In this article we will discuss about the characteristics and factors influencing traffic noise.
Characteristics of Traffic Noise:
Of all the kinds of surface transportation noise, traffic noise, as we have noted, is the most ubiquitous. It is also intrinsically discontinuous: as a vehicle approaches an observation point, the noise level rises, reaches a maximum (peak) level and then decreases as the vehicle moves away.
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However, a steady flow of traffic (e.g., average or dense traffic conditions) generates an almost constant road noise, from which only the noise emitted by certain vehicles (such as trucks and motorcycles) consequently stands out. Thus, traffic noise is caused by a number of vehicles of different characteristics moving together under variable conditions.
The noise levels measured varied enormously according to location, type of street, density of traffic and time of day.
A more recent Paris survey indicated that average noise levels (noise levels exceeded 50 per cent of the time—L50) of 65 dBA or higher were registered during daytime in front of 60 per cent of the buildings surveyed. At night the noise was found to be particularly variable from one location to another, with a 15 dBA difference between the 5 per cent noisiest and 5 per cent quietest streets.
Factors Influencing Traffic Noise:
Unlike the noise emitted by a single vehicle which depends solely upon the vehicle’s own characteristics and on the tyre-road contact, traffic noise, in addition to the characteristics of the different kinds of vehicle involved, depends also upon a number of parameters independent of the vehicle –
1. Traffic parameters-
(a) Speed and density.
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(b) Composition.
(c) Traffic ‘fluidity’ (traffic lights, one-way streets, etc.)
(d) Driver behaviour.
2. Road parameters-
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(a) Road design (tunnels, cuts, embankments or on a level).
(b) Gradients and degree of curvature.
(c) Nature of road surface.
(d) Width.
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3. Environmental parameters-
(a) Distance and height from the road of the recipient of noise.
(b) Presence of natural or artificial screens.
(c) Condition of ground between the road and point of reception (noise absorption).
(d) Reflection of noise from buildings along the road.
4. Weather parameters-
(a) Rain, snow or dry conditions.
(b) Wind direction and speed.
5. Dwelling parameters-
(a) Orientation of living areas.
(b) Attenuation of noise through windows.
(c) Size of windows.
These parameters are not all of equal importance: some affect the observed noise significantly and deserve to be reviewed in some detail. Others affect noise only negligibly.
The average noise levels (level exceeded 50 per cent of the time—L50) emitted by traffic flows of more than 50 vehicles per hour, rise by 3-5 dBA whenever the traffic speed is doubled and by 4-6 dBA whenever the speed is doubled over 24.6 m.p.h. The peak levels increase at a greater rate with increased traffic flow, but the reduction due to distance becomes more pronounced (see the section below on ‘Surrounding Areas’).
As can be expected, the average sound level also rises with the number of vehicles. Increases, in traffic density are indeed a leading factor contributing to higher noise levels. The combined effects of speed and traffic density are exemplified by Fig. 8.4, which shows the results of a computer model for estimating the traffic noise from a lane (or single-lane equivalent) of passenger car traffic. The model has, however, some limitations as it assumes that the traffic flows freely with constant speed, uniform spacing between vehicles and constant power, that is, no hill climbing.
Under normal urban conditions, that is, excluding high-speed highways, the composition of the traffic assumes special importance. The noise level is particularly affected by the proportion of heavy trucks, while the effect of buses is less pronounced. The increase due to trucks also becomes more marked on a gradient. These facts are hardly surprising when it is remembered that an average heavy truck is 10 dBA noisier than a private car.
On superhighways, on the other hand, the noise difference between heavy trucks and automobiles tends to be less marked. This indicates that speed has a greater effect on noise from private cars than from heavy trucks. In other words, noise generated by intense traffic on high-speed roads is little influenced by the proportion of heavy trucks and is relatively constant, whereas noise generated in a city street fluctuates and comprises sound peaks in proportion to the number of heavy trucks travelling on it.
Discontinuous traffic—traffic involving frequent stops and starts causes higher peaks, but is apt to lower the average noise level. Traffic that is continuous but very slow gives a low average level with relatively small peaks. Traffic which flows steadily at high speeds gives the highest average levels.
Noise is affected not only by the absolute speed of traffic, but also by variations in this speed and these are especially noticeable at intersections. When vehicles approach a red light, they slow down and the noise diminishes. When the light changes to green, they accelerate, generating a sharp rise in noise of 15-20 dBA. After the acceleration stage, but while the traffic light system is still in the green phase and the flow resumes, the sound level falls by about 5 dBA.
