How DUI Works

Image Gallery: Car Safety A police officer conducts a field sobriety test at a DUI traffic checkpoint. See more car safety pictures.
Image Gallery: Car Safety A police officer conducts a field sobriety test at a DUI traffic checkpoint. See more car safety pictures.
Joe Raedle/Getty Images

Driving Under the Influence (DUI) is one of the biggest dangers facing motorists today. In 2003, 1.4 million Americans were arrested for DUI, and alcohol-related crashes produce an estimated $45 billion in damages every year. On average, a person is injured in an alcohol-related car accident every 30 seconds [ref]. From 2000 through 2005, 103,213 people were killed in alcohol-related car accidents in the U.S., according to Mothers Against Drunk Driving (MADD), a prominent anti-drunk driving advocacy group.

Some of these statistics may be slightly misleading. MADD classifies any accident in which a driver had consumed alcohol as “alcohol-related” -- even if that driver is not deemed responsible for the accident. For example, someone who drank a beer at dinner and is later hit while pulling into a parking spot may be considered part of an “alcohol-related” accident. Sometimes, pedestrians who have consumed alcohol and are struck by a car are classified as being part of an “alcohol-related” accident, even if the pedestrians weren’t intoxicated or at fault [ref]. Even so, MADD’s numbers do speak to a major problem. One study of fatally injured drivers in several states found that 68 percent of sober drivers and 94 percent of intoxicated drivers were responsible for their accidents [ref]. What this means is that driving while intoxicated significantly increases the risk of getting into an accident. In fact, if a driver’s Blood Alcohol Content (BAC) is 0.15 or higher, he or she is 300 times more likely to get into a fatal car accident [ref].


It’s not just alcohol. In one study published in the New England Journal of Medicine, 45 percent of people stopped by police for reckless driving tested positive for marijuana and 25 percent tested positive for cocaine [ref]. Lack of sleep and stimulants are also dangerous. The impairment caused by a lack of sleep c­an be as significant as being legally drunk. Also, recent studies have shown that talking on a cell phone while driving is as dangerous as driving drunk [ref].

For a long time, the federal government and interest groups have been working to educate people about the dangers of drunk driving. Fortunately, there are many ways to avoid DUI. For example, you can watch what you drink, use a designated driver and -- this one’s pretty obvious -- don’t drink before driving.

In this article, we’ll learn about preventing DUI, how a DUI arrest works, and what the punishment is for a DUI conviction. We’ll also expose some common myths about breathalyzers and alcohol. And though some states use terms like DWI (Driving While Intoxicated) or OMVI (Operating a Motor Vehicle While Intoxicated), for this article, we’ll stick to the term DUI.

Alcohol: Truth and Myth

Each of these drinks contains the same amount of alcohol.
Each of these drinks contains the same amount of alcohol.

Alcohol is a drug that impairs judgment, reaction time, movement, coordination and vision. Though the blood alcohol content legal limit for someone driving a car used to be 0.10 in some states, it is now 0.08 in all states. In Michigan, it will change to 0.10 from 0.08 on October 1, 2013. Even though 0.08 is considered the “legal limit” in most states, courts can still find someone legally drunk with a BAC below 0.08 if the driver is under 21 or shows signs of impairment [ref].

BAC is a measurement of how much alcohol has been absorbed into someone’s bloodstream and is greatly affected by the rate of alcohol consumption. Generally, the faster someone drinks, the more inebriated (drunk) he or she becomes. The type and amount of alcohol does make a difference. One drink equals 12 ounces of beer, five ounces of wine or a 1.5 ounce of 80-proof liquor. (Proof is a measurement of how much alcohol a liquor contains -- 80 proof means 40 percent alcohol, 70 proof equals 35 percent alcohol, etc.) All of these measurements contain 0.54 ounces of ethyl alcohol, the substance that makes you intoxicated.


As alcohol accumulates faster than the body can process it, a person becomes drunk. On average, a person metabolizes, or processes, one drink per hour. Women, who generally have less muscle mass than men, are more susceptible to the effects of alcohol because muscle slows down the absorption process. Women also produce less of the enzyme dehydrogenase, which helps in breaking down alcohol.

