A study of urban motorway noise in Glasgow

Journal o f Sozmd and Vibration (1975) 41(1), 63-72

A STUDY OF URBAN MOTORWAY NOISE IN GLASGOWP R. C. PO'Iq'ER~ AND B. C. Rossw

Y-ARD Linlited, 1395 South Street, Glasgow G14 OXJ, Scotland (Receired 8 August 1974, and ht rerised form 23 Decenlber 1974) Measurements of the noise of the central urban motorway in Glasgow are reported for locations near elevated sections and cuttings, along side roads and on a nearby hill. The typical sound level immediately adjoining the motorway is 75 dB(A), L1o (18-hour), which is in close agreement with the predicted value for free flowing traffic noise. The results show that the traffic on the parallel surface roads and the access roads can dominate the sound levels close to the motorway. The results also indicate that the noise is radiated strongly upward and outward from a road in a cutting, and that any shielding is restricted to positions close to the motorway. The problems of reducing the noise exposure, involving the total separation of people and traffic, are discussed and the Glasgow Corporation approach to reduce the noise of future urban motorways is described.

1. INTRODUCTION The influence of traffic noise in the planning and location of urban motorways is a key factor in providing a socially acceptable road system for modern cities. A comprehensive motorway and expressway system is presently under construction in Glasgow to relieve the city centre of congestion caused by through traffic, and to provide an efficient distribution of vehicles to the main business and residential areas. Figure 1 shows the proposed urban motorway system. The first half of the Inner Ring Road, incorporating the North and West flanks, has been completed, and provides a motorway relief of the city centre. The West flank, including the Charing Cross Section, incorporates all the typical features of an urban m o t o r w a y - cuttings, elevated sections, a major bridge over the River Clyde, and access roads to the business district (Figure 2). It also includes the feature of parallel surface roads, used for local traffic and to distribute the traffic to and from the motorway. Measurements of the sound levels were made at a series of locations on the Charing Cross Section and the North end of the Kingston Bridge to compare the results with predicted levels and to examine the influence of the various structural features. The measurement positions are marked on Figure 2, and the particular parameters associated with each position are indicated. The results, 0nalysis and conclusions are presented in the following sections. 2. NOISE EXPOSURE FROM MOTORWAYS i The definition of an acceptable single number measurement to describe the subjective 9 annoyance of traffic has been extensively studied in recent years [I-5]. As a result, the use of the A-weighted decibel level, exceeded for a stated percentage o f t h e time, has been adopted for legislative purposes in the U.K., although alternative units based on equal energy concepts "j"Presented to the Eighth International Congress on Acoustics, London, July 1974. 1:Now at Bolt Beranek and Newman Inc., 235 Wyman Street, Waltham, Massachusetts 02154, U.S.A. wNow at Department of Navigation, Glasgow College of Nautical Studies, Glasgow G5 9XB, Scotland. 63

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R. C. I~OTTER AND B. C. ROSS

F i g u r e 1. G l a s g o w u r b a n m o t o r w a y system. . . . . , proposed.

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[6] have been proposed as providing a more accurate measure. However, the dB(A) measurements are straightforward to obtain, manipulate and relate to a given situation. The results reported here are generally presented as Llo (18-hour) dB(A) values, where Llo is the sound level that is exceeded for 10 percent of the time, and a reading of Llo is taken within each one hour period from 06.00 to 24.00 hours on a working weekday. These eighteen dB(A) sound levels are then averaged to produce the Lao (18-hour) figure. The recent Department of the Environment Design Bulletin for the design of urban roads [7] specifies an L~o (18-hour) upper level of 70 dB(A) for the noise of traffic outside dwellings measured one metre from the exterior wall; otherwise noise reducing design features should be incorporated. The Land Compensation Act, which applies to England and Wales, sets a maximum level of 68 dB(A) Llo (18-hour) for the noise, again one metre from the exterior wall of a dwelling, as the sound level which must not be exceeded due to a new road without the highway authority having to provide, or pay a grant to provide, appropriate noise insulation. These sound levels are designed to produce an acceptable noise exposure within the residence. With a typical British house construction, providing perhaps 10-15 dB noise reduction with open windows, this gives indoor levels of 55-60 dB(A) Lxo (18-hour). This compares well with recommended mean maximum levels of 55 dB(A) as proposed by the Wilson committee [8] some ten years ago. 3. MEASUREMENT TECHNIQUE The measurements of the Glasgow Urban motorway traffic noise were obtained on nonholiday weekdays, using Bruel and Kjaer instrumentation and a Lockheed Electronics tape recorder. The recordings were made with the wind speed less than 10 km/h and with no precipitation. The analysis was completed by using Bruel and Kjaer instrumentation, including a statistical distribution analyser to give the appropriate Llo levels. An initial survey for the full 18 hours was condficted at the Argyle Street location, which

