Some driver education courses apparently stress the importance of counting drinks and leaving adequate time before driving to avoid being over the limit. The wisdom of encouraging intoxicated people to try and perform complex calculations still escapes me.
Another reason the 'how long would you wait before driving?' type of question is useless is that it presumes the information the driver would rely on would be accurate.
This article by Felix Salmon from Reuters suggests otherwise.
Essentially, people like to think of themselves as sophisticates who go to art-house movies, even if in reality they’re much more likely to sit slack-jawed in front of some reality TV show. In the case of wine, they like the idea of buying something grown-up, with a relatively modest amount of alcohol; when it comes to drinking what’s inside, however, the more heat the better. So wine labels consistently show lower alcohol content than what’s inside.
This is especially true in the new world. Out of 43,908 tested new world wines, 24,561 under-reported their alcohol content, with the reds averaging 14.1% alcohol while claiming just 13.6%, and the whites averaging 13.5% while claiming to be 13.1%.
Interestingly, the smaller number of wines which either over-reported their alcohol content or got it exactly right all reported pretty much the same levels of alcohol: 13.1% or 13.2% for whites, and 13.6% or 13.7% for reds. On average, it seems, wine will just say that it’s 13% if it’s white and 13.5% if it’s red, but in reality it’s likely to be higher than that.
The report Salmon quotes from the American Association of Wine Economists can be found here. It reports that Australian wine is, on average, 0.55 percentage points higher than marked. Not a big difference, but in these matters tiny differences can have huge consequences.
It may make you wonder how accurate the labels on beer and spirits are. And does alcohol distribute itself evenly around a beverage, sink to the bottom or float to the top? (The internet consensus seems to be that alcohol rises above most other liquids, but then divides on whether the effect is noticeable once a drink is poured).
So always read the label, but it might not help much. In Victoria the defence of honest and reasonable mistake does not apply to drink-driving offences: Skase v Holmes (Unreported, Supreme Court of Victoria, 11 October 1995, Vincent J)