3 - TS2010 Assessing Noise Reduction

3.1. Acoustic performance of TS2010

The specification of the TS2010 road surface is provided in Transport Scotland Interim Amendment No 35/15 (Transport Scotland Interim Amendment No 35/15: TS2010 Surface Course Specification and Guidance. Glasgow: 2015.). The road surface comprises gap graded aggregate mixed with polymer modified bitumen, fibres and additives, and is treated with grit immediately post-construction to increase early-life skid resistance. The specification lists a number of Type Approval Installation Trials that have appraised the performance of the road surface, however, despite recommending certain products in the TS2010 range for noise sensitive areas, none of the trials considered acoustics or noise.

To evaluate the acoustic performance of the TS2010 road surface, a noise measurement programme was undertaken as part of an innovation project funded by Transport Scotland. The aim of the project was to take comparative measurements at selected locations to determine how the TS2010 “10mm SMA” product affected noise levels. This Section reviews the TS2010 measurement report (“Assessing the Noise Attenuation Impacts of TS2010 Carriageway Surfacing Over Time”, Scotland TranServ, 2018.).

The TS2010 measurement report describes three sets of noise measurements being undertaken at four sites to cover the following conditions:

• The existing road surfaces, which were typically aged HRA surfaces or aged 10mm or 14mm thin surface course systems;
• Immediately after the TS2010 was laid and gritted, and
• Four weeks after the TS2010 surface was laid.

The survey methodology consisted of three consecutive one-hour noise measurements taken in accordance with the Shortened Measurement Procedure (CRTN) (“Calculation of Road Traffic Noise”, London: HMSO Publications, Department of Transport Welsh Office, 1988.). However, the survey methodology was shown to deviate from the Shortened Measurement Procedure in some instances as the road surface was not dry during some of the measurements. It was also reported that measurements were not taken during the daytime between 10:00 and 17:00 as required by CRTN. Traffic counts were undertaken for the first 10 minutes of each hour, from which hourly traffic flows were approximated. The measured road traffic noise levels from each one-hour period were arithmetically averaged to calculate L_A10,3h in accordance with CRTN.

The results of the measurements indicated the following:

• Decreases of -1.9 to -5.4 dB L_A10 on medium speed roads immediately after the surface was laid and gritted, compared with existing aged thin surface course systems previously located at the same sites. Four weeks after the surface was laid and gritted, the magnitude of the noise decrease changed to -1.5 to -3.1 dB L_A10 relative to the previous aged thin surfacing course systems.
• Decreases of -6.2 to -6.6 dB L_A10 on high speed roads immediately after the surface was laid and gritted, compared with existing aged HRA surfaces previously located at the same sites. Four weeks after the surface was laid and gritted, the magnitude of the noise decrease changed to -5.4 to -5.6 dB L_A10 relative to the previous HRA surfaces at the same sites.

The report concludes that resurfacing an “aged carriageway with TS2010 will always yield a reduction in overall traffic noise, at the sites in this study an average reduction of 4.5 dB(A) was found”. This 4.5 dB(A) value appears to be an arithmetic average of the eight results (two measurements at four sites, one immediately after construction and one 4 weeks after) as reported in Figure 11 of the report (“Assessing the Noise Attenuation Impacts of TS2010 Carriageway Surfacing Over Time”, Scotland TranServ, 2018.).

While these measurements provide an indication of the acoustic performance of the TS2010 road surface, the benefits of the road surface shown in the study, and the figure of 4.5dB(A), are misleading. This is because the results are compared against older HRA, Thin Surface Course (TSC) and mixed road surface types, and it is known from previous TRL research (Published Project Report PPR485 “The performance of quieter road surfaces over time”, Wokingham: Transport Research Laboratory, M Muirhead, R E Stait, and L Morris, March 2010.), studies from Germany (“Measurements on Low Noise Road Surfaces” Author: Wolfram Bartolomaeus, Federal Highway Research Institute, Germany, published in the Inter-Noise 2016 proceedings.), and other research, that the acoustic performance of road surfaces deteriorates with age. Appendix D of the TS2010 report includes photos of the baseline road surfaces, showing surface wear and defects. 

The TS2010 report also describes that one section of TSC (A82 SB) was laid in 2004 and on section of HRA (A77 SB) was laid in 1998. Therefore, the reported noise benefits of the re-surfacing works are as much a reflection of the acoustic performance of the baseline surface as they are of the acoustic performance of TS2010.

The acoustic benefit at each of the test locations is real, on the day of the measurements, but caution should be used when applying these noise reductions to other sites as the measurements have not been normalised for the traffic and meteorological conditions during the surveys. Combined with the absence of detailed information in the appendices about the measurement time periods, there is not enough information in the report to confirm that the test conditions were sufficiently alike for like for a fair comparison.

The report concedes that the study is “still in its infancy” and that interpretation of results are “largely constrained to the short term”. However, it does not draw sufficient attention to the limitations and/or wider use of the measured and reported noise reduction values.

The report mentions improving the methodology for future noise measurements by increasing the number of samples at each position and that this will allow a more accurate portrayal of the noise attenuation properties. More samples would improve the reliability 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 locations. However, it is unlikely to assist significantly with determining the actual low noise performance of TS210 in relation to an HRA reference surface of the same age, with the same traffic flow, speed and composition, and the same environmental conditions.