Road Safety News

'Ready for 20' conference will include interim report of effectiveness of 20mph limits

Monday 18th January 2016

The seventh annual 20mph Places Conference, which takes place on 26 February, will include an interim report on a national evaluation of 20mph limits being carried out for the DfT.

The conference, themed 'Ready for 20', is being organised by 20's Plenty for Us and hosted by the City of London Corporation in the Guildhall. It will demonstrate how 20mph limits are being adopted across the UK and Europe, and is intended to be of interest to local authorities, transport organisations, NGOs and 20mph campaigners.

20’s Plenty points out that wide area 20mph limits are being adopted in most of the UK’s iconic cities and says there is "pressure for 20mph to become the national default limit where people live, work, shop and learn".

The campaign group claims that the recognition of benefits from 20mph limits now go "far beyond road danger reduction" to include a more attractive public realm, public health, active travel, noise, emission and traffic reduction.

A session at the conference titled 'analytical perspectives' will open with the first public presentation by Atkins who have been commissioned by the DfT to analyse and report on the effectiveness of 20mph limit implementations. The presentation will provide a common methodology for analysis of places with maturing 20mph limits and look at the outcomes on a range of issues including road casualties, speeds, noise, active travel, public health and liveability.

The conference will also include session titled 'capital perspectives' whch will look at the roll out of 20mph limits in London and Edinburgh, while a session titled community perspectives will include presentations by the Road Safety Markings Association and the Parliamentary Advisory Committee on Transport Safety (PACTS).


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It's easy for us all to make claim and counter claim. If, for example, anyone has actual measured data, rather than modelling, for vehicles emissions at various speeds and conditions, then let's see it. Do we have any measured air quality data before and after for 30 to 20 limits? In Bristol, for example, there is a very recent press report of 9 out of 10 vehicles ignoring 20 limits, including routine police car patrols etc. All we really can ask for is some comprehensive, properly handled data, rather than unfounded assumptions that 20mph is some sort of magic wand for everything. I know the sort of raw data I want to see, but either it doesn't exist or no one wants to show it.
Paul Biggs, Tamworth

Agree (7) | Disagree (3)


Thanks for providing the full report on the Knowledgebase. Rather than myself make any comment on whether increases from 6.3 to 8.7 (+38%) for pedestrian serious injuries is any more or less valid than the reduction from 39 to 29 (-16%) for all pedestrian injuries, I would encourage readers to look at the whole report themselves. They may well be able to put the findings into the same perspective as Atkins in their Executive Summary.

Whilst there is a huge amount of evidence that urging drivers to slow down with speed bumps does reduce speeds and casualties, we have been lobbying the DfT for many years to do a proper analysis of the effectiveness of wide-area 20mph limits which do not rely on such regular posterior reminders. Whilst many traffic authorities have completed their own pilots, examined the disaggregated results and come out in favour of borough-wide 20mph implementation, the benefit of a standard approach to evaluation by one party has many benefits. Hence we welcomed the DfT commissioning Atkins to make such an evaluation and are delighted that they will be presenting their first public interim report at the conference.

I would therefore encourage road safety practitioners to attend the conference.
Rod King, 20's Plenty for Us

Agree (4) | Disagree (4)

That's still a 2010 report based on after periods of 2 years ending part way through 2009.
David Sharp, Midlothian

Agree (2) | Disagree (0)

Thanks for this Eric - no wonder we struggled to find it! And confusingly it's called both 'interim' and 'final' on the title page. Will see if this is hosted anywhere more centrally (like the DfT website) and then upload to the Knowledge Centre.
Nick Rawlings, editor, Road Safety News

Agree (4) | Disagree (1)

The FINAL report of the interim evaluation is available here
Eric Bridgstock, St Albans

Agree (2) | Disagree (1)

My brief investigations earlier threw up the interim report but nothing else. If anyone has a link to a subsequent report will include that as well.
Nick Rawlings, editor, Road Safety News

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My quotes are taken from the Final Report.
Eric Bridgstock, St Albans

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I see the selective quotes are from an interim report on limited data published more than 5 years ago. Is there a later report which uses the extra data available since then to show the longer-term effects?
David Sharp, Midlothian

Agree (4) | Disagree (0)

For anyone interested, the Atkins' interim report (published in Sept 2010) is referenced in the Road Safety Knowledge Centre:

As well as access to the full report, the Knowledge Centre listing comprises a summary of the key findings.
Nick Rawlings, editor, Road Safety News

Agree (2) | Disagree (0)

To bring some balance, the following text uses extracts from the Second Year Report:

There was a 38% increase in pedestrian KSI and an 11% increase in injured cyclists. The Report also says “although there was a 12% average reduction in KSI nationally, Portsmouth recorded a 6% increase in KSI”. That was despite a 12% reduction in traffic volume within their 20mph area.

From the School Children section “There were more casualties annually in the two years following the introduction of the 20mph speed limit scheme than the annual average for the three years before”.

And virtually the last paragraph of the report’s conclusion states “casualty benefits greater than the national trend have not been demonstrated”.

