The science behind alcohol and driving

The effects of alcohol on driving

Certain brain functions enable us to drive safely, including good vision, effective communication between the eye and the brain and quick reaction times. Many of these functions are hindered by drinking alcohol - you may have blurred or double vision or be unable to process and respond to information quickly enough to avoid or cause an accident.

Blood alcohol levels

A healthy liver processes around one unit of alcohol every hour although you must remember that this is highly variable and dependent on sex, body mass, how much food you have consumed, and how quickly alcohol is absorbed into your blood stream.

  • One pint of 5% strength beer contains around three units of alcohol
  • One small (125 ml) glass of 12% strength wine contains 1.5 units
  • One single measure (25 ml) of 40% strength spirits is one unit.

That means that the alcohol in a pint of beer isn't fully broken down for three hours.

It's worth being aware that wine bars often serve slightly larger measures - for example a small wine can be over two units and a large wine around four units. So if you're drinking in a wine bar then remember to err on the side of caution.

How is alcohol measured once it's absorbed into the blood?

Once the alcoholic drink is in your stomach, the alcohol is quickly absorbed into your bloodstream, where its on-going level will be a balance between the rate at which it's being added to by further drinks, and the rate at which your liver can break it down. Your blood alcohol is measured in milligrams (mg) of alcohol present per 100 ml of blood - the legal limit in the UK is 80 mg.

Testing and penalties

The police can stop you at the roadside and carry out a breath test at any time. If you refuse or your reading shows an excessive alcohol level, you'll be taken to the station for a blood test to confirm your blood alcohol level.

Penalties in the UK include being banned from driving for at least 12 months and a fine of up to £5,000. You can also be imprisoned for up to six months and harsher penalties are imposed if you are found guilty of drink driving more than once.

Play safe

Regardless of the legal limits, any alcohol in your blood will have at least some effect on your ability to drive so the best and safest advice is to not drink at all if you're out driving.

Judging your blood alcohol levels over the course of the evening is a very dangerous tactic - there's no reliable 'formula' because individuals accumulate and break down alcohol at highly varying rates. Add to this the fact that your judgement will be affected anyway if you've had even one drink so treading the line can be a recipe for disaster.

Staying dry

Nominating a trustworthy 'designated driver' is always a great idea, but here are our top tips for staying alcohol-free if the car keys are in your pocket:

Have a big meal beforehand - feeling full will reduce your temptation to drink alcohol

Always have a non-alcoholic drink visible in your hand so that people don't test your resolve by offering you a drink or worse, insisting that you have one

Buy drinks that look - and even taste - like the real thing such as non-alcoholic beer

Steer clear of temptation by avoiding the bar area

Really focus in on social activity such as meeting new people, dancing, listening to the band - there's usually plenty more to do than drink

Take a break every now and then, especially if things get very lively - go for a walk, take a drive, make a call

Remind yourself that you're going to enjoy waking up the next day with a fresh, clear head.


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