Street Traffic

Of all the sources of noise pollution, street traffic is the most prevalent and perhaps damaging source of noise pollution. Indeed, Sharp and Donovan (1979) confirm that "more people are exposed to noise from motor vehicles than any other single source of noise" (p. 32-1). Though this claim is now 20 years old, the prevalence of street traffic has certainly grown since then, and thus the impacts of traffic noise are still a major factor in human society.

Noise that is emitted by street traffic is generated by engines, exhaust systems, tires interacting with the road, and horns. Of these, tires contribute most predominantly to the noise emitted by automobiles, both in the effects on passengers within a vehicle and in the contribution to roadside noise, especially in American-made cars. The other components of traffic noise are significant contributors nonetheless. Exhaust and engine noise, for example, have been implicated as even more prevalent than tire noise in some cases, especially in Japanese and European-made cars. Further, noise produced during acceleration can be as much as 20 dB greater than that produced at cruising speed. (Bugliarello, Alexandre, Barnes, & Wakstein, 1976)

Besides cars, buses and trucks also contribute significantly to traffic noise. Though there are fewer of these vehicles in use than cars, the contribution of buses and trucks to noise pollution is significant nonetheless. For example, according to Burgliarello et al. (1976), "the noise from a heavy truck or bus is equivalent to that from 10 to 15 private cars together" (p. 83). One reason for this is that trucks and buses generally use diesel engines, in which ignition occurs at a higher pressure than in gasoline-burning automobile engine, resulting in an increased amount of airborne vibration emission. Additionally, diesel engines tend to be used at or near maximum power more often than passenger cars and light trucks, further contributing to their levels of noise emission. (Burgliarello et al.)

Motorcycles are another source of traffic noise, and they present a unique situation. First, unlike cars, trucks, and buses, tire noise contributes rather insignificantly to the overall amount of noise produced by motorcycles (Sharp & Donovan, 1979). Thus, the type of engine, acceleration, and other issues that are relevant to the engine system rather than the tires become more important when considering motorcycles as a noise source. Second, unlike passengers in cars, trucks, and buses, the rider of a motorcycle is not shielded by an enclosed compartment from the noise produced by their vehicle. Third, motorcycles can be particularly noisy; whereas cars generally produce noise levels in the range of 67-75 dB, motorcycle noise generally ranges from 72-83 dB, but can reach levels as high as 120 dB immediately behind the cycle. (Burgliarello et al., 1976)

In summary, then, one would expect to find peaks in traffic noise accompanying the presence of trucks, buses, and motorcycles. But there are many other factors besides the type of vehicle that influence the amount of noise produced by street traffic, as well. Examples of these additional factors can be seen in traffic, road, environmental, weather, and building parameters. (Bugliarello et al., 1976)

First, traffic noise is influenced by traffic parameters, which include speed, density, "fluidity," and driver behavior. Traffic speed and density, the first two of the traffic parameters, are both positively correlated with noise production. For example, noise levels increase by 3-5 dB when traffic speed is doubled in the presence of 50 or more vehicles per hour. Traffic fluidity, or the amount of stopping and starting versus continuous flow, tends to exert its effects in the level and duration of peak noise intensities. And of course driver behavior influences noise emission, in that how "hard" people drive their cars also influences peak noise intensities.

Road parameters represent a second additional group of factors influencing traffic noise. For example, the propagation of traffic noise is significantly reduced in tunnels versus open roads. Other factors in this category include road surface (e.g. stone is particularly noisy), gradient (steeper hills cause vehicles to work harder and thus emit more noise), and width (narrow streets lined closely with buildings trap noise and increase its effects; view image).

Another group of factors affecting traffic noise includes environmental parameters, such as the distance and height from the road a person is located, the presence or absence of natural or artificial screens, and the amount of noise absorption due to the condition of the ground between a person and the road.

Weather parameters include factors such as various precipitation conditions (e.g. rain, snow, or dry), which mostly exert their effects indirectly by affecting traffic speed, and wind direction and speed, which are generally more applicable in rural areas since urban areas have many wind blocks such as tall buildings.

A final group of factors affecting traffic noise involve the design of buildings. Namely, people in buildings that are better insulated are influenced less than people in poorly insulated buildings. Other relevant factors include the number of windows in a building and how many of them are open.

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