How much will the
noise levels change after a noise
wall has been built and will the
change be noticeable?
The reduction in noise levels
depends on the separation distance
and elevation difference between the
receptor and the noise wall.
IDOT’s goal is to achieve at least
an 8-dBA traffic noise reduction,
which is considered a substantial
noise reduction. For a
receptor to be considered
“benefited,” at least a 5-dBA
reduction will occur. Changes
in noise levels greater than 5 dBA
are readily perceived by humans.
Why isn’t noise abatement
designed to reduce noise levels
below the NAC?
The noise abatement criteria
(NAC) identifies the noise level at
which noise abatement should be
evaluated. It is not a noise
abatement goal. The objective
of noise abatement is to achieve a
noise reduction that will result in
a noticeable difference from the
unabated traffic noise levels and
can be implemented in a cost
effective way. In typical
noise abatement evaluations, a
substantial noise reduction is
considered to be an 8-dBA traffic
noise reduction. The following
table demonstrates this noise
reduction goal for two sites.
|
Location |
Future Noise Level |
NAC |
Noise Reduction Goal |
Target Noise Level |
|
Site 1 |
69dBA |
67dBA |
8dBA |
61dBA |
|
Site 2 |
78dBA |
67dBA |
8dBA |
70dBA |
Why aren’t
noise barriers proposed in some
cases?
A noise barrier may be proposed
when a noise impact occurs and
the noise barrier will be
feasible and economically
reasonable. A noise
barrier is determined to be
feasible if it achieves at least
an 8-dBA traffic noise
reduction. Issues, such as
driveway access and elevation of
the receptor, may prevent
achievement of an 8-dBA
reduction.
A noise barrier
must also be economically
reasonable, meaning the barrier
option costs less than $24,000
per benefited receptor.
For example, if a noise barrier
will benefit 10 residences, and
the total cost of the noise
barrier is $240,000, then the
noise barrier is economically
reasonable.
If a noise
barrier is not considered
feasible and economically
reasonable for an area, the
noise barrier abatement option
may not be implemented as part
of the project.
How long does the noise wall need
to be?
Generally to be effective, the noise
wall should extend 4 times the
distance between the receptor and
the noise wall. In other
words, if the distance between the
house and the noise wall were 50
feet, the noise wall would need to
extend 200 feet beyond the receptor
in each direction.
Would a berm be as effective as a
noise wall in reducing noise levels?
Earth berms are just as effective
as noise
walls. Studies have shown
that earth berms actually reduce
noise levels to a greater extent
than noise walls due to absorption
and edge effects. However, the
use of berms depends on the space
available. For maintenance
reasons, IDOT requires at least a
3:1 slope on berms. For
example a 12-foot berm with a 3:1
slope would be approximately 72 feet
wide at the base. The
available area for abatement would
need to accommodate this base width.
Why can’t we build a taller
wall to get greater noise reduction?
The barrier height is just one
element that affects the traffic
noise reduction achieved. A
noise wall that breaks the line of
sight between the traffic noise
source and noise receiver reduces
traffic noise up to 5 dBA.
Each additional two feet of noise
wall improves the traffic noise
reduction by approximately 1 dBA;
however, there are diminishing
returns. Beyond a certain
height, incremental changes in
height do not provide additional
reduction in noise level. This
occurs because the wall has already
intercepted a high percentage of
noise energy.
|
Reduction in Sound Level |
Degree of Attainability |
|
5 dBA |
Easily Attained |
|
10 dBA |
Attainable* |
|
15 dBA |
Very Difficult |
|
20dBA |
Nearly Impossible |
Does a noise wall
absorb noise or does noise
bounce off the wall?
This depends on the type of noise
wall constructed. An
absorptive noise wall is
designed to absorb sound and
keep it from reflecting off the
noise wall. A reflective
wall is a wall not designed with
an absorptive material and
consequently, sound reflects off
the wall back toward the source.
The reflected sound level is
less than the sound level coming
directly from the noise source.
This is due to the additional
distance the reflected sound
travels, thereby dissipating the
sound (reducing noise energy).
