Sunday 26 January 2020

People don't know how much they can drink, relative to the drink driving limit

Some new research was highlighted by the New Zealand Herald this week:
Motorists are still confused about "ambiguous" drink-drive laws, with many believing they are fine to get behind the wheel after three drinks.
New research released today shows the average motorist has a significant knowledge gap when it comes to the laws around drinking and driving.
While nearly three-quarters of motorists are confident they understand the rules, only 22 per cent actually know the correct legal adult limit - and 20 per cent believe they can have three or more drinks before driving.
The findings suggest people are basing how much they drink around the number of drinks they think they can have, rather than blood alcohol content which can vary greatly from person to person.
The research was undertaken by DB Breweries, and despite spending a fair amount of time searching, I can't locate it anywhere online. However, even taken at face value based on news reports, it doesn't really add anything new, beyond my own research that was published in 2018 (as I blogged about here), but based on data we collected in 2014:
The headline result is that drinkers generally have no idea of the breath alcohol concentration.
Our research was better than the DB Breweries research that has been reported in the media, because even if someone believes they can have two drinks and still be under the drink-driving limit, the limit is measured in terms of breath alcohol concentration, not in terms of the number of drinks. So, it doesn't matter so much if people can't judge the number of drinks they can have; it matters much more if they can't judge their breath alcohol concentration relative to the limit. And, as the quote above from my earlier blog post notes, people really have no idea (and in our follow-up research last year, which I blogged about last week, we found that informally things have not improved).

We found that drinkers at or below the breath alcohol limit for driving tended to overestimate their breath alcohol concentration, which is probably a good thing if your goal is to reduce drink driving in total. Most people are moderate drinkers, and having moderate drinkers avoid driving therefore limits the total number of drink drivers.

However, the results at the top end suggested that the heaviest drinkers underestimated their breath alcohol concentration. Some of those drinkers even thought they were under the drink driving limit, which is a real worry.

The problem here is not that there are ""ambiguous" drink driving laws" (as the Herald claims). The law is not ambiguous. However, people's understanding of how the limit relates to the number of drinks they can consume and remain under the limit is problematic. There doesn't seem to be an easy solution to this either. The best advice is that, if you are drinking and you are unsure, you should not drive.

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