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The contribution of heavy vehicles to urban traffic noise
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
B. BODSWORTH
Senior Lecturer, School of Engineering and Architecture, Deakin University, Geelong (Australia) and A. LAWRENCE
Associate Professor¢School of Architecture and Building, University of New South Wales (Australia)
SUMMARY
The dominating influence of road traffic on the noise climate of the world's cities is now established and attempts to reduce the problem follow two main lines: The first involves ameliorating the effects of traffic stream noise; the second an attack on the noise levels of individual vehicles. The great expense involved in developing and building quieter vehicles justifies expending considerable effort in establishing the relative noise contribution of the vehicle types found in typical urban traffic mixtures. This paper describes the development of afield method for examining the effects of heavy vehicles such as trucks and buses on the noise profile of the traffic stream. The essential feature of the method involves synchronisation of a recorded voice commentary with the traffic noise. The graphical record of this noise can then be annotated to show what type of vehicles cause the peaks in the overall noise profile.
INTRODUCTION
It has now been accepted that road traffic is the major source of noise in the urban communities of developed countries. This problem is being attacked from two main directions. The first approach involves broad-scale attempts to minimise the effects of traffic stream noise on the community. Techniques used include the erection of barriers, route and traffic flow control, and land-use zoning. In some countries, individuals are compensated to enable them to improve the sound attenuation of traffic noise affected buildings. The second is an attempt to reduce the noise at source by limiting emission levels of new and in-service vehicles as determined by specified test procedures. 57 Applied Acoustics (11) (1978)--© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
58
B. B O D S W O R T H , A. LAWRENCE
All of these approaches are costly and only partly effective. Control of noise at the source by designing and building quiet vehicles is certainly effective, but it is expensive, particularly in Research and Development effort. Further, long lead times in the vehicle industry and the time taken for a generation of vehicles to be phased out exacerbates the problem. Noise-control programmes on existing vehicles tend to be labour intensive; hence they are very expensive and frequently of limited effectiveness. For these reasons, it is important that the greatest community benefit ensues by directing noise-reduction efforts to the vehicles having the greatest impact. This paper reports on some field measurements of noise from typical urban traffic in the Sydney area. There is nothing unique in measuring noise from urban traffic, although it is interesting to discover that typical Australian traffic streams emit similar noise levels to those which have been reported in other countries. However, a novel method of identifying the noise contributions made by individual vehicles present in the traffic stream has been developed, and results of the application of this method are presented here.
METHODS
Site selection Any work aimed at assessing the average effect of particular vehicle types within urban traffic streams faces the immediate problem of site selection. Freely-flowing streams on level grades are relatively rare which suggests that site sampling should include stop-start situations, gradients, speed variations, variable traffic mixes and speeds, built-up and open areas with various traffic-lane configurations. In the work described here, the results obtained at five of the sites are included, selection being ad hoc but based on experience of traffic-noise sampling carried out over a number of years. Vehicle classification Most vehicle noise ordinances permit different levels of noise emission for different vehicle categories, based partly on vehicle type and partly on engine capacity and vehicle size. For example, passenger cars are differentiated from motor cycles, buses and commercial vehicles and the latter are subdivided according to gross vehicle mass and engine size. It is extremely difficult to identify engine capacity and vehicle mass for individual units in a traffic flow of perhaps 2000 to 3000 vehicles per hour; however a good indication may be obtained from the readily observed wheel/axle configuration. For this study the classification method finally adopted showed two types of heavy vehicle in addition to light commercials, cars and motor cycles. The first type included all single driving axle, dual wheeled non-articulated types, these being
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
59