The facts about DUI show that it's a widespread and dangerous problem.
The facts about DUI show that it's a widespread and dangerous problem.

Height, weight and body fat also affect blood alcohol content. Eating high protein foods, such as nuts, meat, pizza and cheese, can help to slow down the absorption of alcohol.

The mistake that people often make is thinking that certain liquors or drinks affect someone differently or that mixing alcohols increases drunkenness. As seen above, different types of drinks can be equally potent. What matters is the alcohol content. Drinking four 12-ounce beers has the same affect as taking four 1.5-ounce shots of 80-proof vodka in the same time period. Some types of beer and wine do contain more alcohol than others, but this is a general guide.

Many people believe that drinking water or coffee, exercising, napping or taking a cold shower will make someone more sober, but only time can help someone sober up. What matters is the body having time to metabolize the alcohol -- not how much someone thinks he or she is sober.

DUI Arrests

Police arrest a woman after she failed a field sobriety test at a DUI checkpoint.
Police arrest a woman after she failed a field sobriety test at a DUI checkpoint.
Joe Raedle/Getty Images

There are many signs that police officers look for when deciding whether or not to pull someone over on suspicion of DUI. Erratic driving, weaving between lanes, going significantly above or below the speed limit, ignoring traffic lights and signs, straddling a lane and unexpected braking all may be symptoms of an intoxicated driver.

After pulling someone over, police officers use four methods to determine if someone is driving under the influence [ref]:


  • Signs of intoxication - bloodshot eyes, flushed face, slurred speech, lack of balance, belligerence
  • Incriminating statements - admitting to drinking or taking drugs
  • Conducting a field sobriety test - making the suspect walk in a straight line, recite the alphabet backwards or spread arms out and then touch his or her nose with eyes closed
  • Chemical test - breath analyzer, blood test, urine test

If a suspect fails any of the above tests, he or she is likely to be arrested.

Anyone suspected of DUI has the right to refuse to answer questions or to take a field sobriety test. If an officer asks a suspect to take a chemical test, the suspect must comply, or he or she will be arrested and could face stiffer consequences. However, as we will explain later, breath analyzers (commonly called breathalyzers) are in fact frequently inaccurate. A blood test is by far the most reliable chemical test, and in some cases, the suspect is offered a choice about which chemical test to take.

If someone is arrested for drunk driving, he or she may face more than just a DUI charge. Depending on the state laws, a suspect could face an additional charge for a high BAC. If the suspect was also involved in a car accident or a child was in the car, there could be charges of reckless driving or child endangerment.

After an arrest, the suspect will probably be released on bail with some sort of temporary license, but practices differ depending on the state and the particular incident. If you or someone you know has been charged with DUI, consult an attorney who specializes in DUI cases.

Punishments for DUI

A breath analyzer like this one might be used in a DUI traffic stop, but some experts question their accuracy.
A breath analyzer like this one might be used in a DUI traffic stop, but some experts question their accuracy.
Photo courtesy

Because laws differ from state to state, there is no consistent punishment for a DUI conviction. Instead, the sentence depends on whether or not the suspect pleaded guilty, BAC at the time of arrest, whether he or she is a first-time offender, if anyone was injured or property was damaged, additional charges, and if other drugs were also involved.

For first time offenders, punishment can include some combination of a fine, jail time, tickets, probation, community service and a suspended driver’s license. Repeat offenders are more likely to lose their license, go to jail, have ignition interlocks placed on their cars (we’ll explain those below) and be ordered to attend alcohol counseling. In some states, drunk drivers who are under 21 will have their drivers licenses automatically suspended for six months to a year.


If you are convicted of DUI, it may be on your criminal record for your entire life. Offenses also go on your driving record, which potential employers and insurers can access. Some states keep computerized records, but others still keep them on paper, so if your job requires lots of driving, it’s probably a good idea to check your own record to make sure it doesn’t contain any errors. You can order a copy of your driving record at this Web site.

Breathalyzers and Ignition Interlocks

Breathalyzer is a brand name that has become a common synonym for breath analyzer, a plastic device with a tube extending out of it that the user blows into. Breathalyzer is just one type of breath analyzer that is commonly used. For information about other types of breath analyzers, check out our article called How Breathalyzers Work.

