20mph speed limit rollout by Lewisham Council (effective September 2016)


The question asked on that study was “do you support the introduction of 20mph speed limits in busy streets”

Not “do you support a borough-wide 20mph limit regardless of road type”

The study is barely relevant.


Seems entirely relevant. 64% supported introduction in residential roads. A large majority. I do not see why Lewisham residents would give a different result in a study of this type. And we learn that accident rates fall even with the 2mph small reductions in speed. And that likely compliance to limits claimed by drivers is increasing over time. These seem some good reasons to stick with this policy and keep monitoring these promising outcomes and driver behaviour.


The data collected from various 20mph experiments over the country has been patchy, and certainly hasn’t always supported the introduction of blanket 20mph limits.


The Manchester one is interesting and recent.
The reason I picked this up this afternoon is that you mentioned the 20mph limit is unpopular as though this were accepted generally and I did not believe this to be true. Among the general population we can now see it is popular - people want lower speeds where they live work and play, and that to me means residential roads too. I do get why many motorists don’t like it though.


More up to date data and interim analysis on the 3 yr national research for the DfT here for interest. http://www.pacts.org.uk/2017/08/atkins-research-on-20mph-limit-areas-interim-results/


As others have said, it’s completely ineffective without enforcement. As a driver, I hate driving up Perry Vale at 20mph with every driver behind me too close, hassling and trying to over take. As a cyclist I am alarmed by the speed that some drivers do on Perry Vale, definitely more than 20 and probably more that 30. It’s pointless having the lower limit if people know it won’t be enforced.


yes I share that experience both as a driver and as a cyclist. I remember some of the very earliest Perry Vale Assemblies where speed was identified as the major issue residents wanted to be addressed. We would have been happy with a 20mph limit but naively expected that it would be enforced in some way.

In Perry Vale I think I drive around up to 25mph - whereas before the limit I would stick at 30 ish. This sometimes gets me tailgated, but I reckon if I support the limit then I should act like it too - maybe I have some responsibility here, and can’t just look to others to make it happen. The more of us who make an effort, the more we can gradually influence others around us to accept this as normal. We know that this limit has great popular support. Why don’t I drive at 20mph ? Because like you I find that seems to induce some drivers to attempt unsafe overtaking. It’s my compromise I suppose.

My own behaviour change seems quite representative of the average driver. Nationally, one of the research studies mentioned above says that:- ‘Evidence suggests 20mph limits result in a decrease in average driving speeds of between one or two miles per hour, and studies of speed limit reductions across the world imply that this reduction in speed would lead to a drop in collisions of between six per cent and 12 per cent.’
For me, that’s one very promising outcome, that is a very significant drop in accidents, with savings for the many agencies and families who get caught up in them.

I think it takes time for public health and social innovations to stick, and to get general public acceptance. Compliance with limits, new or old has never been 100%. There will always be drivers in a tearing hurry, people who just don’t care and late night boy racers. We had another quite serious looking accident in Perry Vale just a week or so ago, and from the look of the car damage I doubt the limit had any impact at all.

Reading around this, traffic management experts have considered evidence and different approaches. The most cost effective way of achieving a mass change in speed is to do signage only across all the roads - which is what Lewisham has done. If it is true that overall collisions are reduced by that amount, then it represents very good social value - in terms of reduced health costs, loss of life, injuries, jobs and so on.

Reinforcing limits through infrastructure (humps, pedestrian crossings etc) is very expensive, and it looks as though they can only be done on a small scale where there is a particular need. eg accident blackspots, rat runs. General enforcement does not look like a priority for the police…

There’s one other outcome that is quoted in these studies; that is though the average reduction is 1-2%. it is on the roads where much higher speeds were previously encountered that the biggest difference happens. In a road like Perry Vale that might mean something like an average of 35 mph before the new limit and somewhere under 30mph with the new limit.
This would be an improvement for the people who live work and play here.