designated 'mediums'. The second group comprised articulated and/or tandemaxled vehicles with buses included as a subgroup within the 'heavies'. Obvious anomalies occurred with the lighter commercials of Japanese origin. Many of these have dual rear wheels and hence are counted as mediums whereas a light commercial classification would be more appropriate to their payload and power. Where possible, the classification of a vehicle is extended to include details of engine type and exhaust configuration, these being recorded as part of a synchronised voice commentary. Counting Traffic counting consumed a major part of the team effort involved in field recording sessions. The technique involved one person for each direction of flow (or a more detailed division of responsibility where turning traffic was involved). The bulk of traffic, private cars, was counted by hand-held counters. Various minority groups such as the heavy commercials were entered as a stroke in the appropriate box on a tally sheet. After a little practice, flows up to 3500 vehicles per hour can be coped with, though in stop-start situations, some loss of accuracy in type classification is likely. At these high flow rates a countin~ period of 10 min is probably close to the maximum if inaccuracies due to lapses in concentration are to be avoided. Voice commentary Quantitative assessment of a noise profile such as obtained from traffic is a controversial subject. In this case, the decision was taken to use the Ll0 level in dB(A) as the primary index although Leq dB(A) is also quoted. It was thus necessary to know as much as possible about the origin of peak-noise levels. During field recording sessions, ore member of the group provided a continuous voice commentary on the traffic flow, concentrating on obviously noisy events such as the passage of a large vehicle, particularly when surrounding traffic was sparse. The provision of a fully comprehensive commentary in heavy flows proved most difficult and may have reduced to perhaps 'heavy and medium, near lanes, now !, and now?'; i.e. basic classification and moment of passing microphone were given top priority, with lane position also important. Other details were provided as the opportunity presented, e.g. exhaust configuration, load condition, vehicle age, engine type. One of the weakest aspects of early commentaries proved to be nonstandard vocabulary; the designing of a set of standard phrases used in order would much improve the quality and quantity of information conveyed. During the later commentaries, a high proportion of peaks were identified (Table 1). At this time, too, attempts were made to photograph significant events in the traffic stream. This caused excessive interference with commentary and synchronised movie filming is now being used for a continuous visual record of selected traffic samples. Videotaping may well be the optimum approach.
B. BODSWORTH, A. LAWRENCE
60
TABLE 1
Site code (No.)
Vehicles per hour
Mediums and heavies by count (per cent)
Mediums and heavies identified on paper c h a r t s (per c e n t ) a
D O W 9 10S1 (1) $2 BOV 23 10SII (2) $21 S12 $22 PAR 25 10 SII (3) $21 S12 $22 GOR S1 (4) $2 PYM SI (5) $6 $7 S12 SI3 S15
3294 3102 1392 1230 1428 1512 3162 3090 3234 3552 180 294 2238 2478 2634 2190 2208 2508
7 9 6 11 8 8 13 13 10 10 40 18 10
90 74 100 100 100 100 78 70 60 74 83 100 89 62 76 100 76 95
and
11
6 7 9 5
° Situations when two or more of these vehicles passed together were not included as the individual contributions could not be assessed.
INSTRUMENTATION AND ANALYSIS
In many cases, noise recordings were made at two or three microphone positions simultaneously, for a separate investigation of the reduction in level obtained by distance or by shielding. However, a reference microphone was always located at a distance of approximately 9 m from the centre of the nearside traffic flow. Typical main-road site configurations consisted of six traffic lanes with a central median strip: during off-peak periods, parking is allowed in the kerb-side lanes, thus traffic is usualty concentrated in the four central lanes. In peak periods, no standing is permitted on the peak flow side and either five or six lanes are in use. The centre of the nearside flow was thus taken, in off-peak periods, to be the line dividing lanes 2 and 3, and in peak periods, to be the centre of lane 2. The output of a precision sound level meter (either Bruel and Kjaer type 2203, linear weighting, Bruel and Kjaer type 2-206, C weighting or General Radio type 1933, linear weighting) was recorded on a Nagra III B or Nagra IV SJ tape recorder; the tape speed was 19 cm CCIR and the microphones were located at 1.2 m above the ground. The recording level was calibrated by recording on a Bruel and Kjaer pistonphone type 4220 or a General Radio calibrator type 1562 as appropriate. The voice commentary was recorded on a separate cassette tape recorder, or in later measurements, on the cue track of the Nagra IV SJ tape recorder. Synchronisation was effected by verbal count-down.
