Setting the bar too low

Having successfully delayed the issue by allowing myself to be diverted from it in my blog Checking the small print (posted 19 May 2012), I really must tackle the candidate that HS2 Ltd has put forward as a train pass-by peak noise threshold for the assessment that is proposed for the environmental impact assessment (EIA).

I mentioned in Checking the small print that the proposed level of this threshold is 85 dB LpAF,max measured at the building façade. HS2 Ltd does not explain where this figure has been plucked from in HS2 London to West Midlands EIA Scope and Methodology Report (here) but we have met it before. This 85 dB figure is quoted in paragraph 8.2.1 on page 52 in Appendix 5.4 to the Appraisal of Sustainability Main Report (here), except that there we aren’t told that it is a façade measurement.

However, the AoS doesn’t explain the origin of this figure either, and so we are left to speculate where on earth it comes from. In my blog Silent night? (posted 3 Dec 2011) I repeated a suggestion made by Professor Colin Waters that the origin of this mystery figure might be Planning Policy Guidance 24: Planning and Noise (here), or PPG24 for short.

If Professor Waters is correct, this document seems to be a strange choice as a source for HS2. For a start, the section of PPG24 that he thinks has been used was produced as a guide to local planners “assessing a proposal for residential development near a source of noise”. As Professor Waters points out, this guidance is “not applicable to the assessment of new railway systems on existing dwellings”. Also, PPG24 is now obsolete, it being one the planning documents that has been swept aside by the shiny new National Planning Policy Framework document; swept aside, but hardly adequately replaced, by the new “developers’ charter”.

The precise location of this presumed source is in Note 1 to the table Noise Levels Corresponding to the Noise Exposure Categories For New Dwellings LAeq,T dB in Annex 1 to PPG24. There are two weaknesses in Professor Water’s postulation that become apparent from examining the text of this note. Firstly, the level quoted in the note is 82 dB, not 85 dB. However, it is expressed in the note as a free field measurement, and 82 dB free field is equivalent to 85 dB façade (see my blog Adding to the confusion, posted on 1 Nov 2011); but if PPG24 is the origin, why did HS2 Ltd chose to convert to a façade measurement? Secondly, the PPG24 note specifies a slow time constant, whereas the threshold in HS2 London to West Midlands EIA Scope and Methodology Report is specified with a fast time constant (for an explanation of these time constants, refer to my blog Checking the small print, posted 23 May 2012).

It is also appropriate to note that, whereas both the PPG24 reference and the AoS citation are made in the context of night-time noise, the 85 dB in the EIA document applies for both day and night.

But, even if we can’t be sure that PPG24 is the source document, it serves to indicate just how high a threshold 85 dB LpAF,max is. The table in Annex 1 defines four noise exposure categories (NEC) to be used in the assessment of a planning application. NEC C is defined, for rail noise at night, as within the range 59 dB LAeq,8hr to 66 dB LAeq,8hr and planners are told that if the noise is in this category:

“Planning permission should not normally be granted. Where it is considered that permission should be given, for example because there are no alternative quieter sites available, conditions should be imposed to ensure a commensurate level of protection against noise.”

Note 1 to the table in Annex 1 of PPG24 says:

“Sites where individual noise events regularly exceed 82 dB LAmax (S time weighting) several times in any hour should be treated as being in NEC C, regardless of the LAeq,8h.”

So PPG24 regards 85 dB LpA,max (façade) as a very high noise level (equivalent to NEC C) and for HS2 Ltd to propose this as a threshold level is totally inappropriate. What is required is a threshold that will work alongside the 50 dB LpAeq,16hr to give an alternative indication of the onset of annoyance, based upon maximum rather than equivalent continuous level.


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