Some more data just came in:

[Poll] Lewisham’s borough-wide 20mph speed limit

Chris, you forgot to quote the bit that says:

A nationwide review of 20mph limits published by the The Royal Society for the Prevention of Accidents (RoSPA) last month concluded: “A large number of evaluation studies have demonstrated a link between the introduction of 20mph zones and a subsequent reduction in casualties. The size of the reductions and the consistency of results over a wide number of areas are further evidence for this link.”

Does a nationwide study not trump a local study?


The article points out that the experience of Bath council and others “is reflective of the national situation.

And of course I respect that RoSPA may have a different take on the matter, but you forgot to quote the caveat about the RoSPA findings in the Telegraph article.

And I believe those RoSPA studies may be outdated, and they have since been forced to revise significantly.

This revision illustrates how inaccurate RoSPA’a studies have been, and how real-world data from Manchester, Portsmouth and Bath (all of which show a decrease in safety outcomes) should concern us all.


But one year on, a report has found that the rate of people killed or seriously injured has gone up in seven out of the 13 new 20mph zones.

Which means that in 6 of the 13 areas the rate has reduced. Although 7 is more than 6, it is hardly conclusive evidence and is based on a small area.

So far all that has been proved is that people, papers, and councils will pick and choose statistics that support their viewpoint. (referred to above as the Texas Sharpshooter)


Reduced? Or stayed the same:

Only in two of the 13 areas did safety improve:

Overall it sounds like the 20mph limits were an embarrassing disaster.


Here’s the full report:



So in three areas KSI stats stayed the same (at a rate of zero) - I’m not sure anybody would have expected that to reduce. Three area have reduced KSI rate and seven have seen an increase. Four of the seven have seen KSI rate rise faster than a control area. Not a good report for the effectiveness of 20mph zone, but it is only one set of data (others also exist but so do contrary-indicating stats).

No doubt the larger DfT study will be met with much interest.


It would be interesting to know if these studies measure levels of adoption. If you change the limit from 30 to 20 and drivers largely ignore it (and its rarely enforced) - then how relevant are the results.


The results are interesting because whether or not the 20mph limits are effective or enforced, we’ll still see the negative side-effects (driver anger, divergent driving styles on the same road, diminished respect for speed limits, distraction of signage and bumps, overtaking etc).

And it looks like the negative side effects might be more than speculative…


Perhaps pedestrians thinking cars are driving at up to 20mph when they are actually driving 50% faster is a recipe for accidents.


Our erstwhile Mayor of London, aka BoJo, saying in 2015, provided untrammelled support for the 20mph limit,

“The Mayor of London, Boris Johnson, said: `Lower speeds have the potential to significantly improve road safety while enhancing the environment for walking and cycling. As well as actively supporting and funding the installation of 20mph zones and limits on borough roads across London, we have also been looking at the TfL Road Network to see where further 20 mph limits could provide significant benefits. These locations will help us to better understand the role that 20 mph limits could play going forward.’”

In the same TfL item this was said,

"The six key commitments are:

  1. To lead the way in achieving a 40% reduction in the number of people killed or seriously injured on the capital’s roads by 2020 - with a longer term ambition of freeing London’s roads from death and serious injury;
  2. To prioritise safety of the most vulnerable groups - pedestrians, cyclists and motorcyclists - which make up 80% of serious and fatal collisions;
  3. To provide substantial funding for road safety, invested in the most effective and innovative schemes;
  4. To increase efforts with the police and enforcement agencies in tackling illegal, dangerous and careless road user behaviour that puts people at risk;
  5. To campaign for changes in national and EU law to make roads, vehicles and drivers safer;
  6. To work in partnership with boroughs and London’s road safety stakeholders to spread best practice and share data and information"

The key element is “reduction in the number of people killed or seriously injured on the capital’s roads by 2020”

To be found here -

It’s simple - and as testing as it may seem - reducing speed increases significantly the survive-ability of those struck by motor vehicles.


Precisely, hence the importance of the studies carried out post-implementation.

That does indeed sound like a simplistic view of the policy.


Empirically simple and of immense value.

With national studies having a significantly greater veracity and accuracy over some of the local outputs.