While breath analyzers are frequently used by police officers in DUI stops, they are known to be inaccurate. The main problem with breath analyzers is that they only estimate how much alcohol is in a person’s blood. The alcohol found in drinks is called ethyl alcohol and is part of the methyl group of compounds. Unfortunately many other compounds, including some found in human breath, are part of the methyl group and detection of multiple methyl compounds raises a BAC reading.

Breath analyzers use a ratio of 2,100:1 to convert breath alcohol into blood alcohol. Unfortunately, this ratio can be anywhere from 1,900:1 to 2,400:1 in some people and also fluctuates over time [ref].

People suffering from diabetes or hypoglycemia produce significant amounts of a compound called acetone. Acetone is sometimes identified as ethyl alcohol by breath analyzers, so someone with diabetes or hypoglycemia can appear legally drunk to a breath analyzer [ref].

The strength or duration of a breath can also affect the breath analyzer reading. In one test, sober subjects ate different types of bread products and then registered 0.05 BAC readings. Other factors that can affect a breath analyzer’s measurement include:

  • Fumes from paint, rubber cement, cleaning fluids, glue, gasoline, etc.
  • Blood or vomit in the mouth (however unappealing that sounds)
  • Electrical interference
  • Smoke in the air or chemicals in a smoker’s lungs
  • Improperly used or poorly maintained devices
  • Physical activity and hyperventilation

Because of the flaws associated with breath analyzers, some states don’t allow readings from handheld models to be presented as evidence in court. South Dakota doesn’t allow readings from any type of breath analyzer, handheld or otherwise. If you’re faced with a situation where an officer wants you to take a chemical test, request a blood test.

Ignition Interlocks

Ignition interlocks are devices placed in cars that test sobriety before a car can be started. The driver breathes into the device, and if any alcohol is registered, the car will not start. Judges frequently order repeat offenders to install the devices in their cars for a period of time.

Unfortunately, anyone who has a willing, sober accomplice can bypass an ignition interlock by just having the passenger breathe into the device (as was famously illustrated in the movie “40-Year Old Virgin”). Several car companies are developing devices that would foil these cheaters. Possibilities include a detection system built into the steering wheel that could detect sobriety through the hands or an alcohol-sensing keychain [ref]. Anti-drunk driving organizations and insurance companies have been among the big supporters of such projects.

Future ignition interlock technology may integrate alcohol-detection devices directly into steering wheels -- you won't even know it's there.
Future ignition interlock technology may integrate alcohol-detection devices directly into steering wheels -- you won't even know it's there.
Photo courtesy stockexpert

The future of preventing drunk driving may be tied to technologies like these, but it is also in education and awareness. Some educators and lawyers complain that anti-drunk driving activism often turns into anti-alcohol activism. The key is drinking safely and in moderation -- remember that even one drink could lead to impairment while driving.

For more information on DUI and related topics, check out the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • “2002 National Victims Assistance Academy.” 2002. U.S. Department of Justice: Office of Justice Programs.
  • “Driving Penalties by State.” 7/23/2004. Insurance Institute for Highway Safety.
  • “DUI Arrest - What happens during an arrest?” DUI FYI.
  • ”DUI/DWI Laws.” Institute for Insurance Highway Safety. March 2007.
  • “National Drive Safely At Work Week.” Federal Motor Carrier Safety Administration.
  • “Information About Driving Under the Influence of Drugs or Alcohol.” 5/29/2006. Dessen, Moses & Rossitto.
  • “MADD Online: General Statistics.” MADD.
  • “Total DUI: DUI Charges Below the ‘Legal Limit.’” 4/12/2006. The DUI Blog.
  • Hanson, David J. “Alcohol Facts and Fiction.”
  • Hanson, David J. “Driving While Intoxicated (DWI/DUI) Information.”
  • Lawrence Taylor, J.D. “DUI Blog.”
  • Wald, Matthew. “New Strategies to Thwart Drunken Driving.” 11/20/2006. San Francisco Chronicle.