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.
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 »