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

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

by Sarah Barth   February 22, 2014  

20mph sign (CC licensed by EdinburghGreens via Flickr)

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.”

23 user comments

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Well, if the 20mph limit did indeed result in more people cycling then it stands to reason that the absolute count of cyclist related incidents would go up.

You can bet your bottom dollar that, if the motoring lobby successfully argued the 20mph limit hasn't helped or, even worse, has caused things to deteriorate, leading to the 20mph limit being abolished, fewer journeys might then be made by bike and the commensurate reduction in the number of cycling related incidents in those areas would be crowed about non-stop by that same motoring lobby.

As the twenty's plenty spokesperson makes clear, an overwhelming majority support inner city 20mph limits. It's only the motoring-entitlement brigade who consistently come out against them and in this particular transport bunfight they are - for once - the minority voice.

seven's picture

posted by seven [100 posts]
22nd February 2014 - 14:10

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I'm no fan of the ABD, to put it mildly (its seems to be kind of a Jeremy Clarkson fan club) but its entirely plausible that the analysis they attack has statistical errors. Its quite commonplace for local authorities, lobbyists, corporations and governments to put out such things. They are always more concerned with whether the conclusion supports their argument than whether its statistically rigourous.

It wasn't in a peer-reviewed journal I am assuming?

But so what? I don't want cars zooming through my neighbourhood. And 20mph zones are nowhere near enough to encourage more walking/cycling, we need a heck of a lot more than that.

Though it sounds as if even the ABD guy is admitting that even this small measure, over this small time period and in this small geographic area did have a small but measurable effect in increasing cycling and walking (just not the implausibly huge effect the council reports).

posted by FluffyKittenofT... [639 posts]
22nd February 2014 - 14:56

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So basically the motoring lobby want to be able to drive as fast as possible without having to look out for other pesky road users. What a surprise.

posted by Mart [95 posts]
22nd February 2014 - 14:56

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If I went into Ladbrokes, and tried to lay a bet that reducing speed limits would ultimately reduce the severity of accidents statistically, they'd tell me,

"Sorry mate, we don't take bets on dead certs".

And yet we have a situation similar to that witnessed for decades within the smoking debate, where we all have to wait for many years before the opinions of so called "experts" gradually become perceived to be sufficiently bizarre that they eventually wither away, allowing a sensible 20mph limit to ultimately become the norm, rather than the exception.

The key to getting change is not to focus so much on the "hard to prove" reduction in injuries, but actually getting business to understand the link between slower speeds and higher trading figures. People understand money far more easily than safety, and if this approach ultimately achieves the same end much more quickly, then this is the strategy we should be using.

"Hey..... Let's be visible out there."

Neil753's picture

posted by Neil753 [451 posts]
22nd February 2014 - 16:06

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I have a slight issue with the statement

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

well, yes they should. Otherwise the same argument could, and at some point probably will, be used for mandating helmet use.

All you need to do is read any non-cycling specific website to see that 'we' want cyclist to wear helmets etc

its a dangerous precedent to set imho

Racer 074 for the 2014 Transcontinental Race; 2,000 miles from London to Istanbul.

http://themartincox.co.uk/2014/03/racer-074-transcontinental-race-2014/

posted by themartincox [319 posts]
22nd February 2014 - 16:30

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A biased organisation re-massages statistics to argue against lowering of speed limits. Not sure why we give these petrophiles oxygen

I could not give a toss frankly. On the estate I live going 20 has reduced speeds overall and does improve the quality of life on the estate.

20 is a very good thing. As a walker, cyclist and a driver, I am in favour of more. I cannot understand why some argue for faster speeds to be allowed on busy narrow urban roads with children etc.

posted by gazza_d [189 posts]
22nd February 2014 - 17:22

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Far greater scepticism needs to be applied to anything put out by the ABD than is on show here. They're a bunch of climate-change-denying, anti-local motorhead lunatics. This, for example, from their home page:

"The ABD calls for the use of sensible speed limits [not based on] vociferous local activists."

In other words, "sod the people who actually live there we want to scream through Cotswolds villages at 60mph"

The rest of their manifesto is the usual 'boo-hoo, us poor motorists' tripe you hear from people who don't have a clue how heavily subsidised they really are.

Death, frankly, is too good for them.

