA criteria, FTA criteria provide a threshold at which noise abatement must be considered. The FTA criteria are a set of complex curves that incorporate a comparison of existing noise levels with predicted project-generated noise levels. 80 85 75 80 Project noise exposure, Category 1 and 2 Project noise exposure, Category 3 70 75 SEVERE IMPACT land uses (dBA) land uses (dBA) 65 70 60 65 IMPACT 55 60 50 55 Note: NO IMPACT Noise exposure is in terms 45 of Leq(h) for Category 50 1 and 3 land uses, Ldn for Category 2 land uses. 40 45 40 45 50 55 60 65 70 75 80 Existing noise exposure (dBA) Figure 10-2. FTA transit noise and vibration impact Source: FTA 1995. The FTA analysis procedure uses a spreadsheet to determine project-generated noise levels, which are then compared to existing noise levels. The results of these analyses are not net noise levels at discrete receptors but impact levels (no impact, impact, or severe impact) at discrete receptors. This information is sometimes used to create impact level contours. These contours can be used along with GIS to determine whether disparate impacts occur based on net noise levels (FTA 1995). 235