DFT 20MPH Evaluation Conclusions

As a result of the recent 20mph Research Studies commissioned by Atkins for the DOT the national campaign group have issued the following responses.

Initial response – the press release is below.

The full conclusions are available here from the national website



Today the government published its long-awaited report evaluating 20mph limit implementations. We welcome the report. It has been a long time coming since 2014 when it was commissioned.

And in that time there have been nearly half a million casualties on streets with a 30mph limit.

The report only evaluated a small number of case studies which in themselves only covered part of an authority. There are some useful indicators in the report, particularly around the negative aspects of police failing to routinely enforce 20mph limits and the need for national engagement and awareness on the benefits of reducing speeds below 30mph in residential and other roads.

However, we have major reservations about the primary data used in the report around speed reductions and the complete failure of the study to look at sufficient casualty figures to be able to draw any conclusions that would be statistically credible. These were key reasons for the commissioning of the report and we are amazed at the choice of data measured which appears to be based on measuring what is available rather than what is meaningful.

The report also fails to address any issues on the cost effectiveness of wide-area 20mph limits in comparison with the traditional physically calmed limits which are often isolated and small in length due to costs.

We believe that the report fails to provide informed and credible answers to the questions posed by local traffic authorities and does not strengthen the evidence base regarding the effectiveness, either positively or negatively, of 20mph limits.

We will be developing a full critique of the report.

Download the headline report or the technical report.

The report showed that police enforcement, or lack of it, was a key driver in compliance levels but the report did not look at how these varied amongst the case studies chosen. Our experience shows us that the key factors in successful speed reduction and outcomes are :-

Whether an implementation is authority-wide.
The level of cross party political support.
The level of multi-agency support
The level of and commitment to police enforcement.
The the level of community consensus change engagement.
We would have preferred evaluation comparing these across case studies to understand the relative influence of such factors.

We also note that the choice and aggregation of speed data may well have blurred the results. GPS speed readings were said to be just 3% of actual vehicle movements and may well be biased due to only being measured on TomTom devices with GPRS feedback or on software update. In addition static sites measurements tend to be at free-flowing positions. We also note that report moved away from the usual average or mean speeds and instead used median speeds which the authors note “helps to dampen the impact of slower moving vehicles”. One has to ask if a report into the effectiveness of interventions to slow vehicles should be based on data which itself “dampens the impact of slow moving vehicles”.

Some key messages were that the lack of enforcement increases non-compliance and needs to be enhanced. Independent Speed Adaptation on “works vehicles” could also condition this. And both of these work so much better when the correct limit of 20mph (as endorsed by WHO, OECD, ETSC, etc) is set for where vehicles mix with pedestrians and cyclists. In addition a national media engagement that “20 is plenty where people are” would consolidate the developing consensus that the report shows exists.

You can also access our Briefing Sheet which includes the Lessons and considerations for national and local decision-makers.

NICE recommends urban speed reduction for better air quality.


NICE recommends urban speed reduction for better air quality. NICE guidance says 20mph limits without physical measures in urban areas help “avoid unnecessary acceleration and deceleration”.

NICE, the National Institute for Health and Care Excellence, reviews evidence on health policies. It has published guidance on Air pollution: outdoor air quality and health in June 2017[1] which strongly supports 20mph limits for smooth driving and speed reduction.  It advises authorities to set

20 mph limits without physical measures to reduce speeds in urban areas where average speeds are already low (below around 24 mph) to avoid unnecessary accelerations and decelerations”

Many authorities have additionally set 20mph speeds on roads with higher than 24mph average speeds for consistency of limits across a community.

20mph limits are increasingly recognised as an affordable tool to tackle air quality action areas. Edinburgh, Glasgow, Hammersmith and Fulham and others have justified their wide area 20mph limits both on health grounds from fewer casualties, and due to reduced acceleration and encouraging modal shift away from car use towards non-polluting methods like walking and cycling – which all improve air quality.

NICE further states

“Where physical speed reduction measures are used to reduce road danger and injuries (20 mph zones – see NICE’s guideline on unintentional injuries on the road), consider using them to encourage drivers to maintain a reduced, steady pace along the whole stretch of road, rather than road humps that may increase acceleration- and braking-related emissions”

Rod King MBE, Founder of 20’s Plenty for Us said:

“Many authorities recognise that 20mph helps them to both meet their air quality as well as ‘duty of care’ responsibilities to the vulnerable[2]. In fact switching to a 20mph limit makes a significant reduction in the most dangerous NOx and PM10 emissions[3]. It is entirely appropriate for NICE to make this recommendation to direct local authorities in their statutory duty to improve air quality and public health.