GLASGOWURBAN MOTORWAYNOISE

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Figure 2. Charing Cross Section of the Glasgow Inner Ring Road. Measurement positions: R, Argyle Street, reference position and effect of distance; A, Avis Car Itire Office,effect of cutting; M, Mitchell Library, effect of cutting; C, Carnarvon Street, effect of cutting and side street penetration; H, Hill Street, effect of elevation; S, Sauchiehall Street, typical city centre street; T, Telephone Company Car Park, effect of side access road. was an ideal measuring position, being at the end of a dead end road with almost all the neighbouring buildings having been demolished as part of a redevelopment scheme. Thus, the only noise source was the motorway and its adjacent access roads. From these results, a time history of the noise through the day on the section of the motorway under study was determined. Then, measurements at the other locations were made on a shorter time basis, with occasional checks back to the reference location to show that this produced consistent results. This method of representative readings has been used elsewhere [9] and is considered acceptable [10]. 4. RESULTS 4.1. CttARACTEROF TIlE NOISE The traffic flow showed two distinct peaks, for the morning and the afternoon rush hour periods, as would be expected of an urban location, near a commercial and shopping district (Figure 3). These results are for the central four through lanes of the motorways, there being an almost equal flow on the adjoining access and surface roads. The noise levels are also

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shown as hourlyLlo dB(A) levels on Figure 3 and follow the traffic flow distribution, although there are additional mid-morning and afternoon noise peaks. These are considered to be due to the extra heavy lorries and other commercial vehicles on the roads during these periods. In the early hours, before the morning rush hours, the range of sound levels is the greatest, with the excursions on a time/level trace being of the order of 20 dB, as the influence of individual vehicles is observed. However, once the traffic density increases, the noise becomes much more continuous and it is only the very noisy individual vehicle, such as an accelerating lorry, that produces any significant excursion (Figure 4). This small amplitude variation is typical of the noise for almost all the 18-hour period considered, in contrast to the wider amplitude variation previously noted for rural free flowing traffic noise situations. The continuous nature of the urban motorway noise is most apparent when compared to the levels observed on Sauchiehall Street, a typical city centre road (4 lanes, narrow pavements and tall buildings). Here the sound levels vary considerably, the noise coming in bursts alternating with quiet periods as the traffic lights regulate the vehicle stream. Thus, where a typical variational cycle for Sauchiehall Street is 40 seconds, that for the more continuous noise of the urban motorways is 10 seconds. 80

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4.2. MEASUREDAND PREDICTEDLEVELS Table 1 lists the L1o (18-hour) results measured at the various locations, and also includes the values corrected to give the value that would be observed 30 metres from the centre line of the motorway on the basis of inverse spreading from a line source, which allows the influence of the various structural configurations to be examined. One set of measurements taken at increasing distances from, and normal to, the motorway is plotted in Figure 5 showing the 3 dB attenuation per doubling of distance. This result differs from that suggested in the Department of the Environment Design Bulletin [7] of 4 dB per doubling of distance based on the free field measurements of Scholes [4]. It can be argued that the peaks of noise, which set the Llo values, are more likely to be controlled by individual or small groups of vehicles, which will act as point sources. However, the measurements presented here are for a reverberant situation, with concrete buildings, bridges, cutting walls and pedestrian walkways all producing reflections. Thus, even the highest sound levels, which set the Llo values, appear to originate from a line source, so that inverse spreading of the noise from the closest measurement positions outwards seems most applicable. This again emphasizes the continuous nature of the noise radiated from the urban motorway. The predicted Llo (18-hour) levels for e a c h measurement position, obtained by using the Department of the Environment's prediction method for free flowing traffic [7] and the measured traffic flow at an estimated mean traffic speed of 75 km/h, also are shown on Table 1. Here, a shielding allowance of at least 5 dB is TABLE 1

