November 5, 2013

Can we bring out the real numbers now?

So, the decision has been made and the blood alcohol limit will be lowered.  Perhaps now we can start using realistic numbers for the impact.  The story in the Herald today shows the problem, although it’s actually much better than anything I’ve seen in the mainstream media previously:

The changes come after a two-year review of the impact of lowering the legal blood alcohol limit by 30mg suggested 3.4 lives would be saved a year and 64 injury-causing crashes avoided.

It would also save $200 million in social costs over 10 years.

“Alcohol impairment is a major cause of road accidents in New Zealand, with an average of 61 fatalities, 244 serious injuries, and 761 minor injuries every year caused by at-fault drivers who have been drinking,” said Transport Minister Gerry Brownlee.

“The social cost of these injuries and fatalities is $446 million – a huge sum in a country of our size.”

In the first paragraph the estimated benefit based on actual research is quoted. That’s a big step forward. The second paragraph is just wrong: the social costs aren’t in addition to the lives saved and injuries prevented; that’s where the social cost numbers come from. And it’s multiplied by ten years.

In the third and fourth paragraphs Mr Brownlee is quoted as justifying the change by quoting total costs of drink driving. The social cost number in the fourth paragraph is 22 times larger than the actual estimated benefit. You’d think that sort of discrepancy would draw some journalistic comment.

And later in the story we are told about a victim of a drunk driver. A driver whose blood alcohol concentration was 190mg/100ml, more than twice the existing legal limit, and who was duly convicted and sent to prison under the old laws. Not the sort of person whose behaviour is likely to be affected by this change.

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Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »

Comments

  • avatar

    I’ve had an initial go at the numbers over at Offsetting. I really need to see their underlying work. MoT tells me that 1.3% of drivers are in the .05-.08 range; their 2011 stats have those drivers in 1.5% of serious crashes (fatalities or serious injury). We’d expect then at best to reduce crashes down to being proportionate: the excess crashes would be gone, not all crashes. We wouldn’t avoid 29 serious crashes per year; we might avoid 4-5.

    At least that’s my first cut at it, without having access yet to their study. Will pester them more….

    11 months ago Reply

    • avatar
      megan pledger

      When people drink they don’t know what BAC number they are – they prettty much have to guess. When the upper limit of the range goes down then that tends to have a chilling effect across the board; not just on those who were in the safe range but now aren’t. Therefore, the bill is likely to have a wider effect than your numbers show.

      11 months ago Reply

      • avatar
        Thomas Lumley

        That’s the sort of thing the government regulatory impact models try to take into account. We don’t know how well they do it in this case, but their estimate is actually lower than Eric’s.

        11 months ago Reply

        • avatar

          Brownlee seemed to be suggesting we’d avoid all 29 crashes rather than just 5 crashes or so. He’s citing about 3 deaths avoided; I think about 1. But we could get that kind of variance easily out of year-to-year differences in accident rates so I’d need to see their numbers.

          Agree with Megan that we could also see some decreases in the > 0.08 range, but boy is that hard to put a number on.

          11 months ago

  • avatar
    Joseph Delaney

    I am not an expert on New Zealand law (by any possible definition of expert, including guy willing to loudly broadcast his ignorance in a pub). But it appears from a cursory search that the penalty for the incremental drink drivers (blood between 50 and 80) will now be a NZ$200 fine.

    Let me observe, as a student of the Seattle streets in the United States, that the possibility of a $200 fine does not seem to deter drivers in the slightest when it comes to speeding. Nor do laws aimed to curbing cellphone use (as many of the same fast drivers are happy to be holding a cellphone) where the fine is US$124.

    Now maybe the dollar figures are incomparable of New Zealanders are more risk averse. But I am unclear if there really will be any accidents prevented or if the real benefit is a small increase in government revenue.

    11 months ago Reply

    • avatar
      Thomas Lumley

      There’s loss-of-license for the second offense.

      11 months ago Reply

      • avatar
        Joseph Delaney

        Ok, and so it is good that I admitted my ignorance up front. I still wonder how effective the deterrence effect will be on drivers who don’t have a first offence, but at least the consequences increase to something that might be a meaningful cost to the driver.

        11 months ago Reply

  • avatar
    Chris Morris

    From Eric’s cut, which seems to pass the sniff test, it would seem that the accident rate is very close to proportionate. With the error bars it could be statistically the same. That means the 3.5 persons is probably an upper estimate.
    Not good lawmaking but emotional stuff.

    11 months ago Reply

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