THE C O N T R I B U T I O N OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
61
The recorded signals were re-played through a Bruel and Kjaer Microphone Amplifier type 2603 and Bruel and Kjaer High Speed Level Recorder type 2305 to which was attached a Bruel and Kjaer Statistical Distribution Analyser type 4420, using a sampling rate of three pulses per second and a writing speed of 100 or 160 mm/sec. The SDA counts were transferred manually to punch card and a computer curve fitting program was used to obtain the Statistical L x levels; the equivalent energy level, Leq was also computed. The voice commentary was re-played simultaneously and the paper chart record annotated with the relevant information regarding vehicle category (Fig. 1 gives an example).
EXAMPLES
The range of sites investigated is covered by the five chosen for inclusion here. Site (1) is described to illustrate the detail noted, in addition to the statistical information and noise recording: The microphone location is quoted in terms of distance to the nearside flow centre for non-clearway conditions, in this case 9 m. The carriageway consisted of 3 x 3m lanes each way, separated by a 3-m grassed median strip. Recording was done over grass on the edge of a golf course; virtually free field conditions. Traffic flow was a compromise between stop/start and free flow, there being light-controlled intersections approximately 200 m each side of the recording position. Rising ground 50 m back from the road provided a convenient vantage point for the verbal commentary. For the other four sites, briefly: Site (2). Four lanes, closely built-up area, up grade nearside, stop/start conditions. Site (3). (i) Six lanes, relatively open, up grade nearside, stop/start conditions. (ii) Six lanes, relatively open, slight down grade nearside, free flow conditions. Site (4). Two lanes, closely built-up area, up grade nearside, slow moving laden container trucks. Site (5). Six lanes, relatively open, steep grade up nearside, mixed freely flowing and stop/start conditions. Table 1 shows the sample location, coded for processing convenience, and indicates the degree of individual vehicle identification by commentary which was achieved for various traffic flows. Table 2 provides an indication of the extent to which the larger vehicles (i.e. 'medium' and 'heavy' classifications) contribute to the peaks in the traffic noise profile. On a purely arbitrary basis, peaks which barely exceed the L10 level have been excluded because it is desired to emphasise those likely to stand out as clearly noticeable, hence the use of (L10 + 5)dB(A). Table 3 compares the percentage of (Lao + 5) peaks attributed to heavy and medium vehicles and the percentage of these vehicles in the total traffic stream. If the figures are averaged it is found that streams contained 10 per cent heavy and
62
B. BODSWORTH, A. LAWRENCE
Empty near medium
Background only, no vehicles Large rattling, truck, far lane Two mediums, far lanes, rattling
Heavy laden slow semi, far lanes
rime (mm) Empty semi upward exhaust
Downhill semi, far lane, upwards exhaust Quiet laden heavy, lane 5
Diesel semi, standing, start accel, ground exhaust
Sample Comments
Fig. 1.
Sound level dB (A)
Typical annotated traffic-noise profile for a major Sydney outlet. Non-clearway; 6 lanes; up grade, stop/start, nearside; 3162 vehicles/hour; 12-8 per cent mediums and heavies.
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
63
T A B L E 2" Site No.
Leq
Llo
NO. peaks > Llo
No. peaks b > (Llo + 5)
(1) S1 $2
74 75
77 78
34 31
13 17
77 59
23 29
100 88
(2) SII $21 S12 $22
76 83 76 82
78 86 79 85
30 28 42 45
12 24 15 27
50 38 66 22
25 29 20 52
75 67 86 74
(3) SII
86 77 84 75
88 80 87 78
14 17 31 30
6 14 4 11
50 64 50 82
17 36 50 9
67 100 100 91
76 76
78 76
15 15
9 9
33 33
67 56
100 89
77 78 76 76 74 78
80 80 79 80 76 80
29 17 28 25 29 17
9 7 13 11 12 7
33 86 15 82 25 57
67 0 39 9 50 29
100 86 54 91 75 86
$21 S12 $22
(4) SI $2 (5) SI $6 $7 S12 S13 S14
Per cent (Llo + 5) peaks due to. Heavy Medium Total vehicles vehicles
°All levels in dB(A). b Figures are not quoted but a substantial majority of (L~0 + 5) peaks not accounted for were due to motorcycles, or to cars with faulty exhaust silencing systems. The above figures are unhelpful as regards relative effects of heavies and mediums; a m u c h larger sample would be required to assess these effects. TABLE 3 Site No.