The second set of parameters influencing the traffic noise level concerns the road itself. Noise is propagated differently according to whether the road is built at the level of adjacent land, on an embankment, in a cut or in a tunnel.
A tunnel is obviously the best from the viewpoint of reducing noise propagation, since noise is no longer transmitted to the surrounding area. Road tunnels are, however, rare, because of construction difficulties and cost.
Roads placed in cuts with vertical or walls or sloping banks show strong noise reduction, especially in attenuating frequencies above 500 Hz, provided dwellings are located sufficiently far away. However, in a dense urban area, where buildings are close to the road, a road in either type of cut would yield hardly any reduction of the noise level in dwellings. Thus, cuts are only suitable for urban expressways, with only low buildings built nearby.
Certain road surfaces especially stone paving (pave) typical of the streets of some European cities play a significant role in generating noise. According to certain estimates, stone-paved surfaces can increase noise by 6-8 dBA, at least when the vehicles move at steady high speeds.
The gradient and width of the road also influence the noise level. A narrow street lined with high buildings is veritable ‘noise canyon’ where, if all other factors remain unchanged; the noise can up to 6 dBA higher than in an open space. Finally, in steep gradient, the noise can be 6 dBA higher, owing primarily to heavy trucks climbing the hill.
When considering the area adjacent to the road, the main parameters are the distance between the source of noise and the observation point, the latter’s elevation, whether screens are present between it and the road and finally the various absorption and reflection characteristics of the ground and neighbouring buildings.
The closer the observer is to the road, the better he can perceive noise peaks against the background noise; the farther he moves away the less the peaks will stand out from the background. Each time the distance from the road is doubled, the noise peaks are reduced by 7-8 dBA and the average sound level (L50) by 5 dBA. Therefore, in an urban area, where the buildings are close to the road and where the discontinuous nature of traffic requires a large number of accelerations, the sound peaks become especially annoying.
The height from which the noise is perceived seems to play but a very minor role in an urban area. This is because, although the observer is higher up and away from the sound source, noise is heard from a larger area. In the case of buildings at some distance from a high-speed road with grass borders, the lower floors may even prove quieter than the upper floors as a result of the absorption effect of the ground. These variations in noise intensity, however, tend to diminish with height.
The other leading parameters concerning the road surroundings are natural or artificial screens. Natural screens along roads usually consist of rows of trees, which reduce noise to a negligible extent. Significant attenuation is provided only by dense and deep plantings which are difficult to achieve in urban areas.
In the case of artificial screens, estimates vary as to how much they reduce noise. Experiments in Germany with 16 different types of shield for attenuating traffic noise have shown sound reductions varying between 15 and 26 dBA, depending on the system used.
In France, simulations of the acoustic effectiveness of various arrangements of screens on a one-twentieth scale model showed that in every case, the attenuation of the average sound level by screens was limited to 15-20 dBA. Further, the acoustic attenuation provided by a screen depends on the distance and especially the height from which the noise is heard.
In any event screens appear to be of real value only along urban roadways or country highways. If they are put up in ordinary streets not only do aesthetic, safety and other problems arise, but the screens must be very high to prevent sound waves from reaching the upper floors of buildings bordering the roads.
Rain can significantly increase the noise emitted by a single vehicle; increases as high as 10 dBA have been observed. On the other hand, when it rains, traffic speed tends to decrease. For this reason, measurements carried out in Paris show that the sound level exceeded 50 per cent of the time (L50) does not change significantly even during rain.
It should also be noted that in countries where winters are severe, studded tyres are commonly used – when used on dry roads; they cause an appreciable increase in noise, especially at high frequencies.
Finally, in urban areas, wind and temperature profiles only slightly affect the transmission of sound along streets.
Noise in an urban area is primarily associated with the annoyance caused in homes. Thus the final group of parameters of concern is the actual design of dwellings and therefore their insulation rather than the propagation of noise. Because the exterior walls of a building attenuate sound appreciably more than do windows, the noise transmitted into a dwelling thus depends on window characteristics.
Ordinary closed windows attenuate sound by some 15-20 dBA. However, even when open or ajar they reduce noise by some 5-10 dBA. With certain types double windows or those fitted with heavy glass reductions from 30 to 45 dBA may be obtained. Thus traffic noise can be reduced substantially by heavily soundproofing.
Sound levels inside a dwelling also depend upon the arrangement of the rooms. Sound differences of more than 20 dBA may be observed between rooms facing the street and those overlooking a courtyard or garden.
