A chance converstion in IRC this morning started me to thinking it might be a good idea to look at the topic of alcohol and driving - not from the usual viewpoint of how much is safe enough to drive but rather to look at HOW LONG afterwards id required.
I am sure many of you will have noticed the increased use of RBT units during the mornings, especially on Saturday and Sunday. This is because an increasing number of people are being caught at this time of day due to the relatively slow rate that alcohol is removed from the body.
We are all probably aware of the campaigns from TAC etc. that talk about 2 standard drinks in the first hour and one per hour after that (for males) and 1 in the first hour and 1 per hour thereafter for females. This is based on the physical alcohol content of a standard drink being 10 grams of alcohol and that being the quantity that the body can metabolise. This is obviously a conservative approach but it is always better to play it safe.
For the purpose of this exercise let us use the TAC definition of a standard drink as being one of the following -
A 425ml light beer ( a bit over a schooner)
A 285 ml full strength beer (10 ounce, midi or pot)
A 100 ml wine (small glass)
30 ml of spirits (a single shot)
As part of my research (thanks FM) there is also a thing in Victoria called a pint - now a pint is supposed to be 740ml but these pints are actually 580ml (go figure).
Now there are a number of factors to be taken into account here - weight and gender are the two obvious ones.
As an example a single standard drink will represent
.021 % of BAC in a 70 Kg male
.018 % in an 80 kg one
.016 % in a 90 Kg one.
This effect starts to diminish beyond that point and at 100 Kg it is .015%.
Additional drinks are pretty well added at the same rate.
Alcohol dissipation also varies with weight as follows:
At 100 Kg = .011 per hour
At 90 Kg = .012 per hour
At 80 Kg = .014 per hour
At 70 Kg = .016 per hour
So we now have the base facts to answer the question asked at the start. All we need to add is what was drunk and how long has elapsed sice the start of drinking.
Let us use an example here - our test subject is a 70 Kg male. He has had 10 pots over a 4 hour period.
First we calculate his alcohol intake -
= 10 x .021 = .21 % BAC (four times the legal limit)
(but we need to fwactor in the four hours dissipation as well)
= 4 x .016 = .064 %
thus his BAC at the end of the session = .146 %
He will dissipate this at the rate of .016% per hour and he needs to drop below .05% to legally drive thus he needs a reduction of..
.14 - .05 = .09%
at his rate of .016 per hour this willk take ...
.09 divided by .016 = 5.6 hours
For those who don't want to perform all these calcs themselkves (especially with a hangover) try this very informative site that I used for some of this info here ...
Hope this helps!