Why is Welsh Government referencing obsolete 1993 guidance on 20mph limits

We note that the Welsh Government has recently issued a document for local authorities “Local air quality management interim policy guidance for Wales”[i].

We are pleased that it is only “interim” and only “guidance” because where it advises authorities on the subject of speed limits and air quality it has failed to take into account so much of the changes that have been taking place in the 21st century on the setting of speed limits in other parts of the UK.

In particular the new Welsh guidance states in Para 5.19 :-

“However, measures to reduce average speeds of traffic in urban areas, whilst generally benefiting noise, may increase emissions of air pollution.”

Firstly, it appears that the government conflates reductions in air quality brought about by traffic congestion with lower average speeds brought about by traffic congestion. This was a common mistake  made by observing that air quality was lower where average traffic speeds were lower on arterial routes and city centres. But what it failed to take account of was that both were outputs as a result of congestion and stop/start driving. When average speeds are lowered by speed limits rather than congestion then there is no such correlation.

In fact this was researched by Imperial College London in their report[i] (An evaluation of the estimated impacts on vehicle emissions of a 20mph speed restriction in central London) for the City of London which said that :-

“The effects of a 20mph speed restriction on were shown to be mixed, with particular benefit seen for emissions of particulate matter and for diesel vehicles. The methodology was validated by consideration of real-world tailpipe emissions test data. It was therefore concluded that air quality is unlikely to be made worse as a result of 20mph speed limits on streets in London.”

In fact regarding PM emissions from diesel vehicles it forecast an 8% reduction for 20mph limits compared to 30mph.

This can  be extrapolated to take into account the mix of petrol and diesel vehicles on the roads such that setting a 20mph limit in terms of NOx and PM emissions is equal to removing half the petrol engine vehicles[ii].

And in Welsh Government report a section specifically on speed limits makes a bizarre reference to 1993 guidance which was made obsolete in 2006, again in 2009 and further in 2013. In Para 8.22 it says :-

“Careful consideration needs to be given to the potential effect on air quality of setting speed limits. Reducing maximum speeds on motorways and congested roads outside towns and cities may improve flow and capacity, thereby reducing emissions. Local authorities can set speed limits by making orders under section 84 of the Road Traffic Regulation Act 1984. Some authorities have piloted experimental variable mandatory 20 mph speed limits in urban areas on road safety grounds. The resulting lower traffic speeds are unlikely to reduce emissions significantly, and may actually increase emissions of some pollutants. For further advice about the relevant regulations, see Traffic Advisory Leaflet 9/99, 20 mph speed limits and zones[iii] .

Perhaps the clue is in the fact that this 1999 document has already been archived. In fact TRL the document uses the 1993 guidance on setting speed limits. This was made obsolete in the 2006 guidance which heralded the adoption of wide –area 20mph limits across complete authorities such as Portsmouth, Islington, Warrington and Oxford. This was replaced in 2009 and yet again in 2013 with guidance that encouraged the widespread use of 20mph limits on residential roads and city centres. By now most of the UK's iconic cities have adopted wide-area 20mph limits for most roads, and 15.5m people live in such places. In fact the 2013 guidance stated regarding emissions[iv] :-

“There may also be environmental benefits as, generally, driving more slowly at a steady pace will save fuel and reduce pollution, unless an unnecessarily low gear is used.”

Our briefing sheet on how 20mph reduce emissions may be found at http://www.20splenty.org/20mph_limits_improve_air_quality


So we ask the Welsh Government :-

  • Why in 2016 is it advising local authorities to look at TRL documents which were based on 1993 guidance which has been made obsolete by 3 more recent changes to national guidance on setting speed limits?


  • Why it has not kept up with modern research which shows that lower 20mph speed limits can make a substantial reduction in PM and NOx pollutants?

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