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drink with car keysAs alcohol affects our reaction times, our vision and coordination, there is a legal drink drive limit, which is based on BLOOD ALCOHOL CONCENTRATION (BAC).

What is BAC?

The amount of alcohol in your bloodstream is called your Blood Alcohol Concentration or BAC. Your BAC depends on how much you drink. The more you drink, the higher your BAC. But there are also many other factors that affect your BAC such as your size, your weight, your gender, whether you've eaten and even how tired you are.

Statistics about DUI in the United States

How is BAC measured?

BAC can be measured with a breathalyser or by analysing a sample of blood. It is measured by the number of grams of alcohol in 100ml of blood. For example, a BAC of .08, the US legal limit for driving for those over 21, means you have .08 grams of alcohol in every 100ml of blood.


BAC graphThinking distance

Alcohol’s effect on the brain slows down a person’s reaction times - they take longer to respond to hazards. So, if a cat ran in front of a car or motorbike, the delay between you seeing it and putting your foot on the brake slows down. The extra distance travelled in that time is called your ’thinking distance’.

Each drink can increase the ‘thinking distance’ by 20%. The risk of someone being in an accident increases by: two times for drivers with a BAC of .05 four times for drivers with a BAC of .08 and twenty times for drivers with a BAC of .15. Drivers who have been drinking also underestimate the distance and speed of other vehicles on the road. Their vision is affected, slowing reaction times further. Drivers who’ve been drinking also overestimate their ability and drive more recklessly too.

Can BAC vary?

The only safe advice is to nominate a non drinking driver for the evening or to arrange to get home by cab or public transport ... and don’t ever accept a lift from someone you suspect is over the drink drive limit. Do everything you can to persuade them not to drive – you could be saving their life or someone else’s. It is impossible to state categorically how many drinks you can consume before exceeding the BAC legal limit, as the speed that alcohol enters your blood stream will vary if you are having a drink on an empty stomach, or drinking with food slowly. Having food in your stomach does not stop you from getting drunk, but it does slow down the rate at which alcohol passes into the bloodstream. A drink will raise your blood alcohol level by approximately .04 BAC or even .05 if you are a woman drinking without food. Remember your blood alcohol level will rise more quickly if you have a drink on an empty stomach, but alcohol will stay in you body longer if you drink with food. On average your liver will break down 1/2 a drink an hour. If you have drunk heavily the night before, you may risk being over the limit the next morning. For more information, visit

Lots of different factors also affect your BAC including:

  • whether you’re male or female
  • if you are drinking on an empty stomach
  • if you are drinking quickly
  • your proportion of body fat (body fat does not absorb alcohol)
  • your metabolic rate (affected by diet, digestion, fitness, emotional state, hormonal cycle, time of day, year etc).
  • if you are tired - this will affect your concentration and absorption
  • individual differences in size, weight and metabolism
  • the percentage of alcohol in the drink
  • the type of alcohol you are drinking (fizzy drinks are absorbed more quickly)
  • the container size
  • the amount of time since your last drink (the body can only break down about 1/2 a drink an hour, your BAC can still be rising because alcohol takes time to be absorbed).

The use of stimulants, such as caffeine won’t affect BAC, but may ‘mask’ the effect of alcohol, making you feel more sober than you really are.

Designated Drivers

The designated driver program asks that motorists always designate a driver when travelling after consuming alcohol or take a taxi. "Designate a driver" can be used anytime people plan to go out and drink alcoholic beverages ensuring a safe drive home.

What does a designated driver do?

  • Drinks only non-alcoholic beverages.
  • Provides safer and sober transportation home for companions.
  • A designated driver provides "life-insurance" for a group’s trip home and helps friends avoid embarrassment, keep their driving licenses and avoid fines and jail terms.

The GfK Custom Research North America Designated Driver Survey 2013 found that 71 million or 32% of American adults aged 21+ had been a designated driver and 19% or 42 million had been driven home by a designated driver in the past year. 84% of respondents thought that promoting the use of designated drivers is an excellent/good way to help reduce drunk driving.

designated driver
miller lite free ride

Drink Drive and The Law

In the United States, all states define driving with a blood alcohol concentration (BAC) at or above .08 as a crime, but specific laws and penalties vary substantially from state to state, with somes states having specific BAC limits for commercial drivers and drivers who have previous DUI offenses.

The Minimum legal drinking age laws prohibit selling alcohol to people under age 21 in all 50 states and the District of Columbia. Keeping and enforcing 21 as the minimum legal drinking age helps keep young, inexperienced drivers from drinking and driving.

42 states, the District of Columbia, the Northern Mariana Islands and the Virgin Islands have administrative license suspension (ALS) on the first offense. ALS allows law enforcement to confiscate a driver's license for a period of time if he fails a chemical test. Most of these states allow limited driving privileges (such as to/from work).

All states have some type of ignition interlock law, in which judges require all or some convicted drunk drivers to install interlocks in their cars to analyze their breath and disable the engine if alcohol is detected. 20 states (and 4 California counties) have made ignition interlocks mandatory or highly incentivized for all convicted drunk drivers, even first-time offenders.

Never mind the law, you’d never forgive yourself if you injured someone seriously – and try getting car insurance or a new job if you’ve got a driving conviction – not easy.

State Laws

More information

Drive Sober or Get Pulled Over

Each year, GHSA's member State Highway Safety Offices (SHSOs) team up with law enforcement agencies and others to combat drunk driving through the National Highway Traffic Safety Administration's (NHTSA's) Drive Sober or Get Pulled Over campaign.

Drive Sober or Get Pulled Over is a nationwide effort to crack down on impaired driving and reduce roadway fatalities. It takes place each year from mid-August through the Labor Day and again during the December holiday season. During these times, police from every state and most U.S. towns and cities step up their efforts to find and arrest drunk drivers.

The crackdown is supported by national advertising campaign to put motorists on notice that if they are caught driving while impaired, they will be arrested. These national ads are targeted at young male drivers and motorcycle riders, who are the most common offenders. States and localities supplement these national ads with their own public awareness and media outreach.

Travelling Abroad?

Drink-drive legislation varies across the world so always be careful if you are travelling. Most countries in the EU have a drink drive Blood Alcohol Concentration of 50mg per 100 ml blood, 30mg less than the US limit of 80mg.

For more information on BAC levels and the law relating to countries worldwide Click here.

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