Per cent (Llo + 5) peaks identified as due to heavy and medium vehicles
(1) SI $2 (2) S11 $21 S12 $22 (3) SII $21 S12 $22 (4) S1 $2 (5) SI $6 $7 S12 S13 Si4
100 88 75 67 86 74 67 100 100 91 100 89 100 86 54 91 75 86
Per cent o f heavy and medium vehicles in traffic stream Identified on chart By count 6 7 6 11 8 8 10 9 6 7 33 18 9 7 5 7 7 5
7 9 6 11 8 8 13 13 10 10 40 18 10 11 6 7 9 5
64
B. BODSWORTH, A. LAWRENCE
medium vehicles, but these categories contributed 85 per cent of the noticeable noise peaks. Table 4 gives some indication of the likelihood of a particular vehicle category emitting noticeable peaks above the Llo noise levels, these L~o levels themselves representing the noisier events in the traffic stream noise.
DISCUSSION
The overall re'suits presented in this paper, as exemplified by Tables I to 4, provide evidence that reduction of the roadside noise levels of medium and heavy vehicles would be most beneficial to the community. In the long term, substantial reductions in the permitted noise-emission levels of new vehicles in these categories are essential, and research and development work to this end should be strongly encouraged. It is also necessary to institute methods of regularly monitoring the noise emission of vehicles in use, in order that all noise-reducing systems are properly maintained. In the meantime, when considering an overall programme to ameliorate noise nuisance, various relatively-low cost measures may be implemented. The following programmes could each have some beneficial effects: (1)
(2)
(3)
Flow smoothing, for example by sequencing of light signals, will in most situations lower engine speeds and thus reduce the noise level. The L~o levels for two positions at site (2) tend to confirm subjective impressions that stop/start effects dominate because of accelerative noise peaks, hence flow smoothing will benefit the overall noise climate. This is particularly noticeable on up grade sections of road. The complete re-routingofheavyvehicles,particularlyduringsensitivetimes of day, is worthy of much planning effort. The maximising of distance between truck routes and the community is most importa.nt. In some cases, it may be possible to obtain some reduction in received noise levels by centre lane routing of large vehicles, although this may cause increased traffic hazards. For years, many major airports have subjected aircraft to noise-abatement procedures during certain hours. A similar type of approach might well be feasible with the largest types of commercial vehicles such as are used for inter-city haulage. There is considerable similarity between these units and aircraft. Both are highly capital intensive, operated by specially-licensed personnel, and controlled by regulating bodies.
Subjective impressions formed during the observation of many heavy vehicles in typical traffic conditions indicate that driving techniques have a marked effect on noise levels. Perhaps these effects should be evaluated and, if appropriate, drivers
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
65
TABLE 4
Site No. (1) S1 $2 (2) S11 $21 S 12 $22 (3) SI 1 $21 S 12 $22 (4) S1 $2 (5) Sl $6 $7 S12 SI 3 S14
Per cent of chart-identified vehicles peaking over (Llo + 5) Medi*,ms Heavies Others 16 47 20 53 64 82 4 9 16 5 86 83 27 0 31 7 27 13
83 62 100 80 100 86 13 13 46 39 100 100 27 75 67 69 75 80
Not Not Not Not Not Not Not Not Not Not
available available available available available available available available available available 0 11 0 14 46 9 25 14
could then be instructed in the techniques of noise-abatement driving for use in designated areas. Obviously there are many problems, not the least being a need for some reliable means of indicating to the driver his relative noise level.
ACKNOWLEDGEMENTS
This work is supported by a grant from the Australian Research Grants Committee. The assistance of graduate students in the M.Sc. (Acoustics) course in carrying out the field work is gratefully acknowledged.
