What Is the Drink Driving Limit in the UK?

The prescribed limits for drink driving in England and Wales under the Road Traffic Act 1988 are:

Breath: 35 micrograms of alcohol per 100 millilitres of breath (the most commonly used test).
Blood: 80 milligrams of alcohol per 100 millilitres of blood.
Urine: 107 milligrams of alcohol per 100 millilitres of urine.

Scotland introduced lower limits in 2014: 22 micrograms per 100ml breath, 50mg per 100ml blood, 67mg per 100ml urine.

There is no equivalent drink driving limit in England and Wales expressed in terms of units of alcohol. The rate at which a person’s body processes alcohol varies significantly between individuals and cannot be reliably predicted from units consumed. This is why there is no reliable unit guide to the limits — the only certain way of knowing whether you are over the limit is not to drink before driving.

Police conduct an initial roadside breath test using a portable device (such as the Lion Alcolmeter) as a preliminary indication. A positive roadside test leads to arrest and an evidential test at the police station using an approved device (the Intoxilyzer or Draeger). The station reading is the one used as evidence in court.

Two breath specimens are required. The lower of the two readings is used. If the reading is between 35 and 50 micrograms per 100ml of breath, the defendant has the right to request a blood or urine sample instead. Where a blood or urine sample is taken, it is divided between the defendant and the laboratory, allowing independent analysis.

Many factors affect the rate at which alcohol is absorbed into the bloodstream and eliminated: body weight, sex, food consumed before and during drinking, the type of alcohol consumed, individual metabolism, and medical conditions. These factors mean that the same quantity of alcohol can produce significantly different readings in different people.

The “morning after” problem is significant. Alcohol consumed the previous evening may still be in the bloodstream the following morning at or above the prescribed limit, even where the driver feels sober. There is no reliable personal test for this — professional evidential breath testers are the only accurate means of determining a person’s alcohol level.