TRL Noise Assessment of Concrete Trunk Roads opened since June 1988


House Of Commons Written Ministerial Statement

Executive Summary

1. Introduction

2. Background

3. Survey methods

4. Analysis and results

5. Summary

6. References


TRL Noise Assessment of Concrete Trunk Roads opened since June 1988



The Secretary of State for Transport (Mr Alistair Darling)

I am today announcing a range of improvements to the strategic road network that will tackle congestion, improve safety and reduce the impact of traffic noise.

Measures that minimise the effect of traffic on the people who live alongside roads is as important as trying to ensure that people can travel safely and reliably. For this reason we will be installing quieter surfacing on over 60% of the trunk road network, including all concrete stretches, within the period of the 10 year plan.

I am now able to announce the timetable for the resurfacing of all stretches of concrete roads and that within the next four years we will resurface the 26 stretches affecting the largest number of people. The remaining stretches will be resurfaced between 2007 and 2011.

The early work will benefit some 11,500 properties and cost in the order of £77 million. Details have been placed in the library, along with copies of the report by TRL Ltd that compares estimated noise levels with actual noise levels.

In the light of the recommendations of the Regional Planning Bodies, the Statutory Environmental Bodies, local authorities and other stakeholders I have asked the Highways Agency to add the following road schemes to the Targeted Programme of Improvements (TPI) at a cost of £637m:

These schemes will deliver a range of benefits including reductions in congestion and improvements in safety as well as relieving communities of heavy traffic. The improvement of the A14 also demonstrates our commitment to support the Government's plans for accelerated growth in the London-Stansted-Cambridge area, as set out in the Communities Plan.

I have asked the Highways Agency to take forward work on the following schemes with a view to TPI entry in the near future:

But I am not convinced that the proposal for a new motorway in open countryside between M6 junction 19 and the M56 is acceptable. I have also considered alternative improvements of M6 junction 20, but I do not believe that they represent value for money.

I have therefore asked the Highways Agency to work with local stakeholders to identify smaller-scale measures for the A556 in Cheshire.

Department for Transport
1 April 2003


Executive Summary


The Government today stepped up its commitment to tackling congestion, improving road safety and reducing noise levels on roads, with a package of new road improvements, announced by Transport Secretary Alistair Darling.

The balanced package of decisions focuses on some of the most congested points on the trunk road network and provides localised solutions to deal with congestion. It includes junction improvements to improve traffic flow and an upgrade of the main east-west road through Cambridgeshire. But the upgrade to the A556 was rejected because of its negative impact on the surrounding environment.

For the first time there is a timetable for the removal of all the concrete surfaces on the motorway and trunk road network. In the first phase alone, over 11,500 homes across the country will benefit from reduced traffic noise, and journeys on these roads will be quieter for motorists too.

Alistair Darling announced:

Alistair Darling said:

"Today's announcement is a further step in our plans to deal with the most congested points on the road network. We remain committed to tackling congestion and reducing the number of accidents on our roads. This package will deliver those benefits and provide welcome relief from heavy traffic for communities who live near busy roads.

"We now have a programme to remove concrete surfaces from the trunk road network that will benefit many thousands of households across the country. We have prioritised the re-surfacing so that the worst affected homes are first. In the first phase to be resurfaced over 11,500 homes will have the blight of traffic noise reduced.

"The improvement of the A14 also demonstrates our commitment to supporting the Government's plans for accelerated growth in the London-Stansted-Cambridge area, as set out in the Communities Plan. Improving transport links is a great incentive for economic development and I am sure that the East of England will benefit from this upgrade to a vital route."

