I-10 Twin Peaks Traffic Interchange, Environmental Assessment

Affected Environment and Environmental Impacts October 2005
Noise mitigation is implemented where the efforts are feasible, reasonable,
effective and desired by the affected community. Feasibility of mitigation
measures is determined by considering factors such as local access constraints,
safety, community aesthetics and cohesion, visual impact, engineering constraints
of height, drainage considerations, and other engineering requirements.
Reasonableness is based on the size of the impacted area (number of structures,
spatial distribution of structures, etc.), the predominant activities exercised within
the area, practicality of construction, and cost. Effectiveness criteria are based on
the amount of noise reduction provided by a barrier (at least 5 dBA) and the
barriers ability to reduce noise levels below 64 dBA. All of these factors were
considered in this analysis. The analysis is summarized in Table 4-8 and is
discussed briefly below by the same areas that were discussed earlier.
Twin Peaks Road (Receivers NR26-NR32A and RS27-RS30)
The homes with predicted substantial increases in traffic noise were along the east
side of Continental Ranch both north and south of Twin Peaks Road. Higher levels
were predicted south of Twin Peaks Road because of the roadway’s slight
curvature to the south. Mitigation was considered on both the north and south sides
of Twin Peaks Road. It is important to note, however, that although these
residences would experience a substantial increase in traffic noise levels according
to ADOT criteria, the resultant noise levels are considerably below the levels at
which mitigation is normally considered (64 dBA).
A wall replacing the existing privacy wall along the east side of Continental Ranch
north of Twin Peaks Road was only marginally effective at mitigating noise.
According to STAMINA, a 20-foot tall wall along the back yards of these
residences would reduce predicted noise levels by 5 dBA or greater at only 3
residences (NR26, NR26A, and NR27). A 20-foot tall wall 309 feet in length
would result in a 5 dBA reduction at 3 residences for an approximate total cost of
$154,500. The cost per benefited receiver would be $51,500 which is greater than
the $35,000 per benefited receiver recommended by ADOT. Because of the
expense, the wall’s height, and the limited number of homes that would benefit
from the wall, this wall would not be constructed.
A wall replacing the existing privacy wall along the east side of Continental Ranch
on the south side of Twin Peaks Road was effective at mitigating noise, but a very
tall wall was necessary. A series of wall heights was examined in this area. Wall
heights of ranging from 15 feet to 20 feet were examined. Because each of these
walls protected varying numbers of homes, the most cost effective wall was
selected. According to STAMINA, a 17-foot tall wall along the back yards of these
residences would reduce predicted noise levels by 5 dBA or greater at 12
residences (RS27, RS27A, RS28, RS28A, RS28B, RS28C, RS29, RS29A, RS29B,
RS29C, RS29D, and RS29E). These results are presented in Table 4-8. A 17-foot
tall wall 802 feet in length would result in a 5 dBA reduction at 12 residences for
an approximate cost of $340,850. In addition, additional right of way would be
acquired for the placement of the wall. Additional right of way costs in this area
Interstate 10 Traffic Interchange at
Twin Peaks/Linda Vista
4-58
Project No.: NH-010-D (AIW)
TRACS No.: 10 PM 236 H5838 01D