Results

Measurement position End of Argyle Street

Site description

Elevated with surface access road Telephone Company Level with access Car Park road traffic; access road traffic eliminated Avis-North Street Vertical cutting 3 m deep Mitchell Library-Vertical cutting 6 m North Street deep End of Carnarvon Street Deep cutting some distance away 36mup Carnarvon Street Built-up street Below Hill Street On and over a slope Mid slope Hill Street looking into deep End of Hill Street cutting 36 m up Hill Street Sauchiehall Street City street Prediction for flat open With reflection and site due to Motorway traffic at 75 km/h traffic [7]

Predicted" Measured Distance Llo (18-h) Llo (18-h) from r Measured due to corrected of M/way Llo (18-h) M]way to 30 m (m) (dB(A)) (dB(A)) (dB(A)) 72

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included for those locations where the motorway traffic was in a cutting or shielded from the observation position by an elevated roadway. The predicted sound level for an open hard surfaced location at 30 metres from the centre line of the motorway also is included in the table. (Note: for this study, we have preferred to work in terms of a reference distance of 30 metres from the centre line of the motorway as the varying width ofthe secondary roads makes it difficult to define a precise edge to the motorway. The predicted levels have been corrected to give values at similar distances.) A direct comparison of the measured results for open positions, such as the end of Hill Street and the Telephone Company Car Park, with the predicted level shows very good agreement, with the 30 metre Llo (18-hour) level being some 80 + 2 dB(A). The differences of the measured values from the predicted values and between the various measured results allows an evaluation of the influence of the physical parameters for each position to the radiated noise. 4.3.

I N F L U E N C E O F ACCESS A N D P A R A L L E L S U R F A C E R O A D S

Access and parallel surface roads are a necessary feature of an urban motorway designed to distribute traffic and serve the immediate neighbourhood. There are parallel surface roads throughout almost the whole length of the section studied, plus several access roads, and these secondary roads carry very heavy traffic flows. These roads can have a detrimental influence on the noise exposure, especially for pedestrians who will tend to be closer to these roads than to the motorway proper. Measurements at one location--the Telephone Company Car Park --were made next to the motorway with only a single access road between the motorway and the measuring position. The traffic on this access road was regulated by traffic lights and so a series of recordings were made, both with and without the access road traffic present. In the latter case, the measurement involved stopping the tape recorder when there was traffic on the access road. The results, as listed in Table 1, show that the access road traffic causes an increase of the L,o level by 3 dB. 4.4. SttlELDING The results for Argyle Street suggest that the 5 dB assumed for shielding from an elevated motorway to ground level may be rather optimistic, especially for locations some distance from the motorway. However, the measured 2 dB attenuation may be influenced by traffic on the surface roads underneath the motorway. Measurements obtained on North Street indicate the shielding effect of a vertically sided cutting, to positions immediately in front of the buildings lining the road system (Table 1).

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The cutting was at its minimum depth of 3 metres by the Avis Car Hire Office, and the sound levels measured here are typically 2 dB higher than those measured in front of the Mitchell Library, where the cutting is 6 metres deep. Although both measurements were affected by" the noise of traffic on the parallel surface road, the shielding effect o f the cutting on the sound levels near to the motorway at ground level is shown. The result for the noise at the end of Carnarvon Street taken at a greater distance from the motorway, which is still in a deep cutting and separated from the roadway by grassed strips and a low earth bank, show even lower sound levels, with a reduction of 11 dB over the predicted open site level.