Notes to Editors

  1. Today's announcement amounts to a £755m package of improvement.
  2. The then Parliamentary Under Secretary of State, Department of the Environment, Transport and the Regions, announced in 1998 the entry of a number of road schemes into the Government's Targeted Programme of road Improvements (TPI), and the setting up of the multi-modal and road-based studies on the most congested parts of the trunk roads network. At the same time he announced that a further seven schemes were not sufficiently advanced to be considered for the TPI but which the Government wished to be progressed by the Highways Agency and, if endorsed by the relevant Regional Planning Bodies would be taken forward as quickly as possible, subject only to the availability of funds.
  3. This announcement today concerns the final two of the "to be progressed" schemes - A57/A628 Mottram in Longdendale, Hollingworth and Tintwistle bypass, and the A45/A46 Tollbar End Improvement and the final three of the road-based studies - M40/A46 Longbridge Roundabout, A5/A483 Shrewsbury to Chester and A38 Derby Junctions. It also concerns a trunk road scheme which was recommended by the Cambridge to Huntingdon Multi-modal study, the dualling to three lanes of the A14 between Ellington and Fen Ditton (CHUMMS).
  4. Mitigating the impact of traffic on the people who live alongside roads is as important as trying to ensure that people can travel safely and reliably. For this reason we will be installing quieter surfacing on over 60% of the trunk road network, including all concrete stretches, within the period of the 10 year plan period.
  5. The Communities Plan was published on 5th February 2003 by the Office of the Deputy Prime Minister. It sets out a long-term programme of action for building successful, thriving and inclusive communities in all regions. Central to the Plan are proposals to deliver an increase in housing provision through indentified growth areas in Thames Gateway, London-Stansted-Cambridge, Ashford and Milton Keynes - South Midlands. These areas are being developed by the relevant local and regional partners in co-operation with Government Departments and Agencies.
    Schemes where the resurfacing will benefit more than 100 properties per km or, where the road was opened after June 1988, the current noise level is 3dB(A) greater than predicted. This is equivalent to the noise increase expected from a doubling of the volume of traffic on a road.
Concrete Road Section Description Concrete Section Length (km) Affected Properties per km
M1 - A1 Link 15.5 8
M1 Junctions 5 - 8 8.0 234
M6 Junction 12 - Penkridge 5.3 126
M20 Junction 1 / M25 J3 1.1 138
M20 Junctions 8-9 20.4 14
M25 Junctions 9 - 10 7.8 227
M27 Junctions 5-7 5.7 145
M60 Junctions 1-2 1.7 277
A5 Milton Keynes (A422-A508) 6.3 163
A11: B1172 to A47 13.7 5
A12 Ingateston - Margaretting 4.4 102
A12 Hatfield Peverel - Witham 5.2 117
A13: A1306 to M25 J30 3.0 273
A14 Claydon - B1067 underbridge 3.0 118
A27 Chichester to Havant 13.8 70
A30 Honiton to Exeter 16.2 4
A34 Peartree - Weston 8.0 11
A47 East Dereham: (Scarning Fen - East Dereham) 3.3 142
A50: Doveridge Bypass 4.0 12
A419/A417 Cirencester & Latton Bypasses 14.2 8
A1 Balderton - Coddington * 3.0 37
A1 North Muskham - Carlton on Trent * 5.7 12
M5 Junction 26-27 Contract 3 * 4.2 5
M42 Junctions 9 - 10 * 5.1 28
M69 Junction 2 - M1 Contract 1 * 4.8 8
M62 Junctions 37-38 9.1 2

Note: Those schemes indicated with an asterisk are included in this table because they are planned for delivery in 2004/05 due to normal maintenance needs. A1 North Muskham - Carlton on Trent is planned to start in 2004/05 and complete in 2005/06.


Concrete Road Section Description Concrete Section Length (km) Affected Properties per km
M6 Penkridge - Jct 13* 3.8 17
M11 Jct 8 - B1038 * 12.1 19
M18 Junctions 5 - 6 2.3 2
M20 Junction 3 - A20 West Kingsdown* 6.6 30
M25 Junctions 10 -11 9.2 26
M25 Junctions 24-25* 8.5 50
M25 Junctions 8-9 7.2 14
M42 Junctions 3a-4 3.5 20
M54: A5 - A41 9.3 22
M271 Junction 1 to A3057 2.8 22
A1 (M) J1 (South Mimms)* 1.9 12
A11 - Barton Mills* 2.8 13
A12 Contract 11: Kelvedon Bypass-Eight Ash Green* 7.7 28
A12 Margaretting - Boreham 13.4 19
A14 B1067 underbridge - Copdock Interchange 3.3 98
A14 Copdock Interchange - A12 8.0 21
A19 Billingham Bypass* 4.2 65
A30 Longrock Bypass 1.8 38
A35 Tolpuddle - Puddletown 7.2 2
A43 Towcester* 3.3 43
A50 Derby Southern Bypass* 0.5 51
A52 Bottesford Bypass 4.8 7
A120 Elmstead - Wix Eastbound 8.0 14
A168/A19 Thirsk Bypass* 8.0 41
A249: M2 - Iwade 7.9 76
A483: B5445 - Welsh Boundary 3.1 0
M3 Junctions 8-9* 9.0 3
M4 Junction 10 slip roads* 0.5 0
M11 Junctions 5-6* 4.7 6
M18 Moorends - Greenland* 4.9 1
M23 Junction 10 - 9 NB 1.3 1
M25 Junctions 27-28* 5.6 3
M26 Junction 2a - M20* 0.8 7
M53 Junctions 11-12 (A56-M56)* 3.9 9
M54: A41 - A449 12.9 7
M56 Junction 15 (M53) - A5117* 3.0 1
A14 A12 - Levington 3.7 1
A14 Elmswell 3.6 5
A30 Whiddon Down to Okehampton 6.9 1
A46 Six Hills - Widmerpool* 9.5 0
A47 East Dereham: Wendling - Scarning Fen 4.3 7
A50 Blythe Bridge to Uttoxeter* 12.4 6
A50: Foston Hatton Hilton Bypass* 3.6 9
A120: A12 - Elmstead 4.5 6
A160: A1077- A180 4.4 0
A180: Habrough to Grimsby* 11.5 9
A180: M180 - Ulceby* 8.0 1
A303 Ilminster Bypass* 8.8 2

Note: Those schemes indicated with an asterisk may be progressed earlier for road safety reasons, as part of routine maintenance, dependent on the results of further surveys.

In addition, the sites below will have noise mitigation work carried out in 2003/04. Resurfacing work is being carried out primarily for road safety reasons, as part of routine maintenance.