MiserableBastard's picture

posted by MiserableBastard [6 posts]
22nd February 2014 - 18:45

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ABD analysis shows 20mph speed limits alone not sufficient to encourage walking and cycling Applause
ABD calls for councils to consider stronger measures such as lower speed limits, filtered permeability and pedestrianization? Laughing

posted by CarlosFerreiro [54 posts]
22nd February 2014 - 19:02

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My own statistically flawed research has found that motorists pay no attention to 20mph limits.

Both Camden and Islington have borough wide 20 limits and I'm routinely overtaken by vehicles when cruising at that speed.

It's pointless without enforcement. Once there is enforcement there may be a corresponding uplift in cycling and walking along with a reduction in collisions and injuries. Until then it's just signs, paint and wishful thinking.

posted by Pauldmorgan [163 posts]
22nd February 2014 - 19:25

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Article wrote:
Having made some adjustments... ..Heymer said... ...cycling increased by about 3%... ..and 9%
He has a point re the stats but I bet he was cheesed off when his numbers were still 3 and 9%.

posted by vbvb [220 posts]
22nd February 2014 - 20:08

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The Duke of Wellington once said "Lies, Damn Lies and Statistics".

Put basically whatever set of figures you have, you can make them say anything you want them to.

For instance.
If 1 cyclist in 100 has a serious accident in a year and you have a cycling population of 1000 cyclists then you will have 10 serious accidents in year 1.
If the cycling population increases to 10 000 and the serious accident rate is 1 cyclist in 50 then you will have 200 serious accidents in a year 2.

So is cycling 20 times more dangerous or 50% safer? The raw figures could be used to support either viewpoint. And how do you measure things like "Do you feel safer?", "Does seeing more cyclists on the road give you confidence?"

Statistics are not science they are opinion.

posted by levermonkey [341 posts]
22nd February 2014 - 20:25

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vbvb wrote:
Article wrote:
Having made some adjustments... ..Heymer said... ...cycling increased by about 3%... ..and 9%
He has a point re the stats but I bet he was cheesed off when his numbers were still 3 and 9%.

That's what most struck me. Considering how minor a step this is, those numbers are actually surprisingly good. I would have expected the effect to be less than that.
To really increase active travel will take a change in the culture, which itself would require more major changes than a few 20mph streets (one word - bollards!), plus a lot more time for people to change their habits in response.
I think the guy has done a valuable service, in doing the work of making a (presumably) correct statistical analysis and finding an unexpectedly encouraging result.

posted by FluffyKittenofT... [639 posts]
22nd February 2014 - 21:28

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Hasn't road.cc got anything better to do than quote the most extremist types of petrolhead fanatics?

posted by ChairRDRF [107 posts]
22nd February 2014 - 21:37

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tbh, you're doing bloody well to manage 20mph in central Bristol ... not sure the change in speed limits is going to make much difference at all.

posted by Sherlock Ohms [14 posts]
22nd February 2014 - 21:44

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Looking at theABD's claims (and I did check them on their web site, as well

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”.

Firstly, they don't deny there was an increase in cycling and walking just the weighting to come up with the headline figure.

I would agree you can't point to speed limits in isolation, infact that's the whole point. 20's plenty in conjunction with other measures helps make cycling seem safer and normal to non-cyclists which increases the numbers cycling (which they admit has happened) - obviously more infrastructure would be better.

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.

Surely it depends what you are trying to measure? Most traffic surveys are aimed at measuring congestion of some sort or other, therefore you would measure in peak times - rush hour when it isn't a holiday.
Walking and cycling will tend to be more leisure based, and likely to take place when people have spare time. Obviously some of us commute by bike, but that's not necessarily were 20's plenty will have an impact.
So the timings seem reasonable.

Final gem from the ABD website
"ABD Welcomes Godfrey Bloom MEP as a Patron" - Rolling On The Floor

posted by RedfishUK [43 posts]
23rd February 2014 - 10:08

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Pauldmorgan wrote:
My own statistically flawed research has found that motorists pay no attention to 20mph limits.

Both Camden and Islington have borough wide 20 limits and I'm routinely overtaken by vehicles when cruising at that speed.

It's pointless without enforcement. Once there is enforcement there may be a corresponding uplift in cycling and walking along with a reduction in collisions and injuries. Until then it's just signs, paint and wishful thinking.