B. BODSWORTH
Senior Lecturer, School of Engineering and Architecture, Deakin University, Geelong (Australia) and A. LAWRENCE
Associate Professor¢School of Architecture and Building, University of New South Wales (Australia)
SUMMARY
The dominating influence of road traffic on the noise climate of the world's cities is now established and attempts to reduce the problem follow two main lines: The first involves ameliorating the effects of traffic stream noise; the second an attack on the noise levels of individual vehicles. The great expense involved in developing and building quieter vehicles justifies expending considerable effort in establishing the relative noise contribution of the vehicle types found in typical urban traffic mixtures. This paper describes the development of afield method for examining the effects of heavy vehicles such as trucks and buses on the noise profile of the traffic stream. The essential feature of the method involves synchronisation of a recorded voice commentary with the traffic noise. The graphical record of this noise can then be annotated to show what type of vehicles cause the peaks in the overall noise profile.
INTRODUCTION
It has now been accepted that road traffic is the major source of noise in the urban communities of developed countries. This problem is being attacked from two main directions. The first approach involves broad-scale attempts to minimise the effects of traffic stream noise on the community. Techniques used include the erection of barriers, route and traffic flow control, and land-use zoning. In some countries, individuals are compensated to enable them to improve the sound attenuation of traffic noise affected buildings. The second is an attempt to reduce the noise at source by limiting emission levels of new and in-service vehicles as determined by specified test procedures. 57 Applied Acoustics (11) (1978)--© Applied Science Publishers Ltd, England, 1978 Printed in Great Britain
58
B. B O D S W O R T H , A. LAWRENCE
All of these approaches are costly and only partly effective. Control of noise at the source by designing and building quiet vehicles is certainly effective, but it is expensive, particularly in Research and Development effort. Further, long lead times in the vehicle industry and the time taken for a generation of vehicles to be phased out exacerbates the problem. Noise-control programmes on existing vehicles tend to be labour intensive; hence they are very expensive and frequently of limited effectiveness. For these reasons, it is important that the greatest community benefit ensues by directing noise-reduction efforts to the vehicles having the greatest impact. This paper reports on some field measurements of noise from typical urban traffic in the Sydney area. There is nothing unique in measuring noise from urban traffic, although it is interesting to discover that typical Australian traffic streams emit similar noise levels to those which have been reported in other countries. However, a novel method of identifying the noise contributions made by individual vehicles present in the traffic stream has been developed, and results of the application of this method are presented here.
METHODS
Site selection Any work aimed at assessing the average effect of particular vehicle types within urban traffic streams faces the immediate problem of site selection. Freely-flowing streams on level grades are relatively rare which suggests that site sampling should include stop-start situations, gradients, speed variations, variable traffic mixes and speeds, built-up and open areas with various traffic-lane configurations. In the work described here, the results obtained at five of the sites are included, selection being ad hoc but based on experience of traffic-noise sampling carried out over a number of years. Vehicle classification Most vehicle noise ordinances permit different levels of noise emission for different vehicle categories, based partly on vehicle type and partly on engine capacity and vehicle size. For example, passenger cars are differentiated from motor cycles, buses and commercial vehicles and the latter are subdivided according to gross vehicle mass and engine size. It is extremely difficult to identify engine capacity and vehicle mass for individual units in a traffic flow of perhaps 2000 to 3000 vehicles per hour; however a good indication may be obtained from the readily observed wheel/axle configuration. For this study the classification method finally adopted showed two types of heavy vehicle in addition to light commercials, cars and motor cycles. The first type included all single driving axle, dual wheeled non-articulated types, these being
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
59
