Portsmouth’s 20mph “Success”


08 Oct 12      Letter to Hampshire Chronicle and Porstmouth Evening News, and 4 year SI graph added

08 Sep 12   Letters in Hampshire Chronicle about Winchester 20mph trials added

21 Aug 12    Bridgstock Summary and response to 20′s Plenty added

Edited 19 March 12 and 3 files added

Anyone who has studied road accident data knows that even for large numbers in national data, a single year cannot provide statistically meaningful indications of trend because so many other factors are involved. This is of course even more true for the much smaller and therefore more volatile numbers of fatalities than for Serious Injuries and even more so compared to Slight injuries.

These other factors include weather year-on-year (a harsh winter followed by a mild one, a wet summer followed by a dry one) while the state of the economy and with it drivers’ gung-ho or careful attitudes play a part, plus of course pure chance and random variations.

I was therefore surprised late in 2009 to read that Portsmouth City Council had held a press conference to announce “encouraging signs” for their 20mph area after only one year. I used FoI to obtain the data and reports and in January 2010 sent a formal complaint to Councillors about what I believed to be serious misrepresentartion of the results. They did not reply, but a lady involved in the project sent a briefing note to all Councillors for them to refer to if they wished to do so. The note contained clear evidence of failure to understand even the most basic principles of accident data analysis.

A sympathetic Councillor copied it to me and I wrote a detailed respone to all Councillors containing a line-by-line rebuttal of that briefing note. Few bothered to reply, though one or two were symapthetic. I need not include here those errors, instead I summarise the most important of them here – all the relevant documents are accessible below in chronological order:

a) They had spent 600,000 despite being told well in advance by the DfT that schemes of this kind, relying only on signs without enforcement, in streets where speeds were already low, were unlikely to achieve anything.

b) They give the impression that the objective was in part to “be seen to be doing something” rather than any real benefit. At a later stage one reply even claimed that casualty reduction had not been an objective!

c) They concentrated in their PR claiming indications of success on falls in slight injuries while largely ignoring increases in serious injuries.

d) They emphasised that speeds had fallen significantly on some roads, but apparently failing to realise that because average speeds had fallen – as the DfT had warned – only by 1 mph, speeds in other streets must have risen signficantly!

e) Despite traffic reduction having been a scheme objective and despite early reports identifying a 12% fall, they  reported reductions in “All” casualties without reference to the fall in traffic. (The significance of this is of course that if as expected some drivers chose to avoid the area their share of accidents was not eliminated but simply transferred to other roads. Any serious student of road casualties knows that traffic volume changes must be taken into account). Curiously, all reference to the initial report of 12% lower traffic disappeeared from later reports and the lady who wrote the briefing note stated that their targets were set without reference to the fall in traffic that had been one of their objectives!

e) For the most part their claims of succeess were based on comparisons with previous years in the same few streets, without adjusting for long term downward trends or notably better results at the same time in the national figures.

f) I copied them after the first year results, later updated two years, a comparison sheet showing that (especially when adjusted for greater falls in traffic in the 20mph area than nationally) Portsmouth’s accident and casualty results were  overall significantly worse than in the rest of the country at the same time.

g) Entirely undaunted by by criticisms and those of other campaigners, they then set off to Conferences around the country attempting to persuade other local authoties to follow their example and – aided incredibly by at least one Road Safety Award for their failure, and seem to some extent to have succeeded – despite my having copied the DfT my detailed criticisms. Presumably because some other Councils do not understand that the figures are at best statistically meaningless and at worst bogus.

There is a great deal of detail in the documents below, but no one at Portsmouth City Council, the DfT or the National Statistics Office was in the least interested in dealing with the many complaints made by me and others that the results at best proved nothing and at worst implied mostly worse or much worse results than elsewhere and that the claims of success were bogus.

On 19th March I added a detailed complaint and analysis by independent researcher and safety professional Eric Bridgstock, see below



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