Wednesday, 16 April 2014

Are 20mph limits the right approach to reducing injury accidents?


As a local councillor I have been bombarded by emails from campaigners calling for blanket 20mph limits on urban roads. The campaign run by a City of York councillor has this to say:

We quite simply campaign for 20mph to become the default speed limit on residential and urban streets. This can be done on most streets without the need for any physical calming and we accept that on some streets it may be appropriate to have a higher limit based on the road, vulnerable road users provision, etc. But any limit above 20mph should be a considered decision based on local circumstances.

This is very different from the introduction of 20mph zones (usually accompanied by traffic calming measures) in particularly high risk places such as village centres and outside schools. What the campaigners call for is a change to the default speed limit in urban areas.

While I have supported 20mph zones (we have one in Harden, for example, that not everyone is keen on), I am sceptical as to whether the introduction of a general lower speed limit will work as the campaigners suggest. The core argument appears to be the 'laws of physics' - essentially that there will be fewer casualties as a result of slower speeds and where collisions occur they cause less harm.

Campaigners also point to the case of Portsmouth where casualties reduced by 22% in the three years following the introduction of 20mph limits. However, the evaluation report (where the 22% figure is found) also showed that 'killed and seriously injured' figures rose during this period. And the reports authors also concluded that:

...overall, improvements in casualty rates were not demonstrably greater than the national trend, and that 20 mph zones were more effective at reducing average traffic speeds.

 This suggests that 20mph limits might not be the best way forward and that the current approach where zones (supported in most but not all cases by physical intervention as well as signage) are targeted to high risk areas is a better option. This reflects the fact that most of our urban roads are pretty safe with very low - even zero - rates of injury accident.

To help us understand this, if we look at the map of Cullingworth's RTAs from 2000-2010, we find that they are entirely (bar one outside the school gates) on the main roads through the village. This indicates a case for looking a 20mph zones in the village centre (although only five of the accidents involved either pedestrians or cyclists) but does not support a general 20mph limit. Looking in more detail at Bradford, we see that the inner city and especially main roads through the inner city are the most lethal for pedestrians. I would take the view that safety cameras are perhaps a more effective means of managing speeds than 20mph limits without accompanying traffic calming measures.

The proposal for 20mph limits seems to me an honest attempt to develop what we might call an 'all-population' solution to injury accidents on our roads - reduced speeds mean fewer accidents and less deadly accidents therefore reduced speed limits everywhere would achieve this end. However, the RTAs are concentrated not spread evenly across the network. So there is no need for the general reduction. Instead, we should use targeted interventions (easily enforceable, local 20mph zones with physical calming measures) in the most high risk locations. And we know these work.

A bit like minimum pricing for alcohol, blanket 20mph limits have an initial appeal. However, they do not address the problem directly and act to reduce speeds where speed reduction isn't needed and don't provide the active calming measures we know are most effective in high risk locations. Lastly, we need to consider who is causing the injury accidents and wonder whether our current enforcement system is entirely adequate:

For every fatal collision, there is a one in two chance that the driver responsible has a criminal record, according to preliminary research by South Yorkshire police.



Barman said...

A 20mph limit would only be followed by a 15mph campaign...

Anonymous said...

More lunacy like Saltaire Road - inflicted with a 20mph limit which is only relevant for 2 hours a day, 180 term-days a year - that's around 4% of the time.
The other 96% of the time, the rest of the world suffers for a tiny minority of bad drivers, a tiny minority of the time. More nanny nonsense.

Speed is not the issue, inappropriate driving is the issue and dumb sledgehammer speed-limits will never solve that.

Bucko The Moose said...

We had a similar campaign in our area and it was very sensationalist. MPs and local paper were campaigning for a 20 limit to stop the - Qoute - Thousands of child casualties on our roads.

I looked at the figures available on the local government website and RTAs have been steadily declining for the last ten years. Also, there were only 2 child fatalities in the same period.

Road safetly seems to be working it self out without any intervention. We seem to be putting way too much emphasis on speed and forgetting all the other reasons that cause traffic accidents.

Campaigners seem to have only one need at the end of the day - Something to campaign about. And if it involves saving the kiddies, no matter how remote the link, so much the better.

20's Plenty for Us said...


Whilst you referenced the report from Portsmouth and in particular drew attention to the "killed and seriously injured rising during the period", you did not mention that the rise was from 19 to 20 or that in the very next sentence to the one quoted the report said "Because the total numbers of deaths and serious injuries and of casualties by road user type and cause are relatively low, few inferences about the scheme’s impacts should be drawn from these figures."

Neither did you mention the conclusion of the report that :-
"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."

Also if you do look at the casualty map for Cullingworth then the largest group of casualties are the "unclustered ones".

Such casualties do not benefit from micro-engineering and need a wide-area intervention.

And I can confirm that the emails you receive do not come from anyone in their capacity of councilor, but as national campaign manager for 20's Plenty for Us.

One of the biggest disadvantages (besides cost) for targeted intervention is that they endorse higher speeds elsewhere. At the end of every small and isolated 20mph zone is a large sign saying that drivers can then go 10mph faster.

Already 13m people live in communities where councillors have decided that 20 really is plenty where people live, work, shop and learn. You can find out more at

Maybe your constituents would be pleased to have a 20mph limit on their streets and community roads.

Curmudgeon said...

How can councils propose to spend money on this when the roads are full of potholes?

phil said...

Is it always about reducing KSI figures? Maybe there are other reasons why we should be reducing speeds.

And often the busiest, most pedestrian unfriendly roads are the "safest" because people don't like walking there. You can't knock down a pedestrian who ain't there.