The latest road casualty figures from the Department for Transport are out and, as is their wont, the pro driving lobby and road safety campaigners have rushed into the fray over their findings.  The Institute of Advanced Motoring (IAM), after analysis of government data, says that the number of serious accidents on 20mph speed limit roads increased by 26% last year.  Slight accidents on 20mph roads have also increased  At the same time last year both serious and slight accident rates on 30 and 40mph roads decreased.  The debate raging is about whether traffic calming measures and the 20mph speed limit actually do reduce road traffic accidents or not. Here are a few thoughts on the matter - what are yours?

The open road

The chief exec of the IAM, Simon Best has said:

“The government and councils need to take stock of the effectiveness of 20mph signs. Recent advice, guidance and relaxation of regulations has all been about making it easier for councils to put 20mph limits in place.

“More and more roads are being given a 20mph limit but they do not seem to be delivering fewer casualties. The IAM are concerned that this is because simply putting a sign on a road that still looks like a 30mph zone does not change driver behaviour. More evaluation and research is needed into the real world performance of 20mph limits to ensure limited funds are being well spent. In locations with a proven accident problem, authorities need to spend more on changing the character of our roads so that 20mph is obvious, self-enforcing and above all contributes to fewer injuries. In Europe, it is long term investment in high quality segregated or shared surfaces that have led to a much safer environment for cyclists and pedestrians.”

Passport to Portsmouth

To examine the facts it is a good idea to have a look at the city of Portsmouth as it was the first council to introduce a universal 20mph limit on residential roads throughout the city in 2007-2008.  Perhaps one salient point here is that Portsmouth’s ‘speed limits’ were not enforced by speed bumps or other traffic calming measures at the time, and so are judged differently from 20mph ‘zones’ where traffic is physically slowed down.

For the purposes of traffic research 'limits' and 'zones' are examined separately in order to see which of these is more effective.

The Association of British Drivers (ABD) says that it has no quarrel with 20 mph limits on narrow or twisty residential streets but does not want a blanket ban on driving over 20 throughout cities as it deems this to be unnecessary.

The ABD says that between 2010 and 2011 in Portsmouth – well after the 20mph had been universally implemented in the town - casualties resulting in death or serious injury actually rose  by 57 per cent .  The national average rise in that period was only 2 per cent.

Unfortunately though there was no control group in the Portsmouth study which is a fundamental flaw as there is nothing to compare the figures to, apart from the national averages.

Although it seems that the study in Portsmouth can’t tell us anything very conclusive on its own, there have been other studies on ‘speed limits’. A  DFT study in 1998 found that speed limit signs alone didn’t really work to slow traffic down, whereas physical traffic calming methods and speed cameras were much more effective. Proving that people’s consciences alone were not enough to slow their speed. This report also found that there was no reduction in injury accidents in lower speed ‘limit’ areas.

Going at 0 MPH - but nice poppies :0)

Passport to Pimlico

However there is also some evidence that 20 mph ‘zones’ do reduce casualties. The British Medical Journal published a study based on figures gathered in London between 1986 to 2006.  There was a long term downward trend in road casualties in general which was factored in, but the study did also produce information from police recorded casualty data, where 20mph ‘zones’ appear to have produced significant casualty reductions.


“The introduction of 20 mph zones was associated with a 41.9% (95% confidence interval 36.0% to 47.8%) reduction in road casualties, after adjustment for underlying time trends. The percentage reduction was greatest in younger children and greater for the category of killed or seriously injured casualties than for minor injuries. There was no evidence of casualty migration to areas adjacent to 20 mph zones, where casualties also fell slightly by an average of 8.0% (4.4% to 11.5%).”

“20 mph zones are effective measures for reducing road injuries and deaths.”

So it would seem that 20mph ‘zones’ which involve traffic calming measures which actually physically make the traffic have to slow down, are more effective in making accidents less severe when they do happen, rather than preventing them altogether.

The facts never lie? But what are they?

Statistics, which you would imagine would give a clear and conclusive picture, never actually seem to quite do that and are always open to interpretation.  The ABD read the statistics as proof that 20 mph speed limits don't achieve their objectives, while It may be that the number of roads that now operate a 20 mph speed limit is increasing so much that is altering the statistics as the volume of traffic on 20mph roads is far higher.  Or simply that many drivers are ignoring the speed limits, especially perhaps where they are used to driving on a particular road at 30mph. So the conclusion is perhaps inconclusive for the time being.

Perhaps more studies on 20 mph are needed before a definitive road safety yay or nay is arrived at – meanwhile the potholes are performing the same function as speed bumps in many areas so ride safely!

Do you have an opinion on the roll out of 20 mph speed limits in the UK? If so we'd love to hear it. Email us at
Posted by Lucy England
for Wemoto News on 23 July 2014 in General News



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