5.12 TRAFFIC NOISE

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Since the early 1950s, the number of cars and trucks on the highways in the

United States has increased greatly, as shown in Table 5-26. Increased

environmental noise levels have accompanied the growth in the transportation

sector. The U.S. Environmental Protection Agency (EPA) and the U.S.

Department of Transportation (DoT) have recognized that significant noise

problems associated with traffic flow exist. These agencies have proposed

standards for allowable noise emission for interstate traffic and for acceptable

noise levels for highway planning and siting.

Empirical relationships have been developed that can be used to predict

the hourly energy-equivalent A-weighted sound level for freely flowing

traffic (Transportation Research Board, 1976). For purposes of the correlation,

the major traffic noise sources were classified as automobiles, medium

Noise Sources 207

TABLE 5-26 Growth of Highway Vehicle Traffic in the United States

Item

Year

1950 1960 1970 1980 1990 2000

Population (millions) 151 181 204 229 257 289

Automobiles (millions) 40.4 61.7 87.0 122.6 172.4 242.5

Autos per person 0.268 0.341 0.426 0.535 0.671 0.839

Trucks/buses (millions) 8.8 12.2 19.3 28.4 43.9 67.2

Motorcycles (millions) 0.45 0.51 1.2 1.8 2.5 3.1

Source: EPA (1971b).

Copyright © 2003 Marcel Dekker, Inc.

trucks (pick-up trucks, for example) and heavy trucks (18-wheelers, for

example). It was found that the noise produced by all types of vehicles

was proportional to the vehicle volume V, vehicles/hour, and inversely proportional

to the equivalent distance from the highway DE, meters, raised to

the 1.5 power. For automobiles and medium trucks, the noise is directly

proportional to the vehicle speed S, km/hour, raised to the 2.0 power. For

heavy trucks, however, the noise was found to be inversely proportional to

the truck speed.

The equivalent distance from the highway to the observer is the geometric

average of the distance from the observer to the centerline of the

nearest traffic lane, DN, and the distance from the observer to the centerline

of the farthest traffic lane, DF, as illustrated in Fig. 5-6:

DE ј рDNDFЮ1=2 (5-74)

The average width of a traffic lane is approximately 3.8m (12.5 ft).

The correlations for the A-weighted equivalent sound level for each

type of vehicle are given as follows:

(a) Automobiles:

LeрAЮ ј 10 log10 V _ 15 log10 DE ю 20 log10 S ю 16 (5-75)

(b) Medium trucks:

LeрMЮ ј 10 log10 V _ 15 log10 DE ю 20 log10 S ю 26 (5-76)

208 Chapter 5

FIGURE 5-6 Illustration of the nearest and farthest lane distances for traffic noise.

Copyright © 2003 Marcel Dekker, Inc.

(c) Heavy trucks:

LeрHЮ ј 10 log10 V _ 15 log10 DE _ 10 log10 S ю 84 (5-77)

The total A-weighted sound level is found by combining the levels due

to the three types of vehicle:

LA ј 10 log10Ѕ10LeрAЮ=10 ю 10LeрMЮ=10 ю 10LeрHЮ=10_ (5-78)

Certain adjustments must be made to the values obtained from the

correlations to account for different highway conditions. If the road segment

near the observer is not straight, then the roadway may be subdivided

into several finite-length segments. The following factor _1 must be added

to each equivalent sound level value to correct for the fact that noise is

generated from a finite length of roadway, instead of from a very long

straight road. The geometry is illustrated in Fig. 5-7, where _ is the angle

(expressed in degrees) subtended by the road segment:

_1 ј 10 log10р_=1808Ю (5-79)

The basic noise level is influenced by the condition of the road. The

normal surface, from which the correlations were developed, consists of

moderately rough asphalt or a concrete surface. For this surface, the adjustment

_2 ј 0. A smooth surface is one corresponding to seal-coated asphalt,

for example. A very smooth surface road is not often encountered, because

it has fairly low friction characteristics. For the smooth surface, the adjust-

Noise Sources 209

FIGURE 5-7 Road segment nomenclature.

Copyright © 2003 Marcel Dekker, Inc.

ment _2 ј _5 dB. A rough surface is one corresponding to rough asphalt

with large voids or for grooved concrete surfaces. For the rough surface, the

adjustment _2 ј ю5 dB. The adjustment should be applied only to automobile

noise LeрAЮ and not to the truck noise. The noise generated by truck

traffic is much less sensitive to the road condition than that generated by

automobiles.

When trucks move upgrade, there is generally a need to change gears,

with the resulting change in the noise emitted from the truck. For downgrade

travel and for automobiles, no adjustment is needed р_3 ј 0Ю. The

adjustment for trucks on an uphill grade (does not apply to cars) is given by

the following expression:

_3 ј

0:6 р%GЮ for %G < 8%grade

5 dB for %G     8%grade

_

(5-80)

Example 5-8. A four-lane highway has the following traffic data:

Automobiles: 1200 autos/hour at 90 km/hr (55.9 mph)

Medium trucks: 240 trucks/hour at 100 km/hr (62.1 mph)

Heavy trucks: 120 trucks/hour at 105 km/hr (65.2 mph)

The observer is located at a distance of 50m (164 ft) from the centerline of

the nearest lane. The distance from the observer to the centerline of the

farthest lane is 65m (213 ft). The road is straight and has an average surface.

The grade is 3% uphill for trucks. Determine the equivalent A-weighted

sound level due to traffic noise.

The equivalent distance between the observer and the highway is given

by Eq. (5-74).

DE ј Ѕр50Юр65Ю_1=2 ј 57:0m р187 ftЮ

The contribution of the automobiles to the noise is calculated from Eq.

(5-75):

LeрAЮ ј 10 log10р1200Ю _ 15 log10р57:0Ю ю 20 log10р90Ю ю 16

LeрAЮ ј 30:8 _ 26:3 ю 39:1 ю 16 ј 59:6 dBA

The medium truck contribution, without the grade correction, is calculated

from Eq. (5-76).

LeрMЮ ј 10 log10р240Ю _ 15 log10р57:0Ю ю 20 log10р100Ю ю 26

LeрMЮ ј 23:8 _ 26:3 ю 40:0 ю 26 ј 63:5 dBA

210 Chapter 5

Copyright © 2003 Marcel Dekker, Inc.

The heavy truck contribution, without the grade correction, is found from

Eq. (5-77):

LeрHЮ ј 10 log10р120Ю _ 15 log10р57:0Ю _ 10 log10р105Ю ю 84

LeрHЮ ј 20:8 _ 26:3 _ 20:2 ю 84 ј 58:3 dBA

The road segment adjustment (straight road) and the surface adjustment

(average surface) at zero. The grade adjustment for the trucks is found from

Eq. (5-80):

_3 ј р0:6Юр3:0Ю ј 1:8 dBA

The corrected values for the truck sound levels are as follows:

LeрMЮ ј 63:5 ю 1:8 ј 65:3 dBA

LeрHЮ ј 58:3 ю 1:8 ј 60:1 dBA

The overall sound level for the traffic noise is found by combining the

three contributions according to Eq. (5-78):

LA ј 10 log10р105:96 ю 106:53 ю 106:01Ю ј 10 log10р5:324 _ 106Ю

LA ј 67:3 dBA

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