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It is well known that an impact at 20mph causes less injury than an impact at 40mph. Injury is proportional to energy, and energy is higher at higher speeds.
The news item clashes with other experience:
Wikipedia (retrieved 12 Aug 2012) By August 2002 Kingston upon Hull had introduced one hundred and twelve 20 mph zones and 190 km of roads subject to a 20 mph limit covering 26% of the city's streets which they described as contributing to "dramatic reductions in road casualties". Total collisions were reduced by 56%, Killed & seriously injured collisions down 90%, child casualties collisions down 64% and all pedestrian collisions down 54% and child pedestrian collisions down 74%.[23]
A report published in 2008 estimated that following the introduction of 20 mph zones in London, a reduction of casualties by 45% and KSI by 57% occurred.[24]
So what is going on?
Well in fact what is going on is that journalists are just lazily copying and pasting crap information sent to them, probably by some idiot motoring lobbyist. The excellent Joe Dunckley's blog points out that "Back when there were fewer 20mph streets, fewer people were injured on 20mph streets, they revealed. Now that there are more 20mph streets, more people are being injured on 20mph streets. This road safety intervention, they concluded, isn’t working."
Simple as that. The journos had taken the raw numbers of people injured on the 20 streets, rather than taking the rate of injuries per mile of streets. Why did they do this? Three possible reasons:
- The motorists lobby is powerful, hospitable and influential
- Many journalists are thick, poorly educated and innumerate
- Many journalists are lazy, and it takes a bit of time to research the rate at which 20mph zones have been applied in the UK.
I spent 18 minutes trying to find how many zones have been introduced. I find here at ROSPA that 450 20mph zones were introduced 1991-99, and that by 2008, no less than 2148 zones had been introduced. I could not find how many were introduced in the period 2010-2011 covered by the press release that triggered all these reports.
The data seems to come from the Department of Transport here.
What we need are stats on the total length of streets that went from 30mph to 20mph in the period 2010 to 2011. These stats may or may not be collected. Local authorities will be able to calculate them, and someone at the DfT can collect them, but that may take a year or two.
It is just possible that there are other factors in play, such as that people feel more free to run about in 20mph zones, but if anyone is going to argue that, they are going to have to substantiate it, in view of the experience in Kingston upon Hull quoted above.
As things stand, we have a major issue of misinformation based on bad handling of statistics on the part of the BBC and many other media outlets. I am going to complain to the BBC here and I hope that you will too.
Here is what I put into the BBC complaints form, so you can copy and modify it.
Report (carried also on radio and possibly TV) says "Casualties on 20mph roads up by quarter in 2011... Motoring organisations have questioned the safety success of 20mph zones".
However, the numbers are not related to the total extent of 20mph zones. These zones are being implemented all the time, so if they are expanding, the amount of accidents that will occur on them can be expected to increase proportionally. A more realistic statistic would be the number of accidents per length of street, or an even more sophisticated stat such as unit length of street usage by vehicles and people.
There is much good evidence that 20mph zones are safer. Your report calls that into question in peoples' minds. A correction is needed. I would also like to know who issues the press release that forms the basis for your report, which is also carried in many other news outlets.
More on this here: www.greenerblog.blogspot.com
Also I will complain to the Press Complaints Commission, even though they are well known for being not just a toothless paper tiger, but is also a moth-eaten toothless paper tiger with grubby marks where Rupert Murdoch has wiped his feet or something on it. Still, we must go with what we've got, because it is simply not good enough for the media to use statistics in such a sloppy way to mislead the public, especially in a way that puts children's lives at risk and promotes the interests of the motoring lobby.
[Update 18th August]
I have received a civil answer from the BBC complaints; It reads:
The data in the story was supplied by the Department of Transport. Prior to publication, and again since, we went back to the department to ask whether they could also supply details of the number of 20mph zones so that we could put these figures in some context. They were unable to do so.
[Update 18th August]
I have received a civil answer from the BBC complaints; It reads:
The data in the story was supplied by the Department of Transport. Prior to publication, and again since, we went back to the department to ask whether they could also supply details of the number of 20mph zones so that we could put these figures in some context. They were unable to do so.
We are satisfied that our story reports the available facts in a measured and even-handed way without suggesting any conclusions should be drawn. In the third paragraph, we point out that most of the casualties suffered minor injuries. And in the fourth paragraph we contrast the casualty levels with the much higher figures reported on 30mph roads. We also spoke to a range of experts.
We do accept it would have been better if we had included a line in the story explaining that the Department of Transport could not provide details on the number of 20mph zones and whether this had increased in the corresponding period. (We cannot simply assume that there has been a huge increase as we have seen many reports of 20mph zones being discontinued in the past two years.)
We have now added this line to the version of the story which can be found in our archive. Thank you again for contacting us.
Which seems fair enough from the BBC's point of view. Why the DfT is putting out partial data is another matter. Maybe they want to stop the expense of creating 20mph zones. I will try to take it up with them.
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