May 2011 20

George Monbiot has written a column this morning about the increase in road casualties in Oxfordshire following the deactivation of speed cameras. The critical point, as he notes is that you can’t learn much from such a small sample:

“So far, the sample size is too small and the period too short to be sure that the deaths and injuries around the county are linked to the switch-off. The experiment would have to run for longer and be conducted over a wider area. Any volunteers?”

After all, Swindon switched off their cameras and saw no increase in accident rates. The problem is that he casts speed cameras the safe option, and switching them off as the experiment. But in many ways speed cameras are themselves an experiment as road safety policy was focused overwhelmingly on enforcing speed limits, with less attention paid to other problems. We wrote a report last year aiming to test how that experiment had worked out.

If you look at the pattern of casualties before and after cameras were introduced, it looks very much like it didn’t go well:

To see if there was a significant change in the trend, we used a statistical test called a Chow Test. The results suggested there was.

Some people got very overexcited when we released our report. They looked at the graph and imagined extrapolating forward the blue pre-1990s trend line to a time when there would be negative deaths. Of course, at some point improvement was going to slow and then stop. But there is no sign that is what had happened in the early nineties. If it was, you would expect improvement to be slowing before speed cameras were introduced – it wasn’t – and to continue to slow afterwards – it didn’t. There is no particular reason to think that, in the early nineties, diminishing marginal returns drove the slowdown in road safety improvement. The line is on that graph to show the extent to which the two trends differ, not to make any grand prediction about an inevitable and eternal trend of road safety improvement.

Just switching speed cameras off doesn’t constitute a road safety policy. But there is good reason to think that the cameras, and the associated monomaniacal focus on speed limits, have been a mistake. We got better results before with a more balanced approach which we should try to reconstruct over time.

Matthew was the Chief Executive of the TaxPayers' Alliance, author of Let Them Eat Carbon and editor of How to Cut Public Spending (and still win an election)

  • Clifford Singer

     At risk of sounding overexcited, we’ve responded to the TPA stats here:

    • Sad But Mad Lad

       I would say “Think Before You Make A Video”.

      A long video making points about the colour of the lines on a chart and then extrapolating a trend as if it would be reality and saying a “complex forumla” was used to hide your ignorace of statistics.

      And deaths on the road in Oxfordshire. Is that really deaths or just KSI figures which includes injuries. And it doesn’t show that the increase was 2 (two) compared the previous period. Statistically insignificant. Also, where were these two deaths? At speed camera sites. I very much doubt it. Were they result of two accidents or one accident?

      The blue line is an extropolation of the trend. In reality the line would waver around a bit and would never get to zero. In reality you can never stop 100% of deaths. To imply that the TPA was stating that is to bring your whole argument into disrepute and show it be a joke, just like your group is.

      What you should have argued about was the change in the trend around the time the cameras were introduced. Is the change directly attributable to cameras or is it just the natural bottoming out of the numbers as it gets harder and harder to stop all deaths on the road (the law of diminishing returns). Correlation is not necessarily causation and all that.

  • Keith Peat

    But during the switch off there were no deaths at any of the sites. Clearly they are using the Killed/seriously injured mixture where serious injury is subjective and a matter of opinion whereas death isn’t.  There is no argument about death and we can assume that  
    injury would be in proportion so why include it at all? When you just use fatals, then it’s a whole different story .

    There are several factors that Monbiot doesn’t know. 1) Speed limits are arbitrary and totally unscientific and set by non-experts. 2) Simply exceeding one of these numbers cannot cause anything any more than not to won’t. 3) Accidents then are not caused by ‘speeding’ but too fast, often below the limits as indeed most accidents are. & 4) Speed cameras, like your speedo cannot see too fast but just a speed. In short speed cameras cannot measure accident causes but simply a speed. 

  • David Atherton

    It looks like Monbiot is basing his opinion on an increase from 60 accidents a year to 62 accidents a year, about 3% and not statistically significant.

    “Data released by Thames Valley Police revealed in the six months after they were switched off there were 70 slight injuries, 13 serious injuries and no fatalities from a total of 62 accidents at fixed camera sites.

    In the same period the year before [August 2009 to January 2010] there were 55 slight injuries, 13 serious injuries and no fatalities from a total of 60 accidents.

  • Anonymous

     The speed camera tax has done much damage to the relationship between the police and the motoring pubic – particularly as, unlike the general public, the police go about their daily business without fear of prosecution.    In 2006 the Metropolitan Police force acquired a staggering 48,222 speeding tickets – in all but 57 cases the Notice of Intended Prosecution was quashed by their Chief Constable. In Hertfordshire the figure was 12,066 with all except 1 case quashed – and in Derbyshire the figures were 7,835 cases with just 2 prosecutions. These statistics prove a number of points: 1. In the Nu-Labour totalitarian state, the police really were above the law. 2. These machines really are about gathering revenue from the hard pressed motorist, because I don’t believe that the driving standards of our police officers are any worse than anyonelse. and 3. It is only a matter of time and miles before even the most vigilant and careful motorists are caught out.

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  • Steve Collins

    The problem is that the alternative is a lot more expensive.

    Paying lots of traffic officers to patrol whole swathes of our road network looking out for speeding or badly driven vehicles will cost an awful lot of money.

    How many of those traffic officers would the £1bn per year cuts from police budgets pay for? 

    • DPU

      Yes but prosecuting and over-slowing thousands of perfectly safe drivers doesn’t resolve that Steve. Setting limits too low, and creating more offenders to photograph is not the answer. We would just keep photographing thousands of accidents without addressing the problem so why are there the speeders in such large numbers? And if speeding causes accidents, why aren’t they all crashing too? Now the authorities know all this and that it keeps them in jobs. Just follow the money.

  • Robert Brown

    Yawn…… Monbiot pontificating again. The useless man is a climate change nazi, just read some of the bilge he contributes to the guardian. He has a serious aversion to reality, cameras were introduced as a  cash generating system, they cannot catch drunken or reckless drivers. Hang the expense, introduce hundreds of unmarked cars with top rated drivers and catch the imbeciles that infest our roads and cause tragedies and expense daily, make people uneasy about breaking the law of the roads instead of doing so with disdain and slim chance of being caught.

  • Rob Fisher

    I love the Taxpayers Alliance and I support your aims. But I think you need to disown that graph. You can’t just go around drawing straight lines on graphs without a theory that explains why the line should be straight: it makes you look like a climate scientist. Eric Raymond eloquently points out the danger of this.

    You say that “The line is on that graph to show the extent to which the two trends differ” but the two trends are probably just figments of your imagination. You could draw any number of straight lines through that graph. About the most you can say is that speed cameras did not accelerate improvements in road safety. There almost certainly are diminishing returns and any number of other factors. There is no reason to expect any linearity anywhere, so the Chow test is probably not applicable.

  • Bill

    Lies Lies Lies and statistics,

    the actual turnout of figures, don’t take into account,

    the improvements in car  safety, Crash bags came in and were being widely used on NEW cars around the same time as speed cams and by now can be found in most cars. 

    the change in the age of drivers, the numbers in the high risk group of 18-24 reduced , due to demographics, cost, and change of habit. Thus less in this number.

    a disproportionate number of deaths were always in the Motorcycle fraternity and a concerted effort to educate (look, think , etc ) has brought about a reduction in this group. Motor cycles , can in the main, avoid speed cams with small or unviewable number plates.
    However the rest of us in cars, still get to pay the wretched speed cam fines , which serve no useful purpose what soever, other than keeping some in food and  water together with their  golden pension , and some such as the ex CC of north wales with a drum to bang. .What damage that man has done.