Speed Camera Policy will result in Speed Cameras 'causing' accidents

Date - 22 June 2004

Some time ago I wrote about the speed camera partnerships being rubbish at statistics and effectively using regression of the mean to inflate their effectiveness. [link] If you've forgotten what regression to the mean is, a brief explanation is at the bottom of this page.

At the time I suggested that either they were being dishonest and using bad statistics to further their cause, or they were simply stupid.

Never attribute to malice what can be explained by stupidity. 
- M.N. Plano

Now they have a big list of sites where cameras were not effective - 743 of them to be exact. [link]. If they remove these cameras - as popular wisdom would suggest you should - and then measure the changes you run into another set of statistics where regression to the mean will have a big effect.

As demonstrated previously, if you divide a population into two brackets, high and low, we expect that afterwards both will regress towards the true mean, so the average of the high numbers will drop, and the average of the low numbers will increase. In this case we expect that at 'unsuccessful' cameras [high accidents] the number of accidents will fall, and that at 'successful' cameras [low accidents] the number of accidents will increase.

Oh Dear

I predict, that purely by the appalling use of statistics practiced by the Safety Camera Partnerships, that they've shot themselves in the foot and that, the statistics for the next few years will show that accidents decrease when cameras are removed and will increase slightly when cameras are left. This does not mean that the cameras are causing accidents, but, If you believe that the current decrease in accident rate is purely due to speed cameras, you must also believe that the subsequent increase I predict will be due to speed cameras.

The Camera Parnerships are screwed, in order to use their simplistic measures 'look at the statistics at the camera sites', you have to accept the conclusions 'remove the cameras that don't work' which bring on the result 'cameras cause accidents'. They're only way to avoid the 'cameras cause accidents' charge is to use proper statistics in the first place, but they've spent far to long ignoring real statistics because the numbers didn't look as good.

I'm a safety camera partnership, what can I do?

Start removing cameras that have an excellent safety record, that will bias the regression to the mean in the other direction.

Hope that cameras really have a massive effect on the accident rate such that more than half of the claimed reduction was real, rather than statistical. If that's the case the statistics will still work out in your favour but not that well.

Do arbtrary major works on the road so you can remove the cameras from the statistics - obviously this will be hard to pull off if you've put a camera on every road in your county.

Arrange a major accident for camera sites just after the cameras have been removed. So, for example, if a camera site had a poor safety record, encourage small children to play chicken with cars at the site. The publicity value of a bunch of squashed six year olds at a site that used to have a camera to your organisation is priceless. You could easily make up an 'economists can prove this is cost effective' argument too - compare the effectiveness of the publicity of a pair of squashed six year olds against £10,000,000 of advertising. This is of course complete rubbish - but probably better than the typical Safety Camera Partnership statistics.

I'm an anti speed camera campaigner what can I do

Stop arguing about regression to the mean, they're not listening, the public doesn't understand or care. Simply push them to remove the cameras that don't work and keep the ones that do and wait for the statistics to fall in your favour. You should also push for the installation of cameras at places with no accidents, e.g. outside schools, hospitals etc. since the accident rate can only go up.

Be prepared, the statistics are going to show that cameras cause accidents, make sure all your publicity material is ready.


Do your statistics right, otherwise your flawed methodology might bite you in the arse.

What is regression to the mean?

It occurs when a random variable X is used as the basis to select subjects to measure a change in X. A simple example is the number of times a dice rolls a six. If you roll ten dice six times each, some will not roll a six, some will roll one, some will roll two, but on average it will be one six per dice. If you then take only the dice that got two or more sixes, on average for those dice that's two sixes per dice. If you redo the test with just those dice the new average will again be one six and it appears that you've reduced the number of sixes per dice - you haven't it's just that your initial selection had an artificially high mean and it's now regressed to the true mean on the second test.

A note about offensiveness

I've been told this article is offensive. It is. It is not offensive because it uses the word 'fucked' in the URL. It is offensive because it seriously presents the option of murdering small children as a cheap form of advertising.

And so it comes to pass

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