I agree that you will still be overtaken doing 20 in a 20 zone, but that doesn't mean that the 20 limit has made no difference. Most drivers will be driving at a lower speed than they would in a 30 limit, and this lower speed differential between car and bike will mean the driver gets more time to respond to hazards and will be less likely to push past recklessly.

Most people don't push limits too far even in the absence of any enforcement, and once enough start to follow the lower limit it becomes the norm.

posted by 3cylinder [61 posts]
23rd February 2014 - 10:47

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Yesterday while driving I was over taken twice in a 30 limit and once in a 40. With no one to police it no one will obey it.

posted by peterben [30 posts]
23rd February 2014 - 11:24

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themartincox wrote:
I have a slight issue with the statement

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

well, yes they should. Otherwise the same argument could, and at some point probably will, be used for mandating helmet use.

All you need to do is read any non-cycling specific website to see that 'we' want cyclist to wear helmets etc

its a dangerous precedent to set imho

Too true.

It's depressing to see the groupthink on this website as bad as on, say, PH sometimes. And how blindingly obvious it is that that groupthink is what holds back progress, since real flaws aren't addressed.

There's a saying about that, I think. Same meat, different taste?

posted by nuclear coffee [114 posts]
24th February 2014 - 10:50

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The comments from 20's Plenty do not endear me to their views, regardless of my vested interests as a road user. That's not a million miles from the kind of one-sided stupidity that states that if you have nothing to hide, you have nothing to fear...

If I could have, say, 6 bikes, would it stop me drooling over others that I don't have?

posted by notfastenough [2948 posts]
24th February 2014 - 11:19

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we've had 20mph borough wide speed limit where I live (London Borough of Camden, NW London) since August 2013.

Police Commander admitted in interview with Camden New Journal (local newspaper) he does not have the resources to enforce this speed limit

every day, I seen numerous motorists speeding, red light gambling, jumping red lights, driving through green pedestrian crossing lights, driving at pedestrians trying to use zebra crossings, etc.

absolutely no traffic Police out there in LB Camden enforcing "the law"

other boroughs are campaigning for 20mph speed limits = TOTAL WASTE OF TIME WITHOUT POLICE ENFORCEMENT!

posted by hampstead_bandit [122 posts]
24th February 2014 - 16:29

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Sherlock Ohms wrote:
tbh, you're doing bloody well to manage 20mph in central Bristol ... not sure the change in speed limits is going to make much difference at all.

To average journey times, no. But the problem is people thinking they need to get up to 30mph, even if it's only for a few seconds. It increases impatience with cyclists and leads to dangerous rapid acceleration then braking.

Also everyone doesn't need to obey it - if you get enough people in cars who do stick to it, everyone else gets stuck behind them.

posted by HKCambridge [109 posts]
25th February 2014 - 15:42

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hampstead_bandit wrote:
we've had 20mph borough wide speed limit where I live (London Borough of Camden, NW London) since August 2013.

Police Commander admitted in interview with Camden New Journal (local newspaper) he does not have the resources to enforce this speed limit

every day, I seen numerous motorists speeding, red light gambling, jumping red lights, driving through green pedestrian crossing lights, driving at pedestrians trying to use zebra crossings, etc.

absolutely no traffic Police out there in LB Camden enforcing "the law"

other boroughs are campaigning for 20mph speed limits = TOTAL WASTE OF TIME WITHOUT POLICE ENFORCEMENT!

What _are_ the economics of speed cameras? Is it true as motorist organisations constantly allege that they are 'revenue raisers'? If they don't in fact have substantially negative running costs I'd say put in a lot more of them.
(I'm guessing that the Clarkson tendency are being less-than-honest about their profitablity or surely we _would_ see a lot more of them, given how strapped-for-cash councils are).

posted by FluffyKittenofT... [639 posts]
25th February 2014 - 16:41

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Where I live I think there's only one 20mph zone, which is about 150 yards long. I've never seen any enforcement at that point (although occasionally there is a mobile speed camera parked 2 miles up the same road at a 30mph zone) and have no reason to believe it makes any difference to local driver behaviour.

Any benefit it might have for cycling is moot in any case, as the majority of roads around the whole area are completely geared in favour of motor traffic, despite the many local schools, and exceedingly unfriendly for both walking and cycling. Dual carriageways are impractical as cycling routes as they are virtual motorways and very indirect, and single carriageways are busy and narrow, often because of parked vehicles and the occasional bus.

posted by congokid [110 posts]
25th February 2014 - 17:47

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