By Alan M. Petrillo
The walls that motorists encounter along some busy stretches
of regional and state highways often have different artistic elements affixed
or built into them, but their chief purpose is the same — mitigating the effect
of traffic noise on nearby residences.
Rick Ellis, manager of the Pima County Department of
Transportation’s Engineering Division, said when the county adds lanes to a
roadway to improve its traffic volume capacity, it always analyzes the impact
that the increased traffic noise might generate.
"Noise isn't
considered a public health or safety issue that's regulated like air or water
quality, but it is an issue that can be a concern when it reaches certain
levels," Ellis said. "The threshold is when you're sitting in an open
space and having a normal conversation it would be at 66 decibels. The federal
standards are at or approaching 67 decibels, while Arizona Department of
Transportation is down to 64 decibels."
Dustin Krugel, public
information officer for ADOT, confirmed that ADOT's decibel criterion is the
most stringent in the United States.
"If future noise levels, 20 years and out, are
estimated to be 64 decibels or higher, that triggers a noise analysis to
determine if a noise wall would be reasonable and feasible," he said.
Krugel noted the Federal Highway Administration requires all
states to use its approved “Traffic Noise Model” when performing a noise
analysis.
"The model accounts for traffic volumes, speeds,
roadway profile grades, terrain conditions, geometry, ground types and
distances from the noise source to the receivers (homes) and is very sensitive
to elevation differential," he added.
Ellis pointed out that Pima County uses rubberized asphalt
exclusively on all its capital improvement projects and most other roadway
projects, which allows the county to take a three decibel credit in noise
calculation computations. However, on federally funded projects, that credit
can't be taken.
"The feds don't give credit for rubberized
asphalt," Ellis said. "But we use it for its longevity, softer ride,
durability and less noise. Two-thirds of the noise from traffic comes from the
interaction of tires on pavement and the rest is engine and exhaust
noise."
Ellis revealed that Pima County performs its noise analysis
based on a computer model projection and takes field readings of noise in six
to ten locations throughout an affected traffic corridor in order to calibrate
that analysis.
"We document the time of day, traffic counts,
atmospheric conditions, temperature and humidity, then plug that information
into the computer to develop the decibel level projection," he said.
"In modeling the future roadway condition, we model for the worst case
scenario of what the noise levels could be."
Ellis stated that if analysis doesn't hit the 66 decibel
level, then we don't have a problem and no further consideration is needed. But
once you cross 66 decibels, we evaluate the performance and cost benefit of a
noise barrier wall.
"Cost, reasonableness and topographic elements come
into play in evaluating the benefits of a noise wall," Ellis said,
"and there must be at least two benefiters (homes) for one to be
erected."
Krugel pointed out that ADOT holds public meetings when
noise walls are being considered to discuss their need and aesthetics, and to determine
the most desirable designs. If a majority of affected residents do not want a
noise wall, for whatever reason, ADOT won't build the wall or walls, he said.
Ellis added that Pima County's procedure is to go out and
solicit acceptance from the affected parties.
"Fifty percent plus one must vote to accept the noise
walls," Ellis said. "The federal standard is 50 percent plus one must
reject the wall."
"The maximum height of noise barrier walls also is
different between Pima County and ADOT. ADOT caps noise barriers at 20-feet
high while our height maximum is 10 feet. Sound travels in the line of sight,
and when the topography is challenging, the wall heights can change, depending
on if the property is higher or lower than the roadway," said Ellis.
Ellis noted that as long as the county could get a five
decibel reduction, it might build a lower wall, for example six or eight feet
high, instead of the maximum height.
"Sound barrier walls typically are constructed of
masonry blocks, although the county sometimes will use pre-cast concrete or do
cast-in-place concrete walls," he said.
Cost is another part of the noise wall equation. Ellis said
the cost per benefitted receptor (home) for a typical noise wall is between $20
and $25 per square foot. ADOT's Krugel puts the cost of its 20-foot-high noise
walls at $33 per square foot.