Drunk driving (drink driving in the UK) or drinking and driving is the act of operating a motor vehicle while intoxicated with alcohol (i.e. ethanol) to a sufficient degree to impair mental and motor skills. In the US alone, 17,419 people died in 2002 in drunk driving collisions, representing 41 percent of total traffic deaths in that country. Over 500,000 people were injured in drunk driving accidents in the US in 2001.

Drunk driving is illegal in most jurisdictions; laws prohibiting it refer, for instance, to driving while intoxicated (DWI), driving under the influence [of alcohol or other drugs] (DUI), or drunk in charge [of a vehicle]. Such laws also apply to boating, likewise often called boating while intoxicated (BWI) or boating under the influence (BUI). Laws also prohibit piloting aircraft while intoxicated.

Such laws tend to define a particular level of alcohol in the blood as the threshold of drunkenness. The most common blood alcohol content (BAC) limit in the United States is 0.08% for the legal definition of intoxication. Only three states still use the more lax, once-common standard of 0.10%. Prior to wider emphasis on drinking and driving in the 1980s, standards of 0.12% were also common. The legal limit for aircraft pilots in the U.S. is set at 0.04% while in the UK it is 0.02% for flight crew and 0.08% for groundcrew.

Many jurisdictions add extra penalties (more jail time and/or a longer DUI program) in cases where the driver's BAC is over 0.20%. This is, at least in part, due to the fact that an average person would have passed out from that much alcohol. To be able to drive at that level, a person has to have gotten drunk regularly for years, to increase his or her alcohol tolerance.

Timeline of a typical DUI arrest

The following is what typically happens when a law enforcement officer has a reason to suspect a driver is intoxicated. Some possible reasons are erratic driving, poor coordination, and/or the presence of the smell of alcohol.

Field sobriety test
The officer will administer one or more field sobriety tests (FSTs). Some common FSTs include having the driver:

• try to walk in a straight line, heel-to-toe.
• tip his or her head back with eyes closed and try to touch the tip of the nose with the index finger.
• stand on one foot.
• reciting all or part of the alphabet.

FSTs are better at determining the level of impairment than they are at estimating the driver's BAC.

Chemical test
If arrested, the driver is brought to the police station, and given one or more chemical tests: breath, urine, and/or blood. Breath test results are usually available immediately and are sometimes given before the actual arrest takes place; urine and blood samples are sent to a lab to determine the BAC. In some jurisdictions, refusing to take a breathalizer test is an offense in itself, often creating an automatic assumption of guilt under the law.

Chemical tests are better at determining the driver's BAC than they are at estimating the level of impairment, but their accuracy is disputed by some; see blood alcohol test assumptions. In any case, tests can only determine the BAL at the time the test is taken, which sometimes can be higher than when the vehicle was actually operated.

After being arrested for drunk driving, Nick Nolte tested negative for alcohol. Later tests showed that he was under the influence of GHB[

Next step
If it is determined that the person is not legally intoxicated, they might be released without any charges. However, many jurisdictions have charges which don't require a particular BAL, and tests for some drugs (such as GHB) will not show up in a test designed for alcohol.

Most of the time, the driver will either be kept in a holding cell (the "drunk tank") until they are deemed sober enough to be released, or sent to jail to wait for their first court hearing (or until they can get bailed out).