Site Type of Work
M1 Junctions 1-2 Resurfacing Concrete Road
M1 J10 - 11, Luton Noise Barrier
M1 near J28 Noise Barrier
M4 J5-6 Datchet, Brands Hill Noise Barrier
M5 Junctions 26 to 27 Contract 2 Resurfacing Concrete Road
M5 Junction 27 south to Willand Resurfacing Concrete Road
M20 Junctions 11a-11 Resurfacing Concrete Road
M20 Junctions 13-11a Resurfacing Concrete Road
M25 Junctions 26-27 Resurfacing Concrete Road
M42 Junctions 2 - 3a Resurfacing Concrete Road
M50 Bromsberrow Heath Noise Barrier
M180 Sandtoft - Junction 2 Resurfacing Concrete Road
A1 Winthorpe - Coddington Resurfacing Concrete Road
A12 Witham - Kelvedon Resurfacing Concrete Road
A36: M27 rdbt to Circuit Road rdbt Resurfacing Concrete Road

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Executive Summary

1. Introduction

The Government's strategic plan for transport, "Transport 2010: The 10 Year Plan" was published in July 2000. As part of this plan, the Highways Agency (the Agency) was set the target of installing quieter road surfaces on over 60% of the trunk road network, including all concrete stretches, by 2010. The Agency needed to establish criteria for deciding priorities for programming surfacing works on the concrete stretches, which in many cases would not normally need to be resurfaced within the plan period.

Having consulted with English local highway authorities and other interested bodies, in October 2001 the Government announced four criteria for prioritising the surfacing of concrete trunk roads with quieter materials. The criteria are:

  1. that wherever possible the application of quieter surfaces will fit in with normal maintenance needs;
  2. that priority will be given to those sites where treatment would benefit the greatest number of people;
  3. that the works will be carried out in such a way as to minimise disruption to general public and users of network;
  4. that priority will be given to roads, opened since June 1988, where actual noise levels have turned out to be significantly higher than predicted at the time of the Public Inquiry.

A list of 17 roads containing concrete sections were identified that needed to be assessed under the fourth criterion. The Agency commissioned TRL Limited to carry out measurements to establish relative noise levels for each of the concrete sections on the list. This report describes the rationale underpinning the method adopted, the sites selected, the measurements undertaken and the results obtained.

1. Introduction

2. Background

Since 1973 the adverse noise effects of road proposals on the surrounding environment and possible ways of minimising these effects have been more comprehensively taken into consideration before the road is built. The basis of assessing the effects of traffic noise was established from the outset, although the methodology has been improved in succeeding years. Because it is necessary to predict the effects of a scheme long before it is put into practice, assumptions have to be made about a number of factors that influence noise levels. In order to simplify the prediction process, the effects of most of these factors have been incorporated into statistical relationships established by measurements of traffic noise under different conditions.

The factors that influence noise levels at locations some distance from a road can be split into two groups: those related to the source; and those related to the propagation path between the source and the receiver. The source-related terms are dominated by the volume and the speed of traffic, together with the proportion of lorries. In some cases the road gradient becomes significant and, since 1988, greater recognition has been given to the influence of the road surface.

Noise sources related to the engine, exhaust etc, are important in towns where traffic often travels at a fairly low speed, and also on long gradients for lorries. When the traffic is travelling faster than 50 km/h, the interaction of the tyres with the road surface becomes the dominant source of noise. Consequently most of the noise affecting communities located alongside high-speed roads arises from the tyre/road surface interaction.

2.1 Noise Predictions

The standard method for calculating road traffic noise in the UK is that contained in the Department of Transport's Technical Memorandum, 'Calculation of Road Traffic Noise' (CRTN). The method, initially developed in the early 70's, was first published in association with the Noise Insulation Regulations in 1973. Revised versions were published in 1975 and 1988 (Department of Transport and Welsh Office, 1988).

The Memorandum is the statutory method for assessing entitlement to insulation of dwellings as prescribed by the Noise Insulation Regulations. CRTN is also widely used to assess the noise impact of highway schemes, including the benefits of any mitigation measures. It also provides data for inclusion in Environmental Assessments when comparing options for new road schemes and presenting the effect of the chosen scheme in the Environmental Statement.

The 1988 revision of CRTN provided formulae that allowed an adjustment to be made for the road surface characteristics. However, these in general can only be applied after the road has been built and the necessary parameters measured. At the planning stage and at Public Inquiry, past practice has been to quote noise levels assuming an average form of surface (ie with no adjustment).

2.2 Roadside Noise Measurements

2.2.1 Traffic noise

Direct measurements of overall traffic noise can be undertaken at the roadside. However, this method does not properly separate the influence of the road surface from the characteristics of the traffic that happens to be passing and so it is not really appropriate to compare sites in this way to meet the needs of this study. Overall levels of traffic noise are influenced by the topography of the road and surrounding area, meteorological conditions, traffic flow, composition and operational characteristics such as traffic speed. Consequently in order to compare the influence of the surface characteristics of different roads, any inter-site variations due to differences in traffic characteristics have to be removed by a process of normalisation. This process is very protracted, requiring many factors to be measured and analysed before different road surfaces can be compared.

