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In recent years, highway traffic noise — the unpleasant,
unwanted sounds generated on our nation’s streets and
highways — has been of increasing concern both to the
public and to local, State, and Federal officials. At the
same time, modern acoustical technology has been providing
better ways to lessen the adverse impacts of highway traffic
noise — click on the “Noise Solution” link
above, for SonicStop’s solution to this problem, as well
as general information regarding acoustical techniques employed
by government agencies, highway planners and designers, construction
engineers and private developers.
The following information describes the
noise problem in basic, measurable terms.
Sound and Noise
As we all know, sound is created when an
object moves: the rustling of leaves as the wind blows, the
air passing through our vocal cords, and the almost invisible
movement of the speakers on a stereo. This movement causes
vibrations or waves in air molecules, like ripples on water.
When the vibrations reach our ears, we hear sound.
Sound is quantified in units called decibels
(dB). For highway traffic noise, an adjustment, or weighting,
of the high and low-pitched sounds is made to approximate
the way that an average person hears sounds. The adjusted
sounds are called “A-weighted levels” (dBA).
The A-weighted decibel scale begins at zero.
This represents the faintest sound that can be heard by humans
with very good hearing. The loudness of sounds (that is, how
loud they seem to humans) varies from person to person, so
there is no precise definition of loudness. However, based
on many tests of large numbers of people, a sound level of
70 is twice as loud to the listener as a level of 60. This
principle is illustrated below.
Causes of Traffic Noise
The level of highway traffic noise depends
on three things:
(1) Volume of the traffic
(2) Speed of the traffic, and
(3) Number of trucks in the flow
of the traffic
Generally, the loudness of traffic noise
is increased by heavier traffic volumes, higher speeds, and
greater numbers of trucks. Vehicle noise is a combination
of the noises produced by the engine, exhaust, and tires.
The loudness of traffic noise can also be increased by defective
mufflers or other faulty equipment on vehicles.
Any condition (such as a steep incline)
that causes heavy laboring of motor vehicles engines will
also increase traffic noise levels. In addition, there are
other more complicated factors that affect the loudness of
traffic noise. For example, as a person moves away from a
highway, traffic noise levels are reduced by distance, terrain,
vegetation, and natural / manmade obstacles.
Traffic noise is not usually a serious problem for people
who live more than 500 feet from heavily traveled freeways,
or more than 100 to 200 feet form lightly traveled roads.
  Determining
Noise Impact
Highway traffic noise is never constant. The noise level is
always changing with the number, type, and speed of the vehicles
that produce the noise. Traffic noise variations can be plotted
on a graph as shown below. However, it is usually inconvenient
and cumbersome to represent traffic noise in this manner.
A more practical method is to convert the noise data to a
single representative number.
Statistical descriptors are almost always
used a s a single number to describe varying traffic noise
levels. The two most common statistical descriptors used for
traffic noise are L10 and Leq. L10 is the sound level that
is exceeded 10 percent of the time.
In the above graph, the shaded areas
represent the amount of time that the L10 value is exceeded.
Adding each interval during which this occurred shows that
during the 60-minute measuring period the L10 was exceeded
6 minutes (1/2 + 2 + 2 + 1 1/2 = 6) or 10 percent of the time.
The calculation of Leq is more complex.
Leq is the constant, average sound level, which over a period
of time contains the same amount of sound energy as the varying
levels of the traffic noise. Leq for typical traffic conditions
is usually about 3 dBA less than the L10 for the same conditions.
The Federal Highway Administration (FHWA) has established
noise impact criteria for different land uses close to highways.
Some of the exterior criteria are illustrated below.
If a project causes a significant increase
in the future noise level over the existing noise level, it
is also considered to have an impact.
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