designated 'mediums'. The second group comprised articulated and/or tandemaxled vehicles with buses included as a subgroup within the 'heavies'. Obvious anomalies occurred with the lighter commercials of Japanese origin. Many of these have dual rear wheels and hence are counted as mediums whereas a light commercial classification would be more appropriate to their payload and power. Where possible, the classification of a vehicle is extended to include details of engine type and exhaust configuration, these being recorded as part of a synchronised voice commentary. Counting Traffic counting consumed a major part of the team effort involved in field recording sessions. The technique involved one person for each direction of flow (or a more detailed division of responsibility where turning traffic was involved). The bulk of traffic, private cars, was counted by hand-held counters. Various minority groups such as the heavy commercials were entered as a stroke in the appropriate box on a tally sheet. After a little practice, flows up to 3500 vehicles per hour can be coped with, though in stop-start situations, some loss of accuracy in type classification is likely. At these high flow rates a countin~ period of 10 min is probably close to the maximum if inaccuracies due to lapses in concentration are to be avoided. Voice commentary Quantitative assessment of a noise profile such as obtained from traffic is a controversial subject. In this case, the decision was taken to use the Ll0 level in dB(A) as the primary index although Leq dB(A) is also quoted. It was thus necessary to know as much as possible about the origin of peak-noise levels. During field recording sessions, ore member of the group provided a continuous voice commentary on the traffic flow, concentrating on obviously noisy events such as the passage of a large vehicle, particularly when surrounding traffic was sparse. The provision of a fully comprehensive commentary in heavy flows proved most difficult and may have reduced to perhaps 'heavy and medium, near lanes, now !, and now?'; i.e. basic classification and moment of passing microphone were given top priority, with lane position also important. Other details were provided as the opportunity presented, e.g. exhaust configuration, load condition, vehicle age, engine type. One of the weakest aspects of early commentaries proved to be nonstandard vocabulary; the designing of a set of standard phrases used in order would much improve the quality and quantity of information conveyed. During the later commentaries, a high proportion of peaks were identified (Table 1). At this time, too, attempts were made to photograph significant events in the traffic stream. This caused excessive interference with commentary and synchronised movie filming is now being used for a continuous visual record of selected traffic samples. Videotaping may well be the optimum approach.
B. BODSWORTH, A. LAWRENCE
60
TABLE 1
Site code (No.)
Vehicles per hour
Mediums and heavies by count (per cent)
Mediums and heavies identified on paper c h a r t s (per c e n t ) a
D O W 9 10S1 (1) $2 BOV 23 10SII (2) $21 S12 $22 PAR 25 10 SII (3) $21 S12 $22 GOR S1 (4) $2 PYM SI (5) $6 $7 S12 SI3 S15
3294 3102 1392 1230 1428 1512 3162 3090 3234 3552 180 294 2238 2478 2634 2190 2208 2508
7 9 6 11 8 8 13 13 10 10 40 18 10
90 74 100 100 100 100 78 70 60 74 83 100 89 62 76 100 76 95
and
11
6 7 9 5
° Situations when two or more of these vehicles passed together were not included as the individual contributions could not be assessed.
INSTRUMENTATION AND ANALYSIS
In many cases, noise recordings were made at two or three microphone positions simultaneously, for a separate investigation of the reduction in level obtained by distance or by shielding. However, a reference microphone was always located at a distance of approximately 9 m from the centre of the nearside traffic flow. Typical main-road site configurations consisted of six traffic lanes with a central median strip: during off-peak periods, parking is allowed in the kerb-side lanes, thus traffic is usualty concentrated in the four central lanes. In peak periods, no standing is permitted on the peak flow side and either five or six lanes are in use. The centre of the nearside flow was thus taken, in off-peak periods, to be the line dividing lanes 2 and 3, and in peak periods, to be the centre of lane 2. The output of a precision sound level meter (either Bruel and Kjaer type 2203, linear weighting, Bruel and Kjaer type 2-206, C weighting or General Radio type 1933, linear weighting) was recorded on a Nagra III B or Nagra IV SJ tape recorder; the tape speed was 19 cm CCIR and the microphones were located at 1.2 m above the ground. The recording level was calibrated by recording on a Bruel and Kjaer pistonphone type 4220 or a General Radio calibrator type 1562 as appropriate. The voice commentary was recorded on a separate cassette tape recorder, or in later measurements, on the cue track of the Nagra IV SJ tape recorder. Synchronisation was effected by verbal count-down.