2.2.2 Vehicle pass by noise

The preferred method of measuring the influence of the road surface on noise generation is detailed in ISO 11819-1. This standard describes a procedure known as the Statistical Pass-By (SPB) method (International Organisation for Standardisation, 1997). The SPB method was originally developed by the Transport and Road Research Laboratory (now TRL Limited) in the 1970's and now, by virtue of an international standard, it is used in many countries for road surface noise assessment purposes.

The main advantage of this application of the SPB method is that it can provide a measure of the influence of the road surface on traffic noise at the roadside. The statistical averaging carried out, and the standardisation of site and measurement conditions ensures that the influence of variations in the vehicles within different classes, and the volume, speed and composition of traffic are minimised. This exposes the influence of the road surface as the main dependent variable.

A disadvantage of the SPB method is that it requires strict site criteria to be met before measurements can be conducted and it can often be difficult to locate measurement sites along a given road. For example, the procedure requires a 100m length of homogenous, straight level road and a 25m radial area around the receiver microphone that is free from any large acoustically reflecting objects such as building facades, fences, embankments. Measurements must also be taken with the road surface in a dry condition and when various meteorological conditions are met.

TRL have derived a modification to the basic SPB method by incorporating a known reflective surface located directly behind the measurement microphone, which overcomes some of the site layout restrictions. The technique, known as the backing board technique, enables SPB measurements to be taken at locations where there are large reflective objects within 25 metres, provided these are behind the microphone.

2.2.3 Assessment of road surface noise in the UK

The SPB method has been incorporated into the noise test provided within the Highway Authorities Product Approval Scheme (HAPAS) for the approval or certification of road surfacing products for use on public roads in the UK (British Board of Agrément, 2000). The HAPAS procedure combines the results of SPB measurements into the expected level of noise arising from a typical trunk road or alternatively a specified class of local road. From this value is subtracted the noise level that the standard noise calculation produces for the same traffic flow assuming the road had an average conventional surface. This difference is called the Road Surface Influence and is essentially the adjustment that needs to be applied to any noise levels calculated by CRTN to account for the use of the chosen surface. The HAPAS procedure is described more fully in section 2.4

2.3 Measurements of Tyre Noise

The SPB method provides measurements of tyre/surface noise influence only at specific points located alongside a road. Unless a very large number of measurements are taken, the method cannot provide an indication of how the noise might vary along a road section. In addition to the limitations imposed by the stringent site selection criteria, such a measurement programme would clearly be protracted and expensive. An alternative approach involves noise level measurements with microphones located close to dedicated test tyres mounted on a vehicle or trailer. The method, currently being standardised by ISO, is known as the close proximity (CPX) method.

The most significant benefits of this method are that measurements can be taken continuously along a whole road length. In principle this overcomes some of the site restrictions associated with the SPB method as well as providing a way of assessing the variation in noise levels along the length of a road. However, the results obtained from measurements taken using the CPX method cannot at present be simply related to traffic noise levels although work is underway to provide this link. The main difficulties are associated with the fact that the results are generally only applicable to the limited number of test tyres used, which may not necessarily be representative of the range of tyres in common use. The CPX method, therefore, should currently be seen as a means of supplementing the standard roadside SPB measurements. The main purpose of using CPX measurements for this study is that they can be used to indicate whether the sites chosen for SPB measurements are representative of the road surface over the whole of the section of interest.

TRL has developed a fully integrated truck based CPX test vehicle known as TRITON to enable CPX measurements to be safely carried out without the need to close road lanes to normal traffic. Further details of TRITON and the use of the CPX method in this study are described in Section 3.3.

2.4 The HAPAS Road Surface Influence Parameter

The HAPAS noise test procedure follows a slightly modified version of the ISO statistical pass-by method described earlier. The main difference is that HAPAS requires measurements to be taken using the SPB method for three categories of vehicles rather than the two specified in the ISO standard. The three categories are light vehicles, two axle trucks and multi-axle trucks. Dependent on the class of road, different reference speeds are used to calculate the noise level for each vehicle category. With weightings applied to represent standard traffic composition, these results are combined into a single ranking of noise emission for given traffic conditions and can be compared with the noise level predicted for the average surface assumed in CRTN.

The ranking for a particular surface is known as the Road Surface Influence (RSI). All the roads included in this measurement programme are defined as high-speed, and the following equation is used to calculate the RSIH level:


where Lveh,L is the normalised level for light vehicles at 110 km/h, Lveh,Ta the normalised level for two axle trucks at 90 km/h and Lveh,Tb the normalised level for multi axle trucks at 90 km/h. The weightings for each vehicle category are derived from the average traffic statistics for motorways and trunk roads in the United Kingdom.

2. Background

3. Survey methods

Given the information contained in the previous section the overall method of approach used in this study was determined. It was clear that in order to provide accurate assessments of current noise levels for the identified road lengths, it was necessary to use the SPB method at the sites identified alongside each road length. Direct measurements of traffic noise (for example at property facades), for the reasons stated earlier, would not be an appropriate measurement method when the aim was to provide the Agency with a level for each road length that could be compared and used for prioritisation for resurfacing purposes. The SPB method is the most appropriate procedure for determining and comparing the noise performance of road surfaces. In addition, it was decided to use the HAPAS formulation to provide a measure of relative traffic noise levels for each road length that could be related to CRTN 88 predicted levels.

