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


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And I believe those RoSPA studies may be outdated, and they have since been forced to revise significantly.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


I am waiting to see a car travelling at 20mph! I have tried to stick to 20 and been horned abused and overtaken probably more likely to cause an accident.


The point is - accidents do and will happen. Nothing here increases or decreases that frequency significantly in the short term.

It has to be said that the point that is most relevant is - a child struck in an accident when a vehicle is doing 20mph has a much larger chance of surviving than a child struck by a vehicle at 40mph.


Thought this might be of interest.


Confused? Is that not stating the obvious?



Initial reading of the reports on the report and the report itself is not clear on the answers to these questions.

Where the KSI figures increased, what was the recorded speed of the vehicles involved.in these incidents.

Were any speed limits being observed ? Did all the incidents happen with vehicles doing 20mph ? Are these data present in the analysis ?


I bloody love the 20mph zone. I really like it. 20mph is a much more civilised speed in cities. Most people in Lewisham don’t drive anyway. I don’t know why everyone’s being so down on it.


I agree. I choose not to drive and as a pedestrian I love the 20 mph. If only there would be better enforcement.


It was an interesting read. It’s good to see multiple interpretations of the data.

Particularly interesting, his criticism of the choice of control area. The council chose a control area where the accident rate increased significantly. This means the worsening accident rates in 20mph zones will overall look better (as the control area also exhibited a worsening too).

The author thinks a different control area should have been chosen. In this case, there’s a good chance the overall worsening of safety in 20mph zones would have looked even worse.

The reason I find this odd is that it’s clear that the author wanted to try to show up the study. Yet he hasn’t shown it up, aside from his (accurate) statement that more data is required to build a strong case against 20mph zones.