THE C O N T R I B U T I O N OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
61
The recorded signals were re-played through a Bruel and Kjaer Microphone Amplifier type 2603 and Bruel and Kjaer High Speed Level Recorder type 2305 to which was attached a Bruel and Kjaer Statistical Distribution Analyser type 4420, using a sampling rate of three pulses per second and a writing speed of 100 or 160 mm/sec. The SDA counts were transferred manually to punch card and a computer curve fitting program was used to obtain the Statistical L x levels; the equivalent energy level, Leq was also computed. The voice commentary was re-played simultaneously and the paper chart record annotated with the relevant information regarding vehicle category (Fig. 1 gives an example).
EXAMPLES
The range of sites investigated is covered by the five chosen for inclusion here. Site (1) is described to illustrate the detail noted, in addition to the statistical information and noise recording: The microphone location is quoted in terms of distance to the nearside flow centre for non-clearway conditions, in this case 9 m. The carriageway consisted of 3 x 3m lanes each way, separated by a 3-m grassed median strip. Recording was done over grass on the edge of a golf course; virtually free field conditions. Traffic flow was a compromise between stop/start and free flow, there being light-controlled intersections approximately 200 m each side of the recording position. Rising ground 50 m back from the road provided a convenient vantage point for the verbal commentary. For the other four sites, briefly: Site (2). Four lanes, closely built-up area, up grade nearside, stop/start conditions. Site (3). (i) Six lanes, relatively open, up grade nearside, stop/start conditions. (ii) Six lanes, relatively open, slight down grade nearside, free flow conditions. Site (4). Two lanes, closely built-up area, up grade nearside, slow moving laden container trucks. Site (5). Six lanes, relatively open, steep grade up nearside, mixed freely flowing and stop/start conditions. Table 1 shows the sample location, coded for processing convenience, and indicates the degree of individual vehicle identification by commentary which was achieved for various traffic flows. Table 2 provides an indication of the extent to which the larger vehicles (i.e. 'medium' and 'heavy' classifications) contribute to the peaks in the traffic noise profile. On a purely arbitrary basis, peaks which barely exceed the L10 level have been excluded because it is desired to emphasise those likely to stand out as clearly noticeable, hence the use of (L10 + 5)dB(A). Table 3 compares the percentage of (Lao + 5) peaks attributed to heavy and medium vehicles and the percentage of these vehicles in the total traffic stream. If the figures are averaged it is found that streams contained 10 per cent heavy and
62
B. BODSWORTH, A. LAWRENCE
Empty near medium
Background only, no vehicles Large rattling, truck, far lane Two mediums, far lanes, rattling
Heavy laden slow semi, far lanes
rime (mm) Empty semi upward exhaust
Downhill semi, far lane, upwards exhaust Quiet laden heavy, lane 5
Diesel semi, standing, start accel, ground exhaust
Sample Comments
Fig. 1.
Sound level dB (A)
Typical annotated traffic-noise profile for a major Sydney outlet. Non-clearway; 6 lanes; up grade, stop/start, nearside; 3162 vehicles/hour; 12-8 per cent mediums and heavies.
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
63
T A B L E 2" Site No.
Leq
Llo
NO. peaks > Llo
No. peaks b > (Llo + 5)
(1) S1 $2
74 75
77 78
34 31
13 17
77 59
23 29
100 88
(2) SII $21 S12 $22
76 83 76 82
78 86 79 85
30 28 42 45
12 24 15 27
50 38 66 22
25 29 20 52
75 67 86 74
(3) SII
86 77 84 75
88 80 87 78
14 17 31 30
6 14 4 11
50 64 50 82
17 36 50 9
67 100 100 91
76 76
78 76
15 15
9 9
33 33
67 56
100 89
77 78 76 76 74 78
80 80 79 80 76 80
29 17 28 25 29 17
9 7 13 11 12 7
33 86 15 82 25 57
67 0 39 9 50 29
100 86 54 91 75 86
$21 S12 $22
(4) SI $2 (5) SI $6 $7 S12 S13 S14
Per cent (Llo + 5) peaks due to. Heavy Medium Total vehicles vehicles
°All levels in dB(A). b Figures are not quoted but a substantial majority of (L~0 + 5) peaks not accounted for were due to motorcycles, or to cars with faulty exhaust silencing systems. The above figures are unhelpful as regards relative effects of heavies and mediums; a m u c h larger sample would be required to assess these effects. TABLE 3 Site No.
