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Bristol data showing walking & cycling is boosted by 20mph zones is ‘flawed’

Alliance of British Drivers say 'elementary' errors make the research meaningless...

A study has found that key evidence that 20mph limits lead to more people walking and cycling could be fundamentally flawed.

Analysis by the Alliance of British Drivers (ABD) of The Local Government Information Unit’s December briefing note,  ‘Area-wide 20mph neighbourhoods: a win, win, win for local authorities’, found that the evidence from Bristol recording a mean increase in walking and cycling of 23% and 20.5% after 20mph limits were introduced were unreliable.

The ABD decided to investigate the figures because they did not tally with a Department for Transport (DfT) finding last January that there was “no conclusive evidence that speed limit changes in isolation from other measures have an impact on walking and cycling”.

ABD traffic management adviser Malcolm Heymer,  a retired local government traffic and road safety engineer, told Transport Xtra that the “statistically invalid”. methods of calculating the data didn’t allow that “the individual percentage changes [were] weighted according to the absolute figures involved to give a true average.

“That such an elementary error could be made in a local authority report is both extraordinary and concerning.”

He went on: “A monitoring report by the council showed that the bottom end of the pedestrian range was actually 1%, not 10%, and the upper figures were taken from survey results that had not been corrected for the rain that affected some count sites in the before period.”

Having made some adjustments to the data himself, Heymer said he believes the Bristol surveys suggest walking and cycling increased by about 3% in one pilot area and 9% in the other.

He also criticised the data gathering, saying the 12-hour counts were undertaken on just one weekday and one weekend day , all in the summer schol holidays.

“Traffic counts are normally held outside holiday periods, due to their greater variability in levels of travel,” says Heymer.

“If the council had undertaken control counts at the same time, in areas where 30mph speed limits were retained, the changes in the pilot areas could have been compared with those in the control areas,” he adds.

Bristol City Council responded: “Our decision to implement 20mph speed limits across Bristol is policy based – if we want safer roads we need lower speeds, if we want lower speeds we need lower limits.

“The safety case for 20mph is clear and as one of our package of measures it will play a role in increasing the number of people walking and cycling.”

Meanwhile, 20’s Plenty, the campaigning group, said councils need not hold public consultations before implementing the 20mph marked zones.

A new briefing paper by the pressure group states: “Consultation is valuable if you can’t predict the outcome, but is costly and takes time. If the question is ‘do you want 20mph limits?’ the majority reply Yes!

“Should public funds be spent asking people questions we know the answers to, or on getting on with increasing safety?

“On 20mph limits there is little point half-heartedly sounding out general opinion and wasting resources and time when surveys consistently show over 70% support it.”

Residents in Brighton & Hove recently persuaded the City Council to drop some plans to introduce 20mph limits. This, say 20’s Plenty, is because the council went about the consultation the wrong way.

“Wholeheartedly recommending 20mph limits to communities post-decision and pre-roll out of signs is the best stage [to engage],” they stated.

Late last year we reported the ongoing stoush between Brighton’s business and motoring groups, and the Green-run council .

The number of crashes involving pedestrians and cyclists on city centre roads had gone up since 20mph restrictions were introduced, although overall crashes were reduced.

In the first six months of the 20mph zone this year there were 129 crashes compared to 145 for the same period in 2012 and 168 in 2011.

Cyclists were involved in 48 crashes in the six months after the 20mph zones were introduced on April 8, and 44 in the same period last year, and crashes involved pedestrians increased from 35 to 40.

Councillor Ian Davey, lead member for transport, said: “It is impossible to know what would have happened if we hadn’t done anything.

“But we do know that measures have been taken over the last few years including lower speed limits and that the number of people killed and injured on roads in the city are going down.

“It’s not realistic for the rate of collisions to continue declining at an increasing pace.

“20mph is not an idea peculiar to Brighton and Hove, one in six UK residents live in a 20mph street.”

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