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R. C. POTI'EK AND B. C. ROSS

(Figure 6). The sound levels are also shown on a corrected basis (inverse spreading) at the reference distance of 30 metres from the centre line of the motorway. They give a local shielding of 9 dB close to and at ground level by the motorway, and show how the noise is radiated upward. Estimated 70 and 75 dB(A) Llo (18-hour) contours are sketched on Figure 6, and are basically in agreement with the curves shown in the Department of the Environment Design Bulletin [7], other than that the 75 dB(A) contour appears to be spread out much further. This result suggests that the total sound energy radiated upwards is somewhat greater than previously supposed. The outline of a high rise building is also drawn on Figure 6 and the results suggest that there easily could be a 10 dB difference in sound levels experienced from the bottom to the top of a 60 metre tall building close to a motorway. A long time (2minute) spectrum average was completed for the recorded noise at three of these locations (Figure 7) and the results show a reduced level at frequencies above 200 Hz for the position closest to the motorway. There is apparently less shielding of the low frequency noise which "spills over" the edge of the vertically sided cutting, in comparison with the subjectively annoying (higher frequency) noise, which is attenuated well close to a deep cutting. The sound levels for the position over the brow of the hill are reduced right across the frequency range by the shielding of the slope, and the equal distance sound levels give the value of this attenuation as 2 dB.

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Measurements taken outside the Mitchell Library had already shown the high levels of noise for pedestrians, even though the motorway is in a cutting. Measurements taken during the afternoon on the first floor inside the building gave an Llo of 56 dB(A) (Figure 8). The sound level for the second floor was 3 dB greater than for the first floor, again showing how the upward radiated noise from the cutting affects the higher stories of buildings alongside the motorway. These results are for a public building of massive construction. The noise experienced within a dwelling of light brick construction with windows and doors, at a similar location next to an urban motorway, could be expected to be somewhat greater, and so a fair amount of insulation would be required. A single measurement taken some 36 metres up Carnarvon Street, a road running normal to the motorway and with buildings on both sides, shows no reduction in the sound level over the measurement at the end of the street, other than the inverse spreading which would be expected because of the increased separation. This result shows how the motorway noise easily penetrates into the surrounding areas.

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5. NOISE EXPOSURE AND CONCEPTS OF URBAN MOTORWAYS In any discussion of noise of urban motorways one must recognize the growing awareness of members of the public of their rights to consultation on any project which affects their environment. People are becoming more sophisticated in their reaction to major public works in their immediate neighbourhood. As well as the visual and physical aspects of a motorway, the influence of noise now is recognized as a major factor in producing an acceptable environment for living and working. The results reported here show that sound levels greater than 70 dB(A) L,o (18-hour) will be experienced immediately alongside any urban motorway, which is greater than the present "acceptable" levels designed to produce satisfactory noise exposure within a dwelling. The measurements show that this "acceptable" figure will not be achieved simply by separation, as the reduction in sound level with distance is small. Placing an urban motorway within a cutting will produce some immediate relief at ground level, but high sound levels still will be experienced at the upper stories of buildings fronting the motorway. Therefore acoustic consideration is required for all surfaces of a building close to and facing an urban motorway. Legislation has been introduced into Scotland, The Land Compensation (Scotland) Act, 1973, under which the highway authorities will be responsible for carrying out protective and remedial works on account ofroad traffic noise. Under discretionary powers, the acquisition of land for noise reduction and acoustic treatment to affected buildings could be necessary for future road developments. The specification of acceptable sound levels and the means and methods for providing the acoustic protection currently are being determined. The reduction of the noise exposure can be achieved in several ways. Increasing the distance between the urban motorway and the population generally will not be an economic or satisfactory solution. Placing the motorway in a less sensitive area, such as an industrial site, will reduce the number of people exposed directly to the noise, but this can really be relevant only for a through motorway, designed to pass traffic away from the city roads. If the purpose of the motorway is to distribute traffic to and from residential and business areas, then it must necessarily run through densely populated regions. In this case, it will be normal to provide access and parallel surface roads to aid this distribution, and the measurements show that the noise of these secondary roads can cause increased annoyance. Providing noise reduction features to existing and future buildings alongside an urban motorway will reduce the internal sound levels. However, the pedestrian still will be exposed to the noise, and this normally will not be an acceptable solution for residential and shopping areas. Putting the motorway and its secondary roads in deep cuttings will be beneficial, although it is still doubtful as to whether the noise will be reduced to acceptable levels for pedestrians close to the cutting. The total separation of people and traffic generally will be required to achieve the necessary noise reduction and this will involve massive structures or earthworks to produce any worthwhile influence. The influence of noise is now included in Glasgow Corporation's policy for the design and siting of all roadworks. The major part of the inner motorway ring road is through Comprehensive Development Areas, and so extra land has been included in the design to provide separation and shielding for noise reduction. The total motorway system is being designed on a corridor route concept, which allows for the altering of land use characteristics and provides protection from noise and visual intrusion as part of the overall planning considerations. The roadways are constructed in cuttings and retaining wall sections wherever possible, and the earth removed then is built into landscaped barriers, with tree belts and shrubs, to provide both acoustical and visual shielding. This approach has been followed since 1968, and the motorways presently under construction should show the benefits of such a design principle, which has been favourably reviewed [11].

