Drink driving limits revisited

By Eric Crampton 30/09/2013 2


A member’s bill proposing a reduction in the drink driving limit was drawn from the ballot.

Last year, I ran some ball-park numbers on what would be needed for it to make sense to reduce the drink driving limit from 0.08 to 0.05. While you would have some reduction in crashes among drivers in that range, you’d also have losses in consumer surplus among drinkers who would have driven home safely at 0.07. My very rough conclusion was that if those in the .05-.079 range suffered reduced consumer surplus of less than $4 per night out, then the policy could make sense, and if those in that range suffered losses greater than $4 per night out, then the policy would fail.

Let’s suppose that the costs of non-fatal accidents are on par with the self-imposed fatality costs incurred by drinking drivers so the $4 is ballpark ok (but note the overestimate problems as it includes people in the 0.08 to 0.105 range). Do you expect that most people enjoying a night out would be willing to accept $4 to be subject to a 0.05 rather than a 0.08 limit? Don’t tell me “Oh, I would, because I never have that much anyway.” This question isn’t for you. This question is for those who go out for the night and either worry that they’ve exceeded 0.05 or know that they’re in the 0.05 to 0.08 range. As you walk into the bar someone offers you $4 and says “You can have this $4 if you can guarantee that you’ll stay under 0.05 tonight.” If most drinkers subject to the risk take the $4, and if the $4 is ballpark correct, then moving to 0.05 makes sense. If you’d have to offer them more than $4, then it’s a value-destroying proposition.

The numbers were pretty rough. I was then waiting for the folks at MoT to provide me with some more recent figures on the proportion of drivers on the road who are in the .05-.08 range. They never got back to me after promising me the numbers, but I didn’t pester them either as I got busy with other things. It looks like I’ll have to start pestering again so I’ll have a chance of running the numbers before the Bill is debated.


2 Responses to “Drink driving limits revisited”

  • I’m following up with MoT for the more recent data on population distributions at different BAC levels.

    At 50 mcg, two pints of decent beer over two hours COULD put you over the limit. While there’s definitely an increase in accident risk from 50 to 80, we really need to put it into cost-benefit to reckon whether dropping to 50 makes sense.

    I expect that the best argument for reducing the drink drive limit from 80 to 50 isn’t the reduction in accidents among those at 70, since there’s hardly any of those. Rather, it would have to hang on some argument that once folks get to 70, they make bad decisions about avoiding getting to 110. But I’ve yet to see any data that could support that kind of argument.