Even small amounts of alcohol affect your driving.[1] Yet some drivers still think it’s safe to have a couple of drinks before getting behind the wheel.
The following information explains just what is in an alcoholic drink and why it is never possible to guess whether you’re safe to drive after having ‘just a couple’.
What is the legal driving limit?
The amount of alcohol in the blood in known as the blood alcohol concentration (BAC). The legal BAC limit for driving in the UK is 80 milligrammes of alcohol in 100 millilitres of blood. However, there is no failsafe guide as to how to stay under the legal limit or as to how much you can drink and still drive safely. There is also no way of converting the BAC limit into how many units a person can have - the concentration of alcohol in the blood will depend of various factors, as explained below.
FACT: ANY AMOUNT OF ALCOHOL AFFECTS YOUR ABILITY TO DRIVE SAFELY [2]
What is a unit of alcohol?
The alcohol content of drinks is measured in units. A UK unit is eight grams (or 10 millilitres) of pure alcohol.[3] The number of units in a drink depends on what it is that you’re drinking. Beer, wine and spirits all contain a different number of units, as do different types of all these drinks.
Some people wrongly think they calculate their safe driving limit by the number of units they consume but the fact is that driving skills are affected after just one unit. This means your risk of being involved in a crash increases after drinking alcohol, even if you are below the legal limit. [4]
As a general guide, the following drinks contain one unit:
- A small (125ml) glass of wine at 9% ABV [5]
- A 25ml measure of spirit at 40% ABV [6]
- Half a pint of ordinary strength lager/ beer/ cider at 3.5% ABV [7]
However, remember that pub measures are usually larger than the above. The following examples show how varying either the % ABV or the size of the drink can affect the number of units you consume:
When drinking wine:
- Most pubs serve wine in LARGE glasses - usually either 175ml (often classed as standard size) or 250ml
- How this alters the units: a 125ml glass of wine at 9% ABV is 1.1 units, but a 175ml glass of wine at this strength is 1.6 units [8]
- The % ABV varies for different types of wine - a glass of house white wine will typically be about 12% [9]
- How this alters the units: a 125ml glass of wine at 12% ABV is 1.5 units; a 250ml glass at this strength is 3 units [10]
When drinking spirits:
- Many pubs serve spirits in 25ml or 35ml measures [11]
- How this alters the units: a 25 ml measure of spirits at 37% ABV is 1 unit, but a 35ml measure of spirits at this strength is 1.4 units [12]
- The % ABV varies for different types of spirits
- How this alters the units: a 25ml measure of spirits at 48% ABV is 1.3 units; a 35ml measure at this strength is 1.8 units [13]
When drinking lager/ beer/ cider:
- The % ABV varies for different types of spirits - many lagers are now around 5% ABV, and several are stronger than this
- How this alters the units: half a pint of lager/ beer/ cider at 3.5% ABV is 1 unit [14]; half a pint at 5.2% ABV is 1.5 units [15]
To find out how many units are in other drinks, including champagne, liqueurs and alcopops, visit The Portman Group’s Drinkaware website, which features a unit calculator.
Alcohol content of some popular drinks:
- A pint of ordinary strength lager (Carling Black Label, Fosters) - 2 units
- A pint of strong lager (Stella Artois, Kronenbourg 1664) - 3 units
- A pint of ordinary bitter (John Smith’s, Boddingtons) - 2 units
- A pint of best bitter (Fuller’s ESB, Young’s Special) - 3 units
- A pint of ordinary strength cider (Woodpecker) - 2 units
- A pint of strong cider (Dry Blackthorn, Strongbow) - 3 units
- A 175ml glass of red or white wine - around 2 units
- A pub measure of spirits - around 1 unit
- An alcopop (e.g. Smirnoff Ice, Bacardi Breezer, WKD, Reef) - around 1.5 units [16]**
What does alcohol do to the body?
Alcohol passes from your mouth into your stomach and small intestine, where it is then absorbed into your bloodstream. From there it is circulated through the liver and on to your heart, lungs and brain. [17]
Alcohol is a ‘depressant’. This means it slows down the reactions in your brain. It can also:
- lower your inhibitions, making you feel reckless
- affect your physical co-ordination
- affect your judgement [18]
The effects of alcohol on the body explains why it is so dangerous to get behind the wheel after drinking. When driving you need to concentrate 100% on the road ahead and the everyday hazards around you - you can’t do this safely if your brain is affected by alcohol.
Alcohol affects people in different ways and at different rates - this is why it is never possible to calculate a safe amount for driving. Some of the factors influencing how alcohol affects you are:
- Your size and weight - if you are small, your blood alcohol concentration will be less than someone who is larger, so you would probably be affected more even if you drank the same amount of alcohol. [19]
- Your gender - women are generally smaller and have less body water and more fat than men. Alcohol doesn’t dissolve easily in fat, so women end up with more alcohol in their blood than men even if they drink exactly the same amount. [20]
- Your water level - alcohol has a greater effect if your body is lacking in water - for example if you drink in summer and are dehydrated. [21]
- Your food intake - If you drink on an empty stomach the alcohol will reach your brain and the rest of your body more quickly. [22]
- What you drink - fizzy drinks and stronger drinks such as spirits are absorbed more quickly. [23]
The LifeBytes website[24] features a section on alcohol aimed at teaching young people the basic effects of alcohol on the body.