The work programme was divided into three main phases. The first phase concentrated on establishing sites along each road where roadside SPB measurements could be conducted. The second phase involved taking the SPB measurements at each road site and the third phase involved the measurement of the variation in noise along the whole length of each road section using the CPX technique.

3.1 Site Selection

Sections of trunk road identified for review according to the fourth criterion were as follows:

Before the measurement programme could begin, each road section was inspected to determine suitable sites for the SPB measurements to be taken. This involved carrying out detailed surveys along each road section noting possible sites and their GPS reference. As part of this process the Agency Route Manager was contacted along with the Agent/DBFO company managing the road on behalf of the Agency. Any issues regarding health and safety and traffic management were discussed at this point and taken into consideration when choosing possible SPB measurement sites.

The acoustical requirements of the test site for an SPB are given in ISO 11819-1. Briefly, the road must be straight and level, with over fifty per cent of the ground cover between the microphone and the test lane having similar acoustic properties to the surface being tested. There must also be no large reflecting objects, such as solid safety barriers or embankments, within a range likely to cause reflections. Given these basic requirements over 100 sites were originally identified as being suitable in terms of either meeting the requirements of the ISO Standard directly or could be adapted using the Backing Board technique (See section 3.2.2).

In order to arrive at the final selection of sites, consideration was given to the condition of the surface and professional judgement was made as to whether the texture at the selected sites was likely to be representative of the road as a whole. Sites were rejected where the texture was clearly not uniform in the proximity of the potential SPB site or where there were obvious defects such as patching or discontinuities in the surface profile. Following consideration of the road surface condition the number of selected sites was reduced to 67 with at least two SPB sites located alongside each of the 17 road lengths.

The sites selected for the study were generally located at lay-bys, slip roads or on the hard shoulder. At a lay-by site, the hard standing of the slip road into the lay-by was used to satisfy the requirement in the ISO standard that the intervening ground between the microphone and test lane must be similar to that being tested. Typically for this type of site the vehicle housing the measurement equipment would be parked at the start of the parking area in the lay-by. At a slip road site, the hard standing of the slip road off is used to meet the requirements of the intervening ground. Traffic management is required at this type of site to ensure the safety of the operators. On a motorway site, the hard shoulder was often suitable to provide the hard standing required between the microphone and measurement lane. As with a slip road site, traffic management is required for the protection of the measurement team and is carefully implemented to avoid any influence on passing traffic.

3.2 Roadside Measurements

Roadside noise measurements were taken at each of the 67 sites identified during the site selection phase. The measurements were nearly all taken using the procedure described in the ISO standard. The TRL Backing Board technique was used in the remaining few cases where site conditions were difficult. Details of these procedures are given below.

It should be noted that all SPB measurements used for the analysis were taken when the road was dry and with wind speed less than 5 m/s. Air and road surface temperatures were monitored and for all the measurements taken were found to be within the ranges specified in the ISO standard.

3.2.1 Standard SPB Measurements

As mentioned earlier the SPB method has been adopted by the International Organisation for Standardisation as the preferred method for comparing noise emission levels from vehicles travelling on different road surfaces (International Organisation for Standardisation, 1997). The method is defined in ISO 11819-1 'Method for measuring the influence of road surfaces on traffic noise- Part 1: The Statistical Pass-by method'. The SPB method is also used in the Highways Authorities Products Approval Scheme (HAPAS) Guidelines document for the assessment and certification of thin wearing course systems for highways (British Board of Agrément, 2000).

The SPB method requires the simultaneous measurement of the maximum A-weighted (A-weighting gives the noise measuring instrument a frequency response approximately equivalent to that of the human ear.) noise level and speed of individual vehicles, which form a representative sample of the traffic travelling on the carriageway. These measurements are taken at a standard distance of 7.5m from the centre of the nearside running lane and the microphone at a height of 1.2m above the level of road surface. During the measurement, the operator selects vehicles that are sufficiently separated from the rest of the traffic stream so that their noise and speed characteristics are not influenced by other vehicles and therefore can be easily isolated and measured. A typical site layout for an SPB measurement is shown in Figure 1 below.

Figure 1. Typical SPB setup (M1-A1 Link, site 2)

Typical SPB setup

As can be seen from the photographs, the microphone is sufficiently far away from any large reflecting objects. The noise analyser and speed measuring equipment is in the surveying vehicle that can be seen parked some distance further along the road.

For the purpose of this study and to conform to the requirements of HAPAS, the vehicles measured at each of the SPB sites were grouped into three categories defined as:

The data set collected for each vehicle group was then used as input to linear regression analysis where, for each vehicle category, the maximum pass-by noise levels are regressed against the logarithm of vehicle speed. The regression lines obtained for each vehicle category were then used to determine the normalised noise levels at the reference speed for the road as defined in the HAPAS guidelines. These reference speeds for a high-speed road are 110 km/h for light vehicles (L), and 90 km/h for the two classifications of heavy vehicle (Ta and Tb).

