Gladiatorial games, part 15

(… continued from Gladiatorial games, part 14, posted on 14 Feb 2016).

I left you at the end of my last posting anticipating some cumulative distribution functions (CDF) of annoyance responses produced in evidence by Rick Methold, the expert acoustics witness put up by Chiltern District Council for its petition hearing in front of the HS2 Select Committee. These appeared as exhibit A1571(59), which Mr Methold advised was “taken from the EU position paper on exposure response from transportation noise”. He also told the Committee that the exhibit used “the same data that was in Mr Thornely-Taylor’s presentation”, which Mr Methold had previously reprised as exhibit A1571(47). He explained, however, that A1571(59) was “just looking at railway noise” and had also been “expanded” to include other noise curves identified in the EU position paper “which aren’t just about highly annoyed, it’s about annoyed and little annoyed” (see footnote 1).

The explanation of why the data used to construct the Thornely-Taylor CDFs and the Methold CDFs can be said to be the same, despite the two experts citing different sources, is a simple one: both can be shown to be ultimately derived from the same meta-analysis of transport noise surveys carried out by Meidema and Oudshoorn (see footnote 2).

Both sources include the same set of “polynomial approximations”, which are equations that allow points on the CDFs to be calculated. I have put these formulas into a spreadsheet in order to produce a table of values, generally in one decibel steps, of the percentage of the population that are little annoyed (%LA), annoyed (%A) and highly annoyed (%HA). The figures in the table agree with A1571(59), but provide better accuracy than trying to read values from the graphical representation (see footnote 3).

When the Committee was shown A1571(59) Sir Peter Bottomley remarked on the nature of the curves between 45dB Lden and 50db Lden, which is of course roughly the area of interest when considering the appropriate level to set the LOAEL (lowest observed adverse effect level) threshold. He said (see footnote 4):

“… what I see from this is the difference between 45 and 50 on highly annoyed is an insignificant change. Interesting change for the annoyed, and you’ve doubled from 10% roughly, to 20% nearly doubled upward two thirds those who are slightly annoyed.”

Although Mr Methold cautioned Sir Peter that the curves represented a best fit for “a horrible spread mess of dots”, he was obviously pleased to hear the Member effectively make his point for him. He told the Committee that Sir Peter was “absolutely right” and that using the annoyed curve, as he would propose, rather than the highly annoyed curve, as HS2 Ltd had done, would provide “a bit more sensitivity to those lower levels, about how people’s annoyance will change from one scenario to another” (see footnote 5).

This is also the reason why the WG2 working group that produced the EU position paper expresses, albeit tentatively, a preference for using the annoyed (%A) curve rather than the highly annoyed (%HA) curve (see footnote 6):

“Initially, %A has been chosen by WG2 instead of the more widely used %HA, because %A is more sensitive to changes in annoyance… at lower noise exposure levels.”

Mr Methold was also able to claim significant support for using the annoyed response in preference to the highly annoyed one from the Department for Transport’s own WebTAG environmental impact appraisal methodology for transport proposal assessment. He pointed out that the WebTAG information includes what he called “an annoyance curve” – actually a data tabulation – that is “closely matched to the annoyed curve on [A1571(59)]” (see footnote 7).

It is tempting to reflect whether HS2 Ltd would have arrived at a different level for the daytime LOAEL threshold if it had used the annoyed response curves, rather than using the research behind the World Health Organisation guidelines, which is based upon the highly annoyed response. The polynomial approximation given in the EU position paper, for highly annoyed response, predicts that 1.5 per cent of the population will be highly annoyed at HS2 Ltd’s chosen value for LOAEL of 50dB LpAeq,16hr (51.5 dB Lden). If the value of LOAEL had been based upon 1.5 per cent of the population being annoyed, as opposed to highly annoyed, my tabulation of the CDF values reveals that LOAEL would need to be set to below 45dB Lden – it is not possible to be more precise than this because, as Mr Methold warned, the polynomial approximations have not, he thought, been “extrapolated down that far” (see footnote 8).

You may say, in response to the above, “That’s all very interesting, but where does it get us with choosing the right value for the daytime LOAEL threshold?”, and you would be right. All that we have established, I think, is that the choice of 50dB made by HS2 Ltd is open to challenge. However, Mr Methold did have some more strings to his bow on this issue, and I will look at that next time.

(To be continued …)

Footnotes:

  1. The EU paper is Position paper on dose response relationships between transportation noise and annoyance, Office for Official Publications of the European Communities, 20thFebruary 2002. Mr Methold’s comments are recorded in paragraph 354 of the transcript of the morning session of the HS2 Select Committee that was held on Wednesday 4th November 2015.
  2. Miedema, H M E and Oudshoorn, C G M, Annoyance from Transportation Noise: Relationships with Exposure Metrics DNL and DENL and Their Confidence Intervals, Environmental Health Perspectives, Vol 109, No 4, April 2001, pp 409-416.
  3. The polynomial approximations are listed in Table 5 of the Meidema and Oudshoorn paper. I should perhaps note that, whilst Table 5 provides formulas to enable the “little annoyed” response data to be calculated, and Mr Methold has done this, the EU position paper only recommends that “the percentage of persons annoyed [%A], or the percentage of persons highly annoyed [%HA] be used as the descriptor of noise annoyance in a population” (refer to section 2.2 of the paper).
  4. See paragraph 361 of the transcript of the morning session of the HS2 Select Committee that was held on Wednesday 4thNovember 2015. Sir Peter’s figures illustrate the difficulty of using the curves in this range. If the polynomial approximations in the EU position paper are employed to calculate the values the ratios of 45/50dB percentages for the three annoyance descriptors are: highly annoyed 0.37, annoyed 0.47, little annoyed 0.57.
  5. See paragraphs 362 and 368 of the transcript of the morning session of the HS2 Select Committee that was held on Wednesday 4thNovember 2015.
  6. Refer to section 2.2 in the EU position paper.
  7. See paragraph 369 of the transcript of the morning session of the HS2 Select Committee that was held on Wednesday 4thNovember 2015. The WebTAG annoyance table is at tab A3.1 in the Excel-based WebTAG Databook. Whilst the WebTAG data points diverge from the Miedema and Oudshoorn %A curve slightly, it is a much closer fit to this curve than to the %HA or %A curves.
  8. See paragraph 364 of the transcript of the morning session of the HS2 Select Committee that was held on Wednesday 4th November 2015. In using 51.5dB as the Lden equivalent of 50dB LpAeq,16hr I have “corrected” Mr Methold’s assumed error that I identified in footnote 6 to part 14. If I had used LpAeq,16hr=Lden + 1.5, as suggested by Mr Methold, the Lden equivalent of 50dB LpAeq,16hr would be 48.5dB. My tabulation shows that this would result in 1 per cent of the population being highly annoyed. If LOAEL was based upon 1 per cent of the population being annoyed, rather than highly annoyed, this would also result in LOAEL being set to a value below 45dB; so the salient point is unaffected by Mr Methold’s putative error.

Acknowledgements:

I wish to thank Michael Woodhouse for his suggestions and comments, which I have found invaluable in preparing this series of blogs.

Exhibit A1571(59) has been extracted from the bundle of evidence submitted to the HS2 Select Committee by Rick Methold and published on the website of the HS2 Select Committee.

Important Note: The record of the proceedings of the HS2 Select Committee from which the quotes reproduced in this blog have been taken is an uncorrected transcript of evidence, which is not yet an approved formal record. Neither witnesses nor Members have had the opportunity to correct the record in such instances, and it may therefore be subject to changes being made in the light of any such corrections being requested.

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