Thus, from the different parameters affecting the propagation, transmission and perception of traffic noise, it is clear that noise, as observed, varies a great deal according to circumstances. Some of the principal factors capable of producing significant change in noise are summarised in Table 8.2.
The table lists various factors producing differences of some 10 dBA, corresponding to a change by a factor of 10 in a the acoustic energy and of approximately 2 in the subjective impression of annoyance. The values given in the table mast, however, are regarded with caution since they are only orientative.
Although train noise is more confined and affects a much smaller portion of the population, it can be very strong in areas adjacent to the rails and it penetrates considerably into developed areas, with sound which is recognisable for several thousand feet from the source.
For instance, a U.S. marine officer at training camp for a summer reported that the officers group had very little difficulty with the 5:30 a.m. reveille because at 4:45 a.m., 5:00 a.m. and 5:20 a.m., a train passed near their barracks and at 5:30 a.m. everybody was awake. This is one possible way to derive positive benefit from train noise, but more often the noise has negative values—interrupting conversation or other activities.
Trains have been observed to cause noise levels outside nearby residences of 90-95 dBA both during daytime and at night. Assuming the average noise attenuation of a house to be some 20 dBA, this gives an interior noise level in a house due to a passing train of 70-75 dBA, both for day and night a level often sufficient to wake a sleeping person, disturb a baby or interrupt normal activities.
On the basis of data from a study of 10 cities with widely varying population, the US estimated that in 1998 two million people in the United States were exposed to peak overall railroad sound levels of 80 dBA or greater. In general, detailed estimates of the number of people who hear various noise levels from trains can be reached by an approach similar to the contour method used in airport noise surveys.
Noise contours can be established for rail lines and the number of people and dwellings within each noise contour by using census data and aerial photographs. Problems arise when two or more railroads are within hearing distance of each other.
i. In Automobiles:
Acoustic insulation in an automobile is of little value if windows are open. By closing the windows, a driver can attenuate the noise by 15-20 dBA. For example, the noise level inside an auto on a high-speed, interstate-type highway, is about 70 dBA with windows closed and 85-87 dBA with windows open. In compact cars the noise can be appreciably higher. Driving alongside a truck may increase the noise level a further 8 dBA and being in a tunnel near a truck a noise level as high as 104 dBA can be observed.
There has been a great deal of work in automobile insulation measures and many new models are advertised for their quietness. This emphasis, however, has been for the benefit of the vehicle occupant; for until spurred by legislation, very little consideration had been given to the exterior airborne noise which reaches people living and working along roadways. From the point of view of the automaker this is reasonable because he is directing the benefits and quality of his product toward the consumer and does not find economic advantage in reducing noise external to the vehicle.
Long-distance truck drivers are exposed to a continuous high noise level. The cabin of a truck without any special sound insulation may have a sustained noise level of almost 100 dBA on an open highway. This suggests that there exists the possibility of hearing loss among truck drivers who regularly operate vehicles not meeting noise-reduction criteria.
ii. In Public Transit Vehicles:
In a modern American bus with rear engine and adequate soundproofing, interior noise of 72-80 dBA is typical (with the bus almost full and all passengers seated). In older buses with the engine in front, less effective soundproofing or in poorer condition, the noise can be 85 dBA and higher.
A large number of bus riders are regular commuters, going to jobs or school and then returning by the end of the working day. It is reasonable to assume that the average time a commuter spends in the transit vehicle is one-half hour for each trip—a total travel time of 1 hour each day. According to the ISO standard for hearing conservation, a person exposed to 85 dBA (interior noise in a noisy bus) for a short time interval, should have a rest period of 3 hours.
Thus, if the bus rider is going from a noisy bus to a noisy job or other noisy destination, there is, in fact, no opportunity for achieving the recommended off- time. If the noise level on the job is the same as on the bus or higher, a worker will, in essence, be adding his commuting time to the exposure time of the noisy job.
Rail transit systems are comparatively less widespread than the urban bus. Particularly in the United States, rail transit systems operate only in a few cities—as subways, elevateds or surface lines. However, these systems carry a large number of passengers: in the United States alone, close to 2 billion per year, with some 900 miles of track, of which approximately 500 miles are at grade.
This figure will increase significantly in the near future as more such urban systems are constructed. The large number of users of urban rail systems is composed primarily of commuters. With noise in American subway cars reaching levels of 85-90 dBA, the exposure which a commuter suffers is substantial and needs to be treated in the same way as the exposure to bus noise. Thus, to an increasing degree, noise has been given consideration in the planning and construction of the new rapid-transit systems.