3.2.2 Backing Board SPB measurements

At a few locations, it was not possible to identify sites that conformed exactly to the strict requirements of the ISO standard. At these sites a modification to the standard SPB measurement method was employed. This method was developed by TRL and is referred to as the SPB Backing Board (SPB-BB) method (Kollamthodi, Phillips and Balsom, 2000). The SPB-BB is very similar to the standard SPB method but employs a recognised acoustic technique to effectively standardise a site when the required site layout conditions cannot be achieved. The technique uses the same sampling and measurement procedure as a conventional SPB measurement but uses a microphone positioned at the closer distance of 5.0m from the centre of the test lane rather than the standard 7.5m position. In order to achieve the controlled acoustic environment needed, the microphone is positioned directly in front of an acoustically reflecting board that is erected at the site. The presence of the board effectively eliminates unwanted reflections from roadside features. Reflections from the board increase the noise levels recorded by a substantial amount, but this is consistent under controlled conditions and an established correction factor can be applied. A typical backing board set up is shown in Figure 3.2.

Figure 3.2. Set-up for measuring noise levels using the Backing Board technique (M18, site 1)

Set-up for measuring noise levels using the Backing Board technique

The value of the correction factor needed to convert noise levels obtained using the Backing Board method to a conventional SPB method at the 7.5m position is 8.1 dB(A) (Kollamthodi, Phillips and Balsom, 2000). This value is subtracted from the SPB values for each vehicle category obtained using the backing board to determine the corresponding standard SPB values.

3.3 Longitudinal (CPX) Measurements

Close proximity (CPX) noise measurements were made to assess the variation in the noise levels along each road section. This would then be used to determine whether the sites chosen for the SPB measurements were reasonably representative of the road section. As described above, the SPB method requires very strict topographical conditions and can only measure the noise levels associated with a relatively short length of road.

In order to carry out CPX noise measurements the TRITON tyre/road noise test vehicle shown in Figure 3.3 was used. This vehicle is capable of taking highly repeatable noise measurements very close to a test tyre and the road surface, thus eliminating any influence from background noise.

Figure 3.3. The TRITON CPX measurement vehicle and tyre / microphone position

The TRITON CPX measurement vehicle and tyre / microphone position

An enclosure on TRITON houses a test wheel/tyre, suspension system and microphone array located in close proximity to the test tyre. The enclosure is designed to attenuate noise generated by the power-train and the tyres of the vehicle. To minimise sound reflection in the enclosure the inner faces are lined with sound absorbing foam. The suspension components within the enclosure are treated with absorptive foam where necessary. A full description of the TRITON test vehicle is given in Phillips, Kollamthodi and Morgan, 2001.

Seven microphones are arranged around the test tyre. During a measurement the tyre is loaded onto the road surface and the microphones record the noise from the tyre. Specially written software controls the data acquisition providing averaged noise levels for every 20m segment of road covered.

CPX measurements are taken in the nearside wheel track, and in addition to measuring the noise level, TRITON is also equipped to record tyre inflation pressure and temperature, air temperature and road surface temperature.

For the purpose of this study TRITON was operated using the 'Survey method' specified in the draft ISO/CD 11819-2 "The Close-Proximity Method" (International Organisation for Standards, Draft 2000). The draft standard specifies two standard tyre types to be used when performing CPX measurements. One of these is designed to have the characteristics of a tyre from passenger cars while the other has characteristics more representative of tyres from heavy goods vehicles. Two test runs at 80 km/h were performed using each tyre and average sound levels determined for continuous 20m segments covering the total length of each road. The results for each segment and for each tyre were first normalised to the target speed of 80km/h using the procedure set out in the draft standard. The normalised results for each segment were then averaged for both test runs and for the two test tyres. Finally, 0.5 dB(A) was added to the resulting values for each segment in order to bring the results into line with that of the more complex "Investigatory method" specified in the draft standard. This procedure requires testing using four different tyres and is a more complex and lengthy approach to determining the tyre/road sound level. This was considered to be an unnecessary refinement for the purpose of this study.

It should be noted that where any individual results are quoted, as opposed to an average for a road, these are in terms of a 100m average (i.e. the average of five 20m segments). This is the standard way for reporting CPXI results where a road section is being investigated.

3. Survey methods

4. Analysis and results

4.1 Results of SPB Measurements

The results of the SPB measurements are shown in the Table 4.1 below.

These are listed by road name and the number of SPB sites included in each road is indicated. The noise levels and RSI values shown are the average from all the SPB sites on that particular road. Of the 67 SPB measurements conducted, one site on the M20 was excluded from the analysis due to the average wind speed during the measurements being outside the permitted level.


