Lambeth 20 Mph Scheme

Lambeth Council are pushing ahead with a borough wide 20 mph speed limit. See for the details.

Anyone who has any views on this should send them to Barbara Poulter at the Council (email address: ) . Here’s some comments this writer has already sent her (and the initial results from the 20mph speed limit in the City of London covered in another recent blog post show what a waste of money such schemes are and can actually increase accidents not reduce them):

Please note that we are not opposed to 20 mph speed limits in all locations – for example where the natural speed of traffic is near that speed. In many residential streets that is the case. However we are opposed to blanket wide area 20-mph limits because they are not a cost effective road safety measure, are not likely to be complied with and needlessly slow traffic.

  1. Let me first refer to your published document entitled “Lambeth Goes 20mph -Guide” which unfortunately contains a lot of inaccuracies.

For example, it states that “driving slower on residential roads has been proven to reduce traffic accidents,……”. Unfortunately there is no such evidence. Perhaps you could care to produce the evidence on that which is of course not supplied in the document concerned. Furthermore you say that “By reducing speeds to 20mph, it will reduce the number of casualties in the borough, improve pedestrian safety, encourage more confidence among cyclists and cut the number of incidents around schools”, but again there is no evidence for those claims.

  1. The facts are these:

a – In general the benefits of 20 mph signed area wide area schemes are grossly exaggerated. The average reduction in the speed of traffic is typically about 1 mph (assuming that there is no bias in the collection of data or other influences that might affect traffic speeds which is a dubious assumption).

Such a speed reduction is not likely to have a significant or measurable impact on road traffic accidents and not have any impact on the general environment of the roads concerned. Neither is it likely to encourage cycling or walking or discourage driving so the general health benefits will be nil – indeed there is no good evidence yet available for any such positive benefits (cities such as Bristol have claimed such benefits but their evidence is statistically dubious in the extreme).

b – The suggestion that a reduction in traffic speed translates into a significant reduction in collisions is not borne out by the real world evidence but is based on a biased analysis of traffic speeds on different types of roads. There has been no proper “controlled” trial of the use of signed only speed limits. The results in Portsmouth (which are mentioned in your document who claim an 8% reduction in collisions) do not provide firm evidence that there is any real benefit. Indeed KSIs in Portsmouth actually rose. I wrote this article on the bias inherent in the claims by Portsmouth that gives more information:

You also refer to data from Nottingham which only covers one year and any road safety engineer will tell you that one year is too short a time to be significant, particularly as there tends to be a short-lived reduction in accidents if the road environment is changed. And as you are no doubt well aware, it is more normal to only consider 3 year before and after periods as showing any significant change.

c – There is no good evidence that 20 mph sign only schemes provide any real, statistically significant, and below trend accident reduction. It is also worth pointing out that the Department of Transport (DfT) have recently commissioned a three year study into the effectiveness of 20 mph schemes as they suggest that current evidence is “inconclusive”. It would be rash of Lambeth Council to spend large amounts of money on any 20 Mph, signed only, schemes before more evidence is available on their financial benefit and effectiveness.

  1. There is no cost/benefit justification provided for the large expenditure of £700,000 on these proposals, money that would be better spent on other road safety measures. The key question is whether the benefits of that expenditure outweigh the costs, i.e. that it is a superior cost/benefit ratio to spending that money on other things.
  2. More evidence. Historically there was a 20-mph speed limit across the whole of the UK before 1930 when accident figures were much higher. Accidents fell after it was removed.
  3. In general the evidence put forward by those who support 20 mph wide area speed limits as a road safety measure is dubious and I would welcome the opportunity to contradict any that you receive. They often rely on selection of the data while ignoring other factors that might affect the results. In practice, their understanding of statistical evidence and the scientific method is weak in the extreme.
  4. So the key question, is whether spending £700,000 on such a scheme is worthwhile, or whether it would not be better to spend it on other road safety measures! Regrettably a proposal to reduce traffic speeds looks both simple and attractive which is why politically it can appear to be sensible. But road safety is a much more complex matter that is not amenable to simplistic solutions. Smaller, focused road safety schemes would be likely to create much more benefit than putting up 20 mph signs everywhere (which will of course be ignored by many road users who will consider it an inappropriate speed for many roads in Lambeth). Imposing a speed limit that is lower than necessary will slow traffic of all kinds, and will not be adhered to unless there is massive expenditure on enforcement (which of course has to be taken into account in the cost/benefit calculations as has the cost of increased travel times).

Finally, let me say that these proposals are being put forward by those who have little understanding of road safety or how to reduce accidents. In reality it is “gesture politics” of the worst kind. It it likely to result in fewer reductions in road casualties, and hence possibly more deaths, by wasting money that would be better spent on other road safety measures.

Roger Lawson

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