Sunday, 1 August 2010

The myth of numbers

Transport Minister Steven Joyce has not agreed to lower the alcohol limit for drivers, to howls from various road safety groups. Apparently, by not reducing the limit from 80mg to 50mg, Joyce has condemned around 15-30 people to death each year, and another 300-600 to serious injuries. Really?

This is the "myth of numbers" again; the idea that if you can measure something, then altering that number will somehow affect the cause. In fact, drivers with alcohol levels between 50 and 80 barely register on the statistics, as this study of over 1000 dead drivers shows. The study was conducted by Dr Helen Poulsen of Environment Science and Research (ESR) between 2004 and 2009.
The group with blood alcohol levels between 50 and 80 had 2% of the fatal accidents. Is that worth a law change? The Minister's own bureaucrats think so.
The Ministry of Transport has calculated that lowering the limit to 0.05g (50mg breath level) and increasing enforcement and public awareness would save 15 to 30 lives a year and prevent 320 to 686 injuries. (NZ Herald)
Before we panic about the Government's callous indifference to the welfare of motorists, let's read that again. "Lowering the limit" - and "increasing enforcement and public awareness" - where did that come from?

We can measure breath alcohol levels (even that is a bit dubious, but let's agree for now) but we can't quantify increasing public awareness and we can't afford to increase enforcement. So we measure the magic numbers and ignore the factors that really would help cut the road toll.

I don't propose that we try this, for many reasons, but I'd be prepared to bet that if we lowered the limit to 50 and kept policing as we do presently, the change to the road toll would be statistically insignificant - less than the margin of error in any survey, in fact.

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