As I mentioned in yesterday's post, back in 2014 I, along with Matt Roskruge and some willing research assistants, spent five nights in November and December surveying every seventh person on the street in Hamilton CBD, and taking a breathalyser reading from every one of them that was willing (which was almost all of them). We also asked each of them to guess their breath alcohol concentration (BrAC) before we took the breathalyser reading, then we compared their guesses with the actual measured BrAC from the breathalyser. The results are now out in a new paper forthcoming in the journal Alcohol and Alcoholism (gated, but you can email me for an offprint), co-authored by myself, Matthew Roskruge (Massey University), Nic Droste and Peter Miller (both Deakin University).
The headline result is that drinkers generally have no idea of the breath alcohol concentration. The key figure is presented below. Each dot represents a combination of an estimated BrAC (what the person guessed it would be) and actual BrAC (what was measured on the breathalyser). If people were good at guessing their BrAC, these dots would be close to the solid, 45-degree line, but as you can see, they are all over the place. Notice also that there are more dots above the 45-degree line than below it - drinkers were slightly more likely to overestimate their BrAC than to underestimate it. The dashed line is the trend, which at least shows that those who had higher guesses were on average more intoxicated (higher BrAC), so the news in terms of guessing is not all bad (it wasn't all just random noise!).
You can also see that the dashed line crosses the 45-degree line at a level of about 487. At BrAC levels above that, drinkers on average tend to underestimate their BrAC, but below that, drinkers tend to overestimate their BrAC. That is probably a good thing since if you want to prevent drink-driving, you want people close to the legal limit (which is now 250 mcg/L) to generally over-estimate their BrAC, increasing the chances that they will be cautious and avoid driving (none of our sample who were over the legal limit admitted to an intention to drive).
Finally, there was a good reason that we ran the survey in November and December of 2014. The legal BrAC limit for driving in New Zealand decreased from 400 mcg/L to 250 mcg/L on 1 December 2014. We hypothesised that drinkers had no idea how close they were to the previous limit, let alone the new limit. It turns out we were right. We didn't even bother presenting those results in the paper, since drinkers were so bad at guessing their BrAC.
However, if you don't know if you're over the limit or not, but you do know that the limit is decreasing, then perhaps that increases the chances that you take the less risky option of not driving, even if you've only had one or two drinks? Based on informal discussions we had with some of the research participants, it appears that might have been what was happening (although, it is also worth noting that there were many people who were completely unaware of the change in the legal BrAC limit, despite plenty of media coverage).