Now a test could make you rethink driving even after that first drink, because even if you're under the limit, you may not be fit to get behind the wheel.
The new face of Australia's growing drink driving problem is 'women who lunch'.
Many women are making the same mistake - playing Russian roulette by misjudging how much they can drink, before exceeding the 0.05 limit. Some are even doing the school run afterwards.More stories from Today Tonight
Professor Ian Hickie from the Brain and Mind Institute says the national drink driving guidelines don't always work and give a false sense of security.
"It depends on what you've had to eat when you're drinking alcohol, it depends on how quickly you absorb that alcohol, and it depends how your body, your liver metabolises alcohol to the rate you actually reach," Professor Hickie said.
The point is no one can tell how much they can drink before hitting, then exceeding, the limit.
To prove it three ladies were brought together to put the theory to the test. The ladies, all in different ages, and of different weights and heights, downed two bottles of wine (or about four standard drinks each) over a two hour lunch.
Government guidelines recommend women have one standard drink an hour if they plan to drive. But according to the Australian Bureau of Statistics the percentage of females caught drink driving is at a record high.
Last year 21 per cent of those convicted were women, up from seventeen per cent a decade ago. 58 per cent of those are under the age of 35.
Part of the problem, according to Paul Nicolaou from the Australian Hotels Association is that "you've got different bottles, different glasses and sizes and different contents of alcohol - it's just confusing for the consumer.
Nicolaou says the mistake many people make is thinking a glass of wine they order in a bar is a standard drink - whereas it's usually one and a half.
"They all have different alcohol contents."
As a rule of thumb a standard glass of wine is about 100ml. Where women also get into trouble is cocktails.
"A mojito is a very popular drink with women and it has two nips of vodka in that - most people think it's one," Nicolaou said.
According to motoring expert John Cadogan "you might think 'I'm just under 0.05, I'm okay to drive'. In fact you're twice as likely to crash at that level."
After having two standard drink over an hour over lunch the ladies are under the legal limit, but when they get behind the wheel their reaction and stopping times are all dramatically affected.
Driving expert Ian Luff says "drinking alcohol and driving is a bad cocktail, it really is dangerous. Even a small amount of alcohol will impair judgement and obviously the more you drink the greater the risk. I just believe don't drink and drive."
This reporter is on Twitter at @PippaGardner7