Tuesday, 5 November 2013

Alcohol's awful superpower - road toll edition

I'd missed this when it came out. From the Herald, regarding the prior Labour bill seeking to reduce the BAC limit:
The bill's policy statement said: "There is demonstrable evidence and research already available which shows enough driver impairment between the proposed 0.05 limit and the 0.08 limit to warrant action. A drug and alcohol expert from the United Kingdom has estimated that this measure could reduce our road toll by two-thirds as it would alter driver behaviour." [emphasis added]
 From the latest road crash stats:


Alcohol or drugs were contributing factors in 30% of fatal crashes and 14% of injury crashes. Note that percentages do not sum to 100% here - many things can contribute to a single crash. 

If we assume that alcohol and all other drugs disappeared, and that every crash that they contributed towards would never have happened, the best you could hope for is getting rid of 30% of fatal crashes and 14% of injury crashes.

Alcohol must have some kind of awful superpower if knocking the BAC down to .05 from .08 can get rid of 2/3 of the road toll. 

Note that this wasn't the Herald screwing up - it's a direct quote from the legislation. Now maybe you could ask just why the Herald didn't stop to think "Hey, waitaminute. Is it even plausible that alcohol's involved in 2/3 of the country's road deaths, let alone that reducing the BAC limit from .08 to .05 could cut the road toll by anywhere near that much?"

I think I understand why there's so much support for reducing the BAC to 0.05. People think that two-thirds of the road toll is caused by booze.

I wonder if Lees-Galloway would like to bet with me on next year's road toll if National goes through with the .05 limit. I'm happy to put some money on that the total 2013 road toll will not be outside the 95% confidence interval you'd get from a simple time trend on the existing data.

HT: TWR