the science of drunk driving

The DUI is the car insurer's natural enemy. We'll clarify how alcohol impacts any driver's abilities, explain how blood alcohol content (BAC) levels are determined, and give you a better idea of what makes drinking and driving so dangerous.

this is your brain on booze

In a sober driver, the brain's neurons send messages all over the body. This includes telling the hands to turn the wheel to avoid an oncoming car or telling the foot to press the brake as you approach a sharp turn.

But once alcohol enters your bloodstream, even at low doses, it diminishes neuron activity, dulling those signals traveling from your brain to your body. This can result in a variety of impairments, including diminished concentration and judgment, a drop in vision of up to 32 percent, and a reaction time that's up to 25 percent slower, according to one report from the University of Texas at San Antonio.

The Montana State Highway Traffic Safety Office broke down the effects of alcohol on a driver by the number of drinks consumed.

1–2 drinks

The first drink or two affect the cerebrum, which manages your judgment, your reason, and your inhibitions.

3–4 drinks

Now the alcohol is affecting your cerebrum more, causing a decrease in fine motor skills and a reduction in reaction time.

5–7 drinks

By this point the booze has reached your cerebellum and decreased your senses, including hearing and vision.

8–12 drinks

Compounding the problems above, this level of intoxication can lead to unconsciousness.

While it may not come as a surprise that 12 drinks is not a recipe for safe driving, the fact that a drink or two can influence your ability is worth considering before you get behind the wheel. The research makes it clear that it's always a good idea to avoid driving after drinking.

who’s drinking and driving in the U.S.?

Check out our infographic to get the latest drinking and driving statistics.

understanding how BAC works

A BAC of .08 means there is .08 grams of alcohol per 100 milliliters of blood. The easiest way to avoid driving under the influence is to make sure your BAC level is zero.

Body weight and BAC

How much you weigh is a major factor in your BAC. In general, bigger people can process more alcohol in a shorter time frame than smaller people because they have more fluids in their bodies.

Gender and BAC

Females absorb alcohol differently than men, according to the National Institute on Alcohol Abuse and Alcoholism (NIAAA). A 140-pound man would have a lower BAC than a woman of the same size after the same number of drinks. The NIAAA cites a few possible reasons, including the fact that women have less "body water" than men and that women may have a lower activity of a certain alcohol-metabolizing enzyme in their stomachs.

Food and BAC

Drinking on a full stomach keeps your body from absorbing alcohol as quickly, but maybe not for the reason you think. Food doesn't "soak up" alcohol, as the common myth goes. When your stomach is full, it passes contents into the small intestine more slowly and, instead, metabolizes some alcohol itself. And keeping food in the stomach longer is a good thing because it has a protective lining that creates slower absorption (up to 3 times slower, according to the NIAAA) than the small intestine.

Time and BAC

Spreading out drinks gives your body more time to metabolize and eliminate the alcohol. Drinking multiple drinks in a shorter time frame does the opposite. The Montana State Highway Traffic Safety Office claims that the average person will eliminate alcohol at a rate of 0.015 percentage points of BAC per hour.

The more time between drinks and the more time after drinks, the lower the BAC.

Other factors that influence a person's reaction to alcohol

Individual reactions to alcohol depend on more factors than those listed above. The Centers for Disease Control and Prevention explains that these elements also play a role:

  • Age
  • Race or ethnicity
  • Physical fitness
  • Use of drugs or prescription medicine
  • Family history

enforcing DUI penalties: the BAC tests

If the police suspect a driver is under the influence of alcohol, determining guilt is relatively easy. Officers can simply use blood tests or breathalyzers to determine BAC. A BAC of .08 is the legal limit in every state for drivers 21 or older, though you can be convicted with a lower BAC in some states if your driving abilities are found to be impaired. For drivers under 21, the law is often zero-tolerance.

Many states have "implied consent" laws, which legally require all drivers to agree to a requested BAC test at the scene. Refusal can lead to a DUI charge.

the science justifies the penalties

A DUI conviction is harshly penalized by all states and by all insurers for good reason. While individual reactions to alcohol vary, nobody is immune to its effects. Alcohol and safe driving just don't mix.

Volunteering to be a designated driver (and actually following through) can help keep drunk drivers off the roads — or, if everyone wants to drink, make sure you have alternate transportation lined up first.

Related links

The true cost of a DUI
Find out how much an average DUI conviction can cost. (Hint: it's a lot.)

DUIs and car insurance
DUIs affect what a driver pays for car insurance because they make a driver riskier to insure.

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