Drink-Driving Guidelines
Ever since the introduction of the breathalyser in 1967, official sources have been extremely unwilling to publish any figures on the amount of alcohol you need to consume to take you over the 80 mg legal limit for driving. The reason for this is that they feel it will encourage drivers to "drink up to the limit". But, in reality, nobody can do that, because of the extremely unpredictable rate at which alcohol is absorbed by the body. Either you play safe, and stay well below it, or you try to drink up to the limit, and run a serious risk of exceeding it.
Of course, everyone knows that the law lays down a limit, not a prohibition, and that a certain amount of alcohol can be consumed without taking drivers outside the law. This is expressed in the popular wisdom that the limit equals two pints. This can be misleading, but contains some truth. Broadly speaking, if a man of average weight consumes two pints of ordinary strength beer of 4% ABV or less, it is extremely unlikely to take him above the 80 mg legal limit, and in reality will probably lead to a maximum BAC of no higher than 60 mg.
The following is an attempt to express this in rather more detail. It is drawn from various sources, including individuals' experiences of being breath tested, but the primary source is a booklet entitled
The Facts about Drinking and Driving, published by the Transport and Road Research Laboratory in 1986, which presumably can be regarded as reasonably authoritative. However,
these guidelines are not a statement of fact, and must not be taken as a guarantee of keeping below the legal limit in any particular circumstances.
Alcohol is normally measured in "units" of 10ml of alcohol. This is the amount of alcohol contained a a half-pint of beer of 3.5% ABV, a single 25ml pub measure of spirits, or a small 125ml glass of light table wine.
The rate of absorption of alcohol into the bloodstream is unpredictable and depends on a number of factors such as the level of hydration, the type of alcoholic drink consumed and whether food is eaten at the same time. As a broad rule of thumb, the alcohol in a drink is fully absorbed about an hour after the drink is finished.
The rate at which alcohol is metabolised and removed from the bloodstream is rather more predictable, and averages out at one unit per hour, starting one hour after the first drink is finished. However, the capacity of the body to metabolise alcohol is finite, and is limited to about 16-20 units per day. If you consistently drink around or above this level, you will probably never be below the limit - and you also need to consider seriously whether you have a drink problem!.
To ensure you run no risk of being "over the limit":
Immediately before driving: - men should consume no more than 4 units, women no more than 3