Metrics matter

Twenty years ago I wouldn’t have been reading a blog on metrics; far less writing one.  I’m not sure I’d really have had much idea of what “metrics” were.

But having chaired HACAN for nearly 20 years I now know metrics matters a lot.  The way noise is measured and the assumptions behind when it becomes annoying are critical factors in the determining Government policy on noise.

The importance of getting metrics right was recognized by the Transport Select Committee in its recent report on the National Policy Statement.  In effect, it suggested the Department for Transport recalculate the number of people which could be impacted by a three runway Heathrow using the most up-to-date metrics.  If this was done it believed ‘an extra 539,327 people would be captured in the annoyance footprint; taking the total number of people in the noise annoyance footprint to over 1.15 million’.  This is considerably higher than either the DfT or Heathrow have acknowledged.

The importance metrics right is the also one of the key messages of the report HACAN published this week (in association with Plane Hell Action) which found that aircraft noise can be a problem over 20 miles from Heathrow – areas where the traditional metrics simply ignored:  

It needs to be acknowledged the real progress there has been in recent years in devising more realistic metrics.  Credit goes to campaigners who gave banged on about outdated metrics for nearly 20 years, the Airports Commission who came to the issue with fresh eyes, Heathrow who came to understand the need for change and to the Department for Transport which moved things forward significantly in its new airspace policy announced in autumn 2016.

Things are not yet perfect – which I’ll come to later in the blog – but we are in a different world from the dark days of two decades ago.  Then the 57 decibel contour was king.  If you were inside the contour, it was accepted you had a noise problem.  Outside of it, you didn’t really count.

So what was so magical about the 57 decibel contour?  It was constructed like this.  Over a 16 hour day, the number of aircraft passing over an area and the noise of each plane were noted.  The noise was then averaged out.  This was then turned into an annual average.  If the annual average was over 57 decibel, the area was within the 57 decibel contour.  

Why 57 decibels?  Because, at the time, this was the level at which the Government argued ‘the onset of community annoyance’ began.  Acousticians were careful to say that it was more subtle than that and that some people became annoyed at lower levels but, to all intents and purposes, the 57 decibel contour became the official cut-off point, used at public inquiries and in industry and government documents to illustrate the numbers impacted by individual airports.  Latterly, it made no sense.  Around Heathrow for example places like Putney and Fulham – both clearly heavily impacted by aircraft noise – were outside the contour.  

Things began to look up when, over a decade ago, the EU required member states to use a different metric known as 55Lden.  It argued that the ‘onset of community annoyance’ started at a lower level.  The difference in numbers impacted at Heathrow was huge:  over 725,000 using 55Lden compared with around 245,000 using 57LAeq. 

The Airports Commission under Sir Howard Davies, although criticized in other areas, moved the metrics debate forward significantly.  It suggested a range of metrics should be used included the ‘N’ metric.  Local communities often feel these are more meaningful to them than the average noise.  So, for example, N60 would indicate the number of flights over 60 decibels that went over an area in any given period.  Heathrow also began to move towards using a suite of metrics.

The culmination of this improved process was the Government’s Airspace Policy announced in autumn 2016.  It effectively ditched the 57LAeq contour and replaced it with the 54LAeq as point where ‘the onset of community annoyance’ starts.  But it went further.  On the basis of a report it had commissioned from the Civil Aviation Authority (CAA), the Department for Transport recognized that around 7% of people could be disturbed when the noise averages out at 51 decibels.  These are more meaningful metrics.  And not far from what the World Health Organisation recommends.

In geographical terms, it takes the annoyance boundary from about Barnes (57 contour) to Clapham (54 contour) to about the Southwark/Lewisham border (51 contour).  As the crow flies, Barnes in 9 miles from Heathrow, Clapham 14 miles and Nunhead (fairly close to the Southwark/Lewisham border), 19 miles.  Similar calculations can be done west of the airport.  

Accurate metrics matter because only when there is a clear idea of the numbers impacted by noise from an airport can realistic policies be put in place to deal with that noise.  Metrics can determine levels of compensation, whether efforts should be made to provide communities with relief and respite from the noise and, indeed, to assess the impact of any new runway.

Campaigners will be pressing for real action based on these more meaningful metrics.  We will also continue to press for still further improvements.  For example, the existing metrics do not reflect the actual noise impact in areas like Ealing or Teddington which only get planes (on easterly departures) about 30% of the year but, when they do, the impact is significant.  A metric that measures only the days areas are overflown would be more meaningful and needs to be added to the suite of metrics used.  This would also capture the problems experienced in places like Reading and Caverham which are currently a little outside the 51 decibel contour when measured over a year.  A metric also needs to be used which reflects the cumulative impact on areas which experience noise from two airports, such as Heathrow and London City.

The dark days when one outdated metric was relied upon do seem to be over.  But the light is not yet shining as brightly as it could be.  

by John Stewart

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