How much alcohol affects your driving?
You don’t need to be over the legal drink-drive limit [25] for your driving to be affected - even a small amount of alcohol can put you at risk behind the wheel.
The effects of alcohol (even a small amount) on driving include:
- slower reactions
- increased stopping distance
- poorer judgement of speed and distance
- reduced field of vision
- over-confidence, making you more likely to take risks [26]
The only safe option is not to touch a drop of alcohol if there is a chance you may be driving.
For how long does alcohol affect the body?
Your liver cleanses the body of alcohol as blood flows through it. After allowing an hour for the body to absorb the alcohol you have drunk, it does this at a rate of about one unit of alcohol per hour. [27] There is no way of speeding this up. If you are planning to drive the morning after drinking, you MUST take this into consideration when deciding how much to drink.
For example, if you drink just three pints of Stella (nine units) or one bottle of 12% ABV wine (nine units) it will take roughly nine hours for the alcohol to leave your system. This means that if you stop drinking at 11pm, the alcohol will not have left your system until 8am. You should not drive before this time and should allow even longer if you think you are still likely to be affected by the alcohol.
A final warning
Almost one in six deaths on the road involve drivers who are over the legal alcohol limit. [28] This means that knowing the alcohol level of your drink is important. With increases in the size and strength of drinks you may be drinking more than you think. Drivers with twice the current legal alcohol limit you are at least 50 times more likely to be involved in a fatal crash than a driver who has not drunk alcohol. [29]
However, even small amounts of alcohol can affect your ability to drive safely. Never drink any amount of alcohol if you know you will be driving later on in the day and always allow plenty of time to sober up if you are driving the morning after drinking.
For more information on drink-driving, the law and the devastation it causes, see Brake’s Drink driving information sheet and case studies of crashes caused by drink-drivers in Brake’s Newsroom.
Sources of further information
[1] THINK! - When Will You Have Had Too Much? (www.thinkroadsafety.gov.uk)
[2] THINK! - When Will You Have Had Too Much? (www.thinkroadsafety.gov.uk)
[3] Alcohol and Health’, Department of Health guidance, (www.dh.gov.uk), How Many Units?, Drinkaware, The Portman Group (www.drinkaware.co.uk)
[4] A Step-By-Step Guide to Alcohol, Bupa, (www.bupa.co.uk)
[5] Mind, Body & Soul: Alcohol, (www.mindbodysoul.gov.uk)
[6] Mind, Body & Soul: Alcohol, (www.mindbodysoul.gov.uk)
[7] Mind, Body & Soul: Alcohol, (www.mindbodysoul.gov.uk)
[8] How Many Units?, Drinkaware, (www.drinkaware.co.uk)
[9] Alcohol and Driving: a toolbox talk, Avoid DVD
[10] Alcohol and Driving: a toolbox talk, Avoid DVD
[11] Alcohol and Driving: a toolbox talk, Avoid DVD
[12] Alcohol and Driving: a toolbox talk, Avoid DVD
[13] Alcohol and Driving: a toolbox talk, Avoid DVD
[14] Alcohol and Driving: a toolbox talk, Avoid DVD
[15] How Many Units?, Drinkaware, (www.drinkaware.co.uk)
[16] ‘Alcohol and Health’, Department of Health guidance, (www.dh.gov.uk)
[17] A Step-By-Step Guide to Alcohol, Bupa, (www.bupa.co.uk)
[18] Department of Health education website (www.wiredforhealth.gov.uk)
[19] What Affects the Amount of Alcohol in Your Blood?, Drinkaware, (www.drinkaware.co.uk)
[20] What Affects the Amount of Alcohol in Your Blood?, Drinkaware, (www.drinkaware.co.uk)
[21] What Affects the Amount of Alcohol in Your Blood?, Drinkaware, (www.drinkaware.co.uk)
[22] What Affects the Amount of Alcohol in Your Blood?, Drinkaware, (www.drinkaware.co.uk)
[23] Department of Health education website (www.wiredforhealth.gov.uk)
[24] LifeBytes, National Grid for Learning (www.lifebytes.gov.uk)
[25] The legal drink-drive limit is 35 microgrammes of alcohol in 100 millilitres of breath, 80 milligrammes of alcohol in 100 millilitres of blood, or 107 milligrammes of alcohol in 100 millilitres of urine. Road Traffic Act, 1988 (c. 52)
[26] THINK! - When Will You Have Had Too Much? (www.thinkroadsafety.gov.uk)
[27] A Step-By-Step Guide to Alcohol, Bupa, (www.bupa.co.uk)
[28] THINK! - Drink Driving, (www.thinkroadsafety.gov.uk)
[29] THINK! - When Will You Have Had Too Much? (www.thinkroadsafety.gov.uk)