Per cent (Llo + 5) peaks identified as due to heavy and medium vehicles
(1) SI $2 (2) S11 $21 S12 $22 (3) SII $21 S12 $22 (4) S1 $2 (5) SI $6 $7 S12 S13 Si4
100 88 75 67 86 74 67 100 100 91 100 89 100 86 54 91 75 86
Per cent o f heavy and medium vehicles in traffic stream Identified on chart By count 6 7 6 11 8 8 10 9 6 7 33 18 9 7 5 7 7 5
7 9 6 11 8 8 13 13 10 10 40 18 10 11 6 7 9 5
64
B. BODSWORTH, A. LAWRENCE
medium vehicles, but these categories contributed 85 per cent of the noticeable noise peaks. Table 4 gives some indication of the likelihood of a particular vehicle category emitting noticeable peaks above the Llo noise levels, these L~o levels themselves representing the noisier events in the traffic stream noise.
DISCUSSION
The overall re'suits presented in this paper, as exemplified by Tables I to 4, provide evidence that reduction of the roadside noise levels of medium and heavy vehicles would be most beneficial to the community. In the long term, substantial reductions in the permitted noise-emission levels of new vehicles in these categories are essential, and research and development work to this end should be strongly encouraged. It is also necessary to institute methods of regularly monitoring the noise emission of vehicles in use, in order that all noise-reducing systems are properly maintained. In the meantime, when considering an overall programme to ameliorate noise nuisance, various relatively-low cost measures may be implemented. The following programmes could each have some beneficial effects: (1)
(2)
(3)
Flow smoothing, for example by sequencing of light signals, will in most situations lower engine speeds and thus reduce the noise level. The L~o levels for two positions at site (2) tend to confirm subjective impressions that stop/start effects dominate because of accelerative noise peaks, hence flow smoothing will benefit the overall noise climate. This is particularly noticeable on up grade sections of road. The complete re-routingofheavyvehicles,particularlyduringsensitivetimes of day, is worthy of much planning effort. The maximising of distance between truck routes and the community is most importa.nt. In some cases, it may be possible to obtain some reduction in received noise levels by centre lane routing of large vehicles, although this may cause increased traffic hazards. For years, many major airports have subjected aircraft to noise-abatement procedures during certain hours. A similar type of approach might well be feasible with the largest types of commercial vehicles such as are used for inter-city haulage. There is considerable similarity between these units and aircraft. Both are highly capital intensive, operated by specially-licensed personnel, and controlled by regulating bodies.
Subjective impressions formed during the observation of many heavy vehicles in typical traffic conditions indicate that driving techniques have a marked effect on noise levels. Perhaps these effects should be evaluated and, if appropriate, drivers
THE CONTRIBUTION OF HEAVY VEHICLES TO URBAN TRAFFIC NOISE
65
TABLE 4
Site No. (1) S1 $2 (2) S11 $21 S 12 $22 (3) SI 1 $21 S 12 $22 (4) S1 $2 (5) Sl $6 $7 S12 SI 3 S14
Per cent of chart-identified vehicles peaking over (Llo + 5) Medi*,ms Heavies Others 16 47 20 53 64 82 4 9 16 5 86 83 27 0 31 7 27 13
83 62 100 80 100 86 13 13 46 39 100 100 27 75 67 69 75 80
Not Not Not Not Not Not Not Not Not Not
available available available available available available available available available available 0 11 0 14 46 9 25 14
could then be instructed in the techniques of noise-abatement driving for use in designated areas. Obviously there are many problems, not the least being a need for some reliable means of indicating to the driver his relative noise level.
ACKNOWLEDGEMENTS
This work is supported by a grant from the Australian Research Grants Committee. The assistance of graduate students in the M.Sc. (Acoustics) course in carrying out the field work is gratefully acknowledged.