Have you ever decided to drive your car after a few drinks and wondered whether you were close to the limit - if it was safe?
If you live in England, Wales or Northern Ireland, you can drink a surprising amount of alcohol before you go above the current drink-drive limits.
But even if you are within those limits, you are up 13 times more likely to kill yourself (and anyone else you crash into) than someone who has drunk no alcohol.
In this week's Trust Me, I'm a Doctor, we look at how the drink-drive levels were set - and whether the current ones are too high.
The UK's drink-drive limit is 80mg of alcohol per 100ml of blood. This level was set in 1967. But in 2001 the European Commission said new safety research suggested the limit should be cut to 50mg per 100ml.
Most European countries changed. The UK didn't.
In 2014, however, Scotland did reduce its legal threshold to 50mg, and Northern Ireland has signalled its intent to do the same - leaving England and Wales with some of the highest limits in Europe.
Prof Richard Allsop was an adviser on the UK's 1967 limits.
They were based on a major US study where researchers looked at all car accidents over the course of a year in Grand Rapids, Michigan.
Based on that data, Prof Allsop calculated that by the time a driver was above the 80mg/100ml level, the risk of involvement in a collision of any kind - from a tiny fender bender up to fatal crash - was roughly doubled. So that is where the drink drive limit was set.
But a 2014 US study that looked at 1,766 fatal accidents over a three-year period indicated a driver was 13 times more likely to be involved in a fatal collision if they were above the 80mg/100ml limit.
Even at the 50mg/100ml level, you were still five times more likely to have a fatal collision, it said.
So how many lives does Prof Allsop think are currently lost due to drink driving?
"The official figure is about 240 a year," he says, "but what is not reflected in that figure is that there are quite a lot of collisions happening where no-one has been driving over the [80mg/100ml] limit but, nevertheless, they are having collisions that they would not have had if they not been drinking - and the best estimate that we can make of the hidden drink drive deaths is about another 60.
"I'm sure we should lower the limit - down to 50mg/100ml at first, then perhaps after a time down to 20mg/100ml, which is what they have in Sweden."
A spokesman for the Department for Transport, which controls the alcohol limit for driving, said: "Drink driving is completely unacceptable, which is why there are tough penalties and rigorous enforcement in place for those who do this.
"The government currently has no immediate plans to lower the drink-drive limit.
"However, we keep this policy area under constant review and will always welcome robust and accurate evidence on this subject."
Trust Me I'm a Doctor continues on BBC Two on Wednesday, September 19, at 20:00.