4 - Overview Discussion
This Section provides a summary of the findings from the literature review (Section 2) and a summary of the TS2010 noise reduction report review (Section 3), along with a brief discussion.
4.1. Literature Review - Summary
Numerous studies have been undertaken over the last 10-15 years which show that reductions in traffic noise can be achieved by replacing HRA or DAC road surfaces with an SMA or TSMA road surface product. These results vary in relation to factors such as: the aggregate size used; the precise product specification; and the reference surface and reference conditions against which the noise reduction performance is determined. The number of variables involved make direct comparisons between datasets difficult, however an indicative summary of the findings from the literature review is provided in Table 4-1 below.
| This Report Section Reference | Reported Noise Reduction dB(A) | Comments |
|---|---|---|
| 2.2 | -4.8 | 14mm SMA relative to 20mm HRA reference (aged?). Primary source of data (Abbot & Nelson, 2001) - not reviewed. |
| 2.2 | -4, -5 & -6 | Predicted initial reduction, at high speeds, relative to HRA (aged?), for 14mm, 10mm and 6mm TSMA, respectively. |
| 2.8.1 | -1.9 | Reduction achieved by going from 11mm SMA reference surface to 8mm SMA. |
| 2.8.2 | -2 | 6mm SMA relative to DAC 0/16 |
| 2.8.3 | -2 to +1 | SMA 5mm to 16mm, relative to Dutch reference HRA/SMA 11mm mix at 80kmph. Initial performance. |
| 2.8.4 | -2 | 8mm and 11mm SMA without loose chippings, compared to a smooth concrete reference surface. |
| 2.8.5.1 | -3 | SMA and OGA relative to Dense Grade Asphalt. |
*The list is not exhaustive
Based on the literature review, it is understood that SMA/TSMA has a relatively smooth driving surface due to the ‘stone skeleton’ created by the interlocking aggregate. This creates a negative texture which provides voids for the air, trapped between the tyre and road surface, to dissipate, which in turn reduces ‘air pumping’ noise. These attributes contribute to road surface noise reductions for SMA, of around 2 to 3 dB(A), and up to 6 dB(A) for TSMA, when compared to DAC and HRA road surfaces.
How best to measure and derive road surface noise reduction factors (corrections) is discussed in many of the referenced documents and is approached differently in different countries. However, the SPB measurement method is widely used and is the most popular method in the UK for deriving an RSI, which is required by the DMRB HD213/11 methodology for determining surface correction. It is understood there is currently limited capacity in the UK for undertaking and analysis CPX measurements. SPB measurement can also be used to determine the corrections required for input into the CNOSSOS-EU prediction methodology, which as of 31st December 2018 became an EU requirement under the END (2015/996/EC European Noise Directive) for strategic noise maps. However, this may require a change in the Environmental Noise Regulations 2006 before becoming a requirement in Scotland [TBC]. It is recommended that this is considered when determining the best way to confirm the noise reduction performance of TS2010.
4.2. TS2010 Noise Reduction Review - Summary
The majority of the indicative noise reduction performances, summarised in 4,1 above, relate to ‘generic’ SMA road surfaces. The TS2010 road surface course is a specific product which is purported to have lower noise levels than ‘generic’ SMA, although, this was not one of the primary goals of its development.
Based on findings of PRR443 (Published Project Report PPR443 “A review of current research on road surface noise reduction techniques”, Transport Research Laboratory, P G Abbott, P A Morgan and B McKell, January 2010.) the initial noise reduction performance of TS2010, at high speeds, is expected to be in the region of -4 to -6dB(A). The TS2010 noise reduction report shows eight different noise reduction values, two at each of the four different sites varying between 1.5 to 6.6 dB(A), with an arithmetic average of 4.5 dB(A) reported as the initial performance. Given the wide range in the: reported noise reduction results; different baseline road surfaces; traffic conditions, and environmental conditions, there is limited confidence in the single figure of 4.5 dB(A) reported. These initial findings are as much a reflection of the various acoustic performances of the aged baseline road surfaces as they are of the acoustic performance of TS2010, notwithstanding the other limitations mentioned.
The CRTN style roadside measurements that have been undertaken and reported, are the simplest way to measure road traffic noise and, providing tyre/road noise dominates over engine noise, provides reasonable proxy for the acoustic performance of the road surface. However, as stated in the Noise Evaluation report (“Sub Task 3 – Noise Evaluation”, Collaborative Research Programme, Highways England, Mineral Product Association and Eurobitume UK, November 2017.), there are too many variables which are not accounted for here such as “vehicle speed, traffic composition, measurement averaging time and the environment which can all have a significant effect on the results” and therefore, “whilst this method can be useful for directly comparing the relative acoustic performance of different test sections laid next to each other and subject to the exactly the same traffic stream, it is not appropriate for any other application”.
The TS2010 measurement report (“Assessing the Noise Attenuation Impacts of TS2010 Carriageway Surfacing Over Time”, Scotland TranServ, 2018.) discusses improving the methodology for future rounds of noise measurements, by increasing the number of samples at each position and stating that this will allow a more accurate portrayal of the noise attenuation properties. More samples would improve the reliability and repeatability of measured noise level data. However, this will not change the conditions and accuracy of the pre-construction baseline measurements, against which the TS2010 measurements are being compared. Further measurements at these four locations, using the same, albeit improved, methodology could continue to provide an indication of the how the acoustic performance of TS2010 changes over time at these specific locations. However, it is unlikely to assist significantly with determining the actual low noise performance of TS2010 in relation to an HRA reference surface of the same age, with the same traffic flow, speed and composition, and the same environmental conditions.