"Wider benefits" would need to be overwhelming to counter these negative effects on casualties.
Eric Bridgstock, St Albans

Agree (5) | Disagree (9)

The Atkins Report which Paul Biggs is presumably referring to noted:-

"Comparing the 3 years before the scheme was implemented and the 2 years afterwards, the number of recorded road casualties has fallen by 22% from 183 per year to 142 per year."

"In conclusion, early figures suggest that the implementation of the 20 mph Speed Limit scheme has been associated with reductions in road casualty numbers. The scheme has reduced average speeds and been well-supported during its first two years of operation."

What he may be imprecisely referring to is that the number of serious injuries increased from 19 to 20. Of course, Atkins know a little about statistics and given the small numbers recognised that "few inferences about the scheme's impact should be drawn from these (KSI) figures".

So what we have is that Paul ignores the conclusion that Atkins say should be recognised and he recognises the observation that Atkins say should be ignored.

On the subject of where people live, work, play and shop, this is no more a blanket limit than the current 30mph 'so-called' national limit. Traffic Authorities are perfectly able to decide exceptions which we actually recommend they do.

Paul is also wrong to assume that at the same RPM in different gears that the fuel consumption and emission will be the same. Motor vehicles have sophisticated engine manangement systems which measure out the fuel injected into a cyclinder in proportion to the load on the engine. At lower speeds and when not accelerating from 20 to 30mph then this load (and the fuel injected) is hence much less.

Regarding speed vs emissions then he has to put aside the 1990's charts which show emissions vs average speed. These were very much based on measurements of roads where congestion (too many cars) were equally causing both lower air quality and lower average speeds. There was hence a correlation between ave speed and air quality but not a causation. Modern day engines are just as efficient at a steady speed of 20mph as 30mph.

I am not sure of Pat's background, but maybe if he/she were to look at our website and consider what we actually call for he/she would find that we campaign very much based on the wide benefits of 20mph limits. A breadth which enables many more sources of funding than can be justified for many road safety specific interventions.

The conference on 26th February is an ideal opportunity to learn more on this issue. I would encourage attendance.
Rod King, 20's Plenty for Us

Agree (6) | Disagree (5)

Hi Hugh
If Atkins have the answer, I will queue up to see if they have a spare magic wand for me to borrow. Unless speeds are already in the low 20s or new residential roads are designed specifically to encourage compliance (as per Manual for Streets 2) then new 20mph limits on existing roads need engineering for compliance to have a chance. For existing roads with higher speeds, reducing speed limit by signs only i.e no engineering and no police presence to enforce, is likely to be pretty ineffective.
Pat, Wales

Agree (16) | Disagree (3)

In October 2010, Atkins reported an increase in casualties in Portsmouth's 20mph limits, but I'm as skeptical about claimed increases as I am about claimed reductions. I'm very open to persuasion in favour of 20mph, where appropriate and where most people are in favour. But 'live, shop, work and learn' is a blanket approach. The question remains what real effects do changing a number from 30 to 20 have on speeds, casualties, emissions, noise etc? A simple experiment in my own car suggests the same engine rpm at both 20 and 30mph - hence the same engine noise and emissions. I've yet to see a speed v emissions graph that shows 20mph to be the most efficient speed. The resultant day long congestion in the likes of 20mph Cotteridge in Birmingham can only be bad for everyone.
Paul Biggs, Tamworth

Agree (9) | Disagree (5)

Good observations Pat and a good assessment of what 20 zones are about. Many residential roads do not have a history of any sort of (reportable) collisions and talk of reductions (or not) in these roads can be a bit of a red herring and misses the point.
I think it's more important to focus on and emphasise the wider benefits achievable in trying to induce a different mindset in the drivers using these sort of roads. Such benefits are very hard to measure and assess however and I've no idea how Atkins would set about doing this - also, any incidents that do occur tend to be low level in severity and therefore don't appear in any statistics.

I think the principle of lower speeds via limits in residential roads is a no-brainer, however the problem will always be, as Pat says - achieving compliance. I would hope that Atkins will address this in their report.
Hugh Jones, Cheshire

Agree (4) | Disagree (6)

I gather that many 20mph speed limit schemes were installed with the objective (presumption?) of road casualty reduction. Both sides of the debate can produce plenty of statistics to prove that it did or did not achieve that outcome, especially if area wide trends and RTTM are factored in.

However, I’m more interested to see pro 20 campaigners in the future:
(a) promote 20mph schemes primarily on the “other” benefits like "attractive public realm, public health, active travel, noise, emission" rather than casualty reduction, which often seemed to be a bit of a fig leaf cover for many of the schemes installed to date.

(b) how they deal with “non-compliance” i.e. those drivers that deliberately don’t reduce their speeds and know that they are unlikely to be caught due to the dearth of police on the streets to enforce the said 20 limits.
Pat, Wales

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You may be right Artio or you may be wrong in your presumption. The only way to find out for sure, is to sign up and go along.
Martin: Suffolk

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It may include an interim report on a national evaluation of 20mph limits but I doubt the organisers are expecting anything but support for their stance on this issue and local authorities, having introduced them at great expense, are unlikely to report they wasted their money and that 20 mph limits are ineffective.

Agree (13) | Disagree (7)