ENVIRONMENTAL CONSEQUENCES
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METHODS FOR ASSESSING IMPACTS
Soundscapes and Noise Quality
Relevant Criteria
The Federal Highway Administration has adopted noise abatement criteria (23 CFR
772) that establish hourly A-weighted decibel (dBA) levels for various land-use
activity categories. Noise levels were considered to have an impact when they
approached (within 1 decibel) or exceeded the criteria, or when the predicted
noise levels substantially exceeded the existing noise levels. A substantial
increase is defined as a predicted noise level of 10 to 15 dBA greater than the
existing noise level. Table 5 summarizes the noise levels for various land-use
categories. Due to the types of activities and serene quality of the park,
Activity Category A was used with a criterion of 57 dBA Leq(h).
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TABLE 5. FEDERAL HIGHWAY ADMINISTRATION NOISE ABATEMENT CRITERIA |
| |
|
|
| Activity Category |
Criteria dBA L eq(h)1 |
Description of Activity |
|
|
|
A |
57 (exterior) |
Lands on which serenity and quiet are of extraordinary significance and serve an
important public need and where the preservation of those qualities is essential
if the area is to continue to serve its intended purpose |
|
B |
67 (exterior) |
Picnic areas, recreation areas, playgrounds, active sports areas, parks,
residences, motels, hotels, schools, churches, libraries, and hospitals |
|
C |
72 (exterior) |
Developed lands, properties, or activities not included in Categories A or B,
above |
|
D |
N/A |
Undeveloped lands |
|
E |
52 (interior) |
Residences, motels, hotels, public meeting rooms, schools, churches, libraries,
hospitals, and auditoriums |
1 dBA Leq(h) = A-weighted average noise level over a 1-hour period.
Source: 23 CFR 772.
Noise levels are described by a logarithmic scale in units of decibels. The
human ear perceives noises of different frequencies in different ways. The dBA
approximates human perception of the overall noise spectrum and is, therefore,
used in most noise studies. Small changes in noise levels of 3 dBA or less are
not noticeable by the average person. Because the dBA scale is logarithmic, a 10
dBA increase in noise level is generally perceived as a doubling of the sound.
Sensitive Receptors
The only known noise receptors in the project study area would be potential
hikers along the Pacific Crest Trail (which crosses the roadway near the east
terminus of the project), Annie Creek Canyon Trail (located 1,320 feet (402
meters) from the east terminus of the project), and a northern spotted owl
activity center near the west boundary (located 2,952 feet (900 meters) from the
roadway). Park staff presumes that the wilderness area around the project
corridor is not heavily used by visitors (Mac Brock pers. comm. 2002). For the
purposes of this study, reference locations of 50 and 100 feet from the roadway
centerline were used in estimating noise levels.
Construction Noise
Construction projects would be accomplished under the implementation of either
alternative B or C. Equipment and vehicles involved in milling the pavement,
road base preparation, paving, and finishing activities would generate the
primary source of noise. Construction noise would be intermittent and short term
in duration.
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Negligible – an action that could change the ambient noise environment, but
the change would be slight and result in an increase of 3 dBA or less.
-
Minor – an action that would result in readily apparent changes in the noise
environment with an increase or decrease of 4 to 9 dBA and affect few sensitive
receptors.
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Moderate – an action that would result in readily apparent changes in the
noise environment with an increase or decrease of 10 or higher dBA and affect
few sensitive receptors.
-
Major – an action that would result in readily apparent changes in the noise
environment with an increase or decrease of 10 or higher dBA and affects most of
the sensitive receptors.
