ha, I first read that as 80ml of alcohol per 100ml of blood...
but then I realised my reading mistake...
not meaning to piss on your chips about the 12 hours before driving... but I've heard this general fable before...
when you are drinking, it takes 1 hour for a unit of alcohol to be absorbed into your system, and two hours after that for it to clear...
For this example we'll assume that 1/2 pint is one unit, (true for beer of about 4.5% ABV (9% proof))
so you drink a half pint, you can probably drive, 1 unit of alchol in an average adult male is roughly the limit.
if you have a whole pint, it takes 1 hour per unit to be fully absorbed, and then 2 hours per unit to disipate.
so from the start of your first drink it's 1hour for the first unit, 2 hours for the second unit, 2 further hours for the first unit to go, and a further 2 for the second unit to go.
so you have 1 pint of beer at (and finish it) 6pm, you should be clear to drive 4 hours later. 10pm
and have no residue in your blood by midnight...
and we're talking about 1 pint of quite weak beer!
as a more realistic example, imagine that you finish work at half five, and go to the pub, again you start drinking, though this time starting at 6, you drink 2 pints an hour, (still quite slow) before taking a taxi home at 9, (so an early night, but you've drank 6 pints, (2 between 6 and 7, 2 between 7 and 8, 2 between 8 and 9). to make the numbers add up I'll assume you drink at a constant rate, 1/2 pint per 15 minutes.
6pm 0 consumed 0 blood -
7pm 4 consumed 1 blood -
8pm 8 consumed 2 blood -
9pm 12 consumed 2 blood - 1 gone
(stop drinking)
10pm 12 consumed 3 blood -
11pm 12 consumed 3 blood - 2 gone
12am 12 consumed 4 blood -
1am 12 consumed 4 blood - 3 gone
2am 12 consumed 5 blood -
3am 12 consumed 5 blood - 4 gone
4am 12 consumed 6 blood -
5am 12 consumed 6 blood - 5 gone
6am 12 consumed 7 blood -
7am 12 consumed 6 blood - 6 gone
8am 12 consumed 6 blood -
9am 12 consumed 5 blood - 7 gone
10am 12 consumed 5 blood -
11am 12 consumed 4 blood - 8 gone
12pm 12 consumed 4 blood -
1pm 12 consumed 3 blood - 9 gone
2pm 12 consumed 3 blood -
3pm 12 consumed 2 blood - 10 gone
4pm 12 consumed 2 blood -
5pm 12 consumed 1 blood - 11 gone
6pm 12 consumed 1 blood -
7pm 12 consumed 0 blood - 12 gone
you finished drinking at 9pm one day, but could still be 9 times over the limit by 8 the next morning, further to that when you go home at 5 you've only just reached the legal limit for being able to drive! and by 7 the next evening you should have no alcohol in your system (assuming that you haven't started drinking again).
drinking heavily in the early evening so that you can get drunk and have an early night so that you'll be good to drive in the morning isn't as effective as you'd think. -that's why over Christmas they actually catch most drink drivers in the morning after.