Table 4.1. Average noise levels by road
Road No. of SPB sites Lights, 110km/h (dB(A)) Ta, 90km/h (dB(A)) Tb, 90km/h (dB(A)) RSIH
M18: Junction 5 - 6* 2 85.8 89.1 92.5 1.3
M20: Junction 9 - 8 8 88.1 90.1 93.3 3.0
M23: Junction 10 - 9 Northbound* 2 84.4 87.3 89.5 -0.7
M1-A1 Link 4 90.7 91.7 94.7 5.2
A11: Besthorpe - Wymondham Bypass 5 88.5 89.9 92.8 3.1
A13: A1306 to M25 J30 2 85.9 87.7 90.4 0.6
A27: Chichester - Havant 6 90.1 91.3 94.3 4.6
A30: Honiton - Exeter 6 89.2 90.2 93.1 3.7
A34: Peartree-Weston 4 91.2 92.2 95.4 5.7
A35: Tolpuddle / Puddleton 4 88.1 89.3 92.2 2.6
A50: Doveridge Bypass 3 89.9 91.5 94.5 4.5
A50: Derby Southern Bypass 2 85.7 88.9 91.9 1.0
A50: Foston - Hatton - Hilton Bypass 3 85.1 86.8 88.9 -0.4
A52: Bottesford Bypass 2 87.2 88.4 91.1 1.7
A249: M2 - Iwade 4 86.8 88.9 91.8 1.6
A419/A417: Cirencester & Latton Bypasses 6 90.5 91.1 94.5 5.0
A483: B5445 to Welsh Boundary 3 88.0 89.8 92.4 2.6

* Corrected for use with the Backing Board

The SPB measurements performed on the M18 and M23 were completed using the backing board method. On the M18 the use of the backing board was necessary since a safety fence was installed alongside the hard shoulder for the entire length of the concrete section which precluded standard measurements being conducted. The setup was as shown in Figure 3.2.

The SPB-BB method was also used on the M23, in an attempt to lessen the influence of aircraft noise from Gatwick Airport. From previous work on this section of road, it was known that noise levels associated with aircraft movements largely prevented ambient noise levels from rising or falling 6dB(A) between vehicle pass-bys as required by the ISO Standard. Using the backing board the vehicle noise levels are 8.1dB(A) higher than when a normal SPB is performed. It was hoped that this increase in the recorded pass-by levels would ensure the passing vehicles are by far the dominant noise source and so would reduce the influence of any noise from aircraft. While this approach did not entirely eliminate the problem of aircraft noise, it enabled the measurements to be taken with a greater degree of accuracy and confidence.

In Appendix A, the results from the individual SPB sites are shown. More detailed results and statistics for each vehicle category are shown in Appendix B. This shows the statistics from the linear regression between noise level and the logarithm of vehicle speed, along with the number of vehicles sampled.

All of the roads in the survey, including the A52 that was a single carriageway road, were subject to the national speed limit and can therefore be classified as high speed roads for the purposes of HAPAS and the ISO standard 11819-1. However, both documents also require that the average speed measured for each vehicle category be within a set range of the reference speed used for the calculations. The ISO standard, to which all measurements were performed, states that the reference speed for heavy vehicles (85 km/h) shall be within the range of plus-or-minus one standard deviation from the actually measured speed. For light vehicles the reference speed is 110 km/h and the measured average speed must fall within plus-or-minus one-and-a-half standard deviation of this. As can be seen from the tables in the Appendix, all the SPB measurements meet these two criteria.

The HAPAS guidelines that are used for road surface classification purposes have a tighter restriction on speeds. This states that the average measured speed for all three categories (Light, Ta and Tb) must be within plus-or-minus one standard deviation of the reference speed 110 km/h for light vehicles and 90 km/h for both heavy vehicle categories. From the tables shown in the Appendix it can be seen that not all roads meet with this criteria. However, these guidelines are in place to ensure that the approval of a new surface is tightly controlled. For the purpose of this study, the RSI values calculated from the HAPAS guidelines are being used solely as a way to combine the results from each vehicle category to provide one figure, which can than be used as a ranking tool. Since all the SPB sites conform with the requirements of the ISO standard, it is considered acceptable to use the RSI ranking despite some of the measurements not quite matching the speed requirements specified in the HAPAS guidelines.

Appendix C lists the meteorological and other site conditions from each SPB site during the measurements. As shown, the average wind speed at site 4 on the M20 was above 5m/s and therefore the noise data taken at this site has been excluded from further analysis.

4.2 Results of CPX Measurements

Measurements were taken to determine how representative the levels measured at the SPB sites were of the entire road section. These measurements were made using the Close-Proximity method (CPX). The results from the CPX noise measurements taken using the TRITON vehicle are shown in Table 4.2 below. The CPXI (Close-Proximity Index) value at a particular SPB site is calculated from an average of the 20m segment level with the SPB site and then the adjacent two points on each side of this point (i.e. a 100m average centred on the SPB site). These values are then averaged with the other SPB sites on that road. The table also shows the number of standard deviations that the overall average CPXI for each road differs from the average for that at the SPB sites. This can be regarded as a measure of how representative the SPB sites were of the whole road length.