Generally, the increase in sound
levels due to reflections is not
perceivable and therefore
negligible.
 If
I live one block from the
highway, will a noise wall
reduce the noise in my yard?
Noise walls are generally most
effective within 200 feet of the
noise wall, and one block is
generally a distance of 700
feet. This means there may
only be minimal reduction but
the magnitude will depend upon
several factors, such as the
distance from the roadway, the
elevation of your home relative
to the roadway, and the presence
of buildings between the roadway
and your home.
What about reflective
noise?
Reflective noise is generated
when sound waves reflect off a
hard flat surface back toward
the source and the opposite
side. This situation may
occur when a noise wall is
constructed on one side of the
highway, potentially reflecting
noise to the opposite side.
However, the increase in noise
level on this opposite side of
the highway is typically less
than 3 dBA, which is not a
perceivable change.
What is the cost of a
noise wall?
The average unit cost of noise
wall construction used for the
barrier evaluation is $25 per
square foot (including materials
and installation). This
cost is based on Illinois
construction costs, and walls
built. In areas where
there are utilities or drainage
issues that may need to be
addressed, additional costs may
be incurred. Typical noise walls
cost $1,500,000 per mile.
How did IDOT determine
the value of $24,000 per
benefited receptor as
economically reasonable?
According to FHWA, an acceptable
cost/residence index for
determining the reasonableness
of abatement is $15,000 to
$50,000/residence. IDOT
considers $24,000 per benefited
receptor as reasonable. In
some instances, IDOT may allow
exceptions to this criterion to
close gaps between two barriers,
provide continuity to similar
adjacent land uses, extend
barriers to the next
natural/manmade feature or to
address other significant area
noise level contributors.
Can alternatives to
IDOT standard noise wall
materials be used?
Based on material
testing, IDOT has currently
approved three types of
materials for noise barriers:
• Construction using concrete;
• Construction using composite
materials; and
• Earth Berms.
Other materials may be
considered if they meet IDOT’s
criteria for noise abatement
wall materials and are approved
by the Illinois Highway
Development Council.
“Non-standard” noise wall
designs, such as alternative
patterns for a concrete wall may
be considered, but any costs
exceeding that of a “standard”
noise wall must be funded by the
sponsor.
What are some of the
positive and negative impacts of
noise wall construction?
• Positive Impacts
–Easier conversation
–Better sleeping conditions
–Windows open more often
–Outside more in summer
–More privacy
–Increased safety
–Decreased stranded motorist
instances
• Negative Impacts
–Restricted view
–Feeling of confinement
–Loss of air circulation
–Loss of sunlight and lighting
–Eyesore if barrier not maintained
–Graffiti
Can’t we just plant
trees/vegetation to help reduce
noise levels?
-
Vegetation would need to be at
least 200 feet wide and 18 feet
high to reduce noise levels by
10 dBA.
- In
most cases, 200 feet of space
between the roadway and
receptors is not available.
-
Vegetation/trees can potentially
help screen the highway traffic
from view.
Can I get sound
insulation for my house?
FHWA only participates in
sound insulation of public use
or nonprofit institutional
structures. IDOT’s policy
is established to address
exterior areas where frequent
human activity occurs. In
addition, IDOT is prohibited
from spending state money
outside of state right-of-way.
Therefore, sound insulation of
private residences is typically
not an abatement option.
Why can’t IDOT buy my house?
If the project impacts your property to the extent that it is not viable or usable, IDOT may buy your house. However, if the proposed project does not affect the property land use, the purchase of the house on the property is not likely.
Why can’t IDOT prohibit trucks along roads or reduce speed limits? Won’t that reduce noise levels?
Both of these options may reduce noise levels; however, several factors must be considered. If the road is a main route into and out of a city, or if there are commercial and industrial businesses along the route, a prohibition of trucks would have adverse economic impacts. Also, truck traffic cannot be prohibited on state marked routes. Lowering the speed limits may slightly reduce traffic noise levels, but the speed reduction would lower the capacity of the roadway, thereby increasing delays, air pollutant emissions, and the overall cost of transporting goods and services. Speed limits are determined by the roadway design and speed studies.
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