On Monbiot on speed cameras

Monbiot has written an excellent article (as though he has any other kind, the bastard) on speed cameras in the UK. Specifically the fact that they work, and that people who say that they don’t invariably sacrifice good analysis at the alter of their own recklessness and selfish libertarianism.

Last week the transport minister Jim Fitzpatrick said he intends to double the penalty for drivers who break the speed limit by a wide margin. This means that people could lose their licences after committing just two offences. The papers are furious. The petrolheads have called for a petition which “will get as big a response as the road pricing one.”

Yes it’s brave, but not quite as brave as you might think. Despite endless attempts by the media to trivialise it, an RAC survey reveals that 62% of drivers still regard speeding as a serious offence. Even more surprisingly, 82% of British people surveyed approve of speed cameras, and the percentage has risen slightly since the mid-1990s. There is a genuine silent majority here, which is rarely represented in the media.

Please bear in mind that I am leaving out his endnotes as I go (his references can be found with his article).

My interpretation of this is that the doubling of the penalty is a function of this “silent majority” (a term I tend otherwise to dislike, co-opted as it most often is by Tory/Conservative/Liberal/Republican racists – Liberal referring to the Howard government), rather than deterrence: re-doubling the number of speed cameras would do that.

Monbiot spends some time criticising the BBC and their programme Top Gear (about which the less said the better, other than that it is petrol-head/hoon-legitimising utter waste of spectrum). Then to the interesting parts:

In Saturday’s Telegraph, Christopher Booker and Richard North published a long article appropriately titled “Speed cameras: the twisted truth”. A sharp decline in the death rate on the roads suddenly slowed down in the mid-1990s. They attribute this to the government’s new focus on enforcing the speed limits, especially by erecting speed cameras. What they fail to mention is that deaths started falling sharply again in 2003, after the number of speed cameras had doubled in three years.

They use similarly selective data to argue that there is no evidence that cameras have reduced deaths even at the spots where they are deployed. They hang their case on an oversight in a government report published in 2003. The report claimed that the accident rate had fallen by 35% where cameras had been installed. Booker and North rightly observe that it had failed to account for a statistical effect called “regression to the mean”. There might have been an abnormal blip in the accident figures, which would have returned to background levels of their own accord. The truth, they maintain, is that “speed cameras actually increased” the rate of accidents.

But what Booker and North fail to tell their readers was that in 2005 the government conducted a new analysis, which took account of regression to the mean. The new figures showed an average reduction of 19% for collisions which caused deaths or injuries after speed cameras had been installed.

Not to pass up some decent statistics. The government’s report found statistically significant reductions in Killed or Seriously Injured and Personal Injury Collisions (KSIs and PICs – their terms of art):

results1

Overall,

regression2

Eco 145 students: confidence intervals! Everyone else: when a 95% confidence interval is entirely negative (positive) the effect being estimated is statistically significantly likely to be negative (positive). I.e. irregardless of those ‘blips’ to which people best ignored like to refer.

The report itself is quite impressive. Specifically, they found that:

  • Vehicle speeds were down– surveys showed that vehicle speeds at speed camera sites had dropped by around 6% following the introduction of cameras. At new sites, there was a 31% reduction in vehicles breaking the speed limit. At fixed sites, there was a 70% reduction and at mobile sites there was a 18% reduction. Overall, the proportion of vehicles speeding excessively (i.e. 15mph more than the speed limit) fell by 91% at fixed camera sites, and 36% at mobile camera sites.
  • Both casualties and deaths were down – after allowing for the long-term trend, but without allowing for selection effects (such as regression-to-mean) there was a 22% reduction in personal injury collisions (PICs) at sites after cameras were introduced. Overall 42% fewer people were killed or seriously injured. At camera sites, there was also a reduction of over 100 fatalities per annum (32% fewer). There were 1,745 fewer people killed or seriously injured and 4,230 fewer personal injury collisions per annum in 2004. There was an association between reductions in speed and reductions in PICs.
  • There was a positive cost-benefit of around 2.7:1. In the fourth year, the benefits to society from the avoided injuries were in excess of £258 million compared to enforcement costs of around £96 million.

Top Gear really is a stupid programme.

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1 comment so far

  1. crywithme on

    Wow?


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