Table 4.2. CPXI noise levels by road
Road CPXI for road dB(A) CPXI at SPB sites, dB(A) Difference* (std. dev.)
M18: Junction 5 - 6 101.5 101.3 0.5
M20: Junction 9 - 8 102.1 102.4 0.4
M23: Junction 10 - 9 Northbound 100.4 100.5 0.4
M1-A1 Link 102.5 102.5 0.0
A11: Besthorpe - Wymondham Bypass 101.5 102.2 0.8
A13: A1306 to M25 J30 100.9 101.0 0.2
A27: Chichester - Havant 102.7 102.8 0.2
A30: Honiton - Exeter 102.1 102.1 0.1
A34: Peartree-Weston 103.6 103.7 0.1
A35: Tolpuddle / Puddleton 101.4 102.0 1.1
A50: Doveridge Bypass 102.3 102.1 0.3
A50: Derby Southern Bypass 99.7 100.0 0.4
A50: Foston - Hatton - Hilton Bypass 99.7 99.9 0.6
A52: Bottesford Bypass 102.4 102.4 0.1
A249: M2 - Iwade 100.4 100.6 0.3
A419/A417: Cirencester & Latton Bypasses 102.4 102.4 0.1
A483: B5445 to Welsh Boundary 101.9 101.7 0.3

* This is the difference, in terms of standard deviations, between the CPXI value measured at the SPB sites and the CPXI value for the entire road.

The selection of SPB sites was often restricted due to the need for each site to conform to the strict ISO requirements. On four roads, the SPB measurements were carried out at sites indicated from the CPX measurements to have lower than average noise levels for that road. However, in all of these cases the measurements were within one standard deviation of the overall average. Along the remaining 13 roads the SPB measurements were conducted at positions with a CPXI level higher than the average for the road, with all but one (the A35) being less than one standard deviation.

During the TRITON measurements on the M18, lane one on the southbound side was affected by slowing and stationary traffic approaching a set of roadworks on the nearby slip road. It was therefore not possible to operate the TRITON vehicle at the test speed along this lane. The results from the TRITON survey quoted therefore only refer to the northbound side of the M18.

An example of a CPXI trace is shown below in Figure 4.1. Ideally the SPB sites would be located where the CPXI traces indicate as being representative of the entire road and/or be evenly spaced along the road length. However, as mentioned earlier, it is not always possible to find a site for an SPB measurement at any point along a given road. This example, the A50: Foston - Hatton - Hilton Bypass, shows three SPB sites located along the road. In this case the sites were all located at lay-bys.

Figure 4.1. Example of CPXI trace

View larger image


4. Analysis and results

5. Summary

A program of noise measurements has been completed on seventeen lengths of concrete road. These seventeen were those listed under criteria four of the Government's announcement on resurfacing of concrete trunk roads.

A total of 66 Statistical Pass-By (SPB) measurements have been taken to the standard required by ISO 11819-1 and RSI values determined for each section from the results obtained.

The TRITON (CPX) measurements showed the SPB measurements to have been taken at fairly representative locations along each road. Except for one road section, the SPB measurements were taken at places where the CPXI value is within plus or minus one standard deviation of the average for the whole section.

Table 5.1 below lists the 17 road sections, the section length and the overall RSI for that road.

Table 5.1. Average RSIH and length for each road

Road Road length (km) RSIH
M18: Junction 5 - 6 2.3 1.3*
M20: Junction 9 - 8 20.4 3.0
M23: Junction 10 - 9 Northbound 1.3 -0.7*
M1-A1 Link 15.5 5.2
A11: Besthorpe - Wymondham Bypass 13.7 3.1
A13: A1306 to M25 J30 3.0 0.6
A27: Chichester - Havant 13.8 4.6
A30: Honiton - Exeter 16.2 3.7
A34: Peartree-Weston 8.0 5.7
A35: Tolpuddle / Puddleton 7.2 2.6
A50: Doveridge Bypass 4.0 4.5
A50: Derby Southern Bypass 0.5 1.0
A50: Foston - Hatton - Hilton Bypass 3.6 -0.4
A52: Bottesford Bypass 4.8 1.7
A249: M2 - Iwade 7.9 1.6
A419/A417: Cirencester & Latton Bypasses 14.2 5.0
A483: B5445 to Welsh Boundary 3.1 2.6

* Corrected for use with the Backing Board

5. Summary

6. References

British Board of Agrément (2000).
Guidelines document for the assessment and certification of thin surfacing systems for Highways.
SG3/98/157, British Board of Agrément, Garston, 2000.

Department of Transport and Welsh Office (1988).
Calculation of Road Traffic Noise (CRTN). HMSO, London.

Franklin, R E, D G Harland and P M Nelson (1979).
Road surfaces and traffic noise. RRL Laboratory Report LR 896. Transport and Road Research Laboratory, Crowthorne, 1979.

International Organisation for Standards (1997). ISO 11819-1.
Acoustics - Method for measuring the influence of road surfaces on traffic noise - Part 1: The Statistical Pass-by Method. Geneva, 1997.

International Organisation for Standards (Draft 2000). ISO/CD 11819-2
Acoustics - Method for measuring the influence of road surfaces on traffic noise - Part 2: The Close-Proximity method.

Kollamthodi, S, S M Phillips and M H Balsom (2000).
Factors affecting SPB measurements: development of the backing board method. Unpublished Project Report PR/SE/164/2000. TRL Limited, Crowthorne, 2000.

Phillips, S M, S Kollamthodi and P A Morgan (2001).
Classification of low noise road surfacings. Inter-noise 2001, International Congress and Exhibition on Noise Control Engineering, The Hague, The Netherlands, 2001.

6. References


Appendix A

Table A1. SPB results by site

Appendix B

Table B1. Light vehicle statistics

Appendix C

Table C1. SPB site details and meteorological conditions