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R . C . POTTER AND B. C. ROSS

6. CONCLUSIONS The measured sound levels of the Charing Cross Section of the Glasgow Urban Motorway system are in general agreement with the predicted values based on free flowing traffic. The character of the noise is more continuous with less amplitude fluctuations than has been reported for rural situations, due to the higher traffic densities, the smaller variations in traffic flow and the noise reflections from the works associated with the urban motorway. The sound levels immediately alongside the motorway are higher when surface and access roads are present. Although cuttings do provide some immediate shielding at ground level, the noise radiated upward would appear to be higher than previously supposed, with a resulting greater exposure to the upper stories o f nearby high rise buildings. The attention paid to noise aspects in the overall planning of the urban motorways of Glasgow, with the inclusion of adequate noise shielding, has been outlined. ACKNOWLEDGMENT The authors acknowledge the assistance and co-operation of the Glasgow Corporation, Mr J. Armour, C.Eng., F.I.C.E., F.I.Mun.E., F.R.T.P.I., Master of Works and City Engineer. REFERENCES 1. D. R. JOHNSONand E. G. SAUNDERS1968 Journal of Sound and Vibration 7, 287-309. The evaluation of noise from freely flowing road traffic. 2. I. D. GgtrrrrHs and F. J. LANGDON1968 Journal of Soundand Vibration 8, 16-32. Subjective response to road traffic noise. 3. W. E. SCHOLES1970 Applied Acoustics 3, 1-21. Traffic noise criteria. 4. W. E. SCHOLESand J. W. SAR~EYr 1971 Applied Acoustics 4, 203-234. Designing against noise from road traffic. 5. T. J. SCHOL'rZ1972 Community Noise Ratings. London: Applied Science Publishers Ltd. 6. D. W. ROBINSON1969 NationalPhysicalLaboratory Aero Report 39. An outline guide to criteria for the limitations of urban noise. 7. Department of tile Environment 1972 Design Bullethl 26, New Houshzg and Road Traffic Noise. London: Her Majesty's Stationery Office. 8. Command 2056 1963 Noise: [inal report of the committee on the problem of noise. London: Her Majesty's Stationery Office. 1971 reprinted. 9. J. E. WESLER1973 Journal of the Acoustical Society of America 54, 985-995. Community noise survey of Medford, Massachusetts. 10. C. DtJERDEN1974 Noise Controland Vibration Reduction 5, 117-123. The noise regulations 1973. 11. The Economist 1974 May 25, 92-93. The urban motorway mess.