How Alcohol Affects Drivers

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Alcohol impacts a person’s ability to drive. Less than 2% of drivers on the road at any given time have been drinking (Association for the Advancement of Automotive Medicine, How Drunk are U.S. Drivers?). However, alcohol is a factor in about 1/3 of car crashes. (Forbes, Drunk Driving Statistics 2022).

Our DUI accident attorneys explain how alcohol affects drivers.

The Effects of Alcohol on Driving

To drive safely, a person must evaluate and respond to the environment around them. They must watch for traffic and other hazards. They must control their speed and execute driving maneuvers safely. Alcohol impairs this ability in several respects.

Reaction times

Often the difference between an accident and a near miss is a fraction of a second. With alcohol making it harder for a person to react quickly, that may mean drinking can contribute to a crash. A person who is sober may have time to react to a dangerous situation. The person who has been drinking may not notice and react until it is too late. (UK Essays, Effect of alcohol on reaction time).

Altered judgment

A driver must make judgment calls many times in a single trip. For example, they must gauge the safe following distance between them and the vehicle in front of them. They must judge when it is safe to execute a lane change. Alcohol use impairs this skill. When a person cannot make sound decisions on the road, they put themselves and others at risk. (Brevers, Impaired decision making under risk in individuals with alcohol dependence).


Driving requires concentration. As many as 13% of all traffic crashes involve distracted driving. (CDC, Distracted driving). Distracted driving isn’t just texting and driving. Distracted driving is the inability to concentrate. A driver cannot concentrate normally when they are under the influence of alcohol. By depressing the central nervous system, alcohol makes it harder for the brain to focus on driving. (Destination Hope, The effects of alcohol abuse on memory and concentration).

A study in the Journal of Studies on Alcohol and Drugs found that alcohol impacts cognitive performance at lower levels than it impacts motor performance. Reaction times decreased among those consuming intoxicants even if there was no effect on motor reactions. (Hernandez, Alcohol impairs the cognitive component of reaction time).


Drunk driving interferes with a person’s ability to perform two tasks well at the same time. Even at a bodily alcohol content as low as .02%, a person’s ability to do two things at once may be decreased. For example, the person may be able to maintain their speed but not also maintain their lane. They may be able to maintain their lane but not react if a vehicle violates a traffic law. This lack of coordination and diverted attention affects driving and the safety of those on the roads. (University of Michigan Health, How alcohol impairs your ability to drive).


To correctly perceive the environment around them, a driver needs to be able to see. That’s why a vision test is required to get a driver’s license. Drinking impacts sight. Double vision, blurry vision and weakened eye muscles can all impair a person’s ability to drive safely after drinking. (Optimax, The impacts of high alcohol consumption on your eyesight).

Alcohol depresses the central nervous system. Alcohol releases γ-aminobutyric acid (GABA). GABA is an inhibitory neurotransmitter. By disrupting the brain’s communication, the person’s ability to think and respond in a normal way is impacted.

Many factors may determine the length and severity of disruption including the size of the person, age, the amount consumed over what period, food in the person’s stomach, medication and personal health. (Healthline, Why does alcohol make you drunk?).

Alcohol Affects Everyone Differently

Alcohol impairment, by percentage

While everyone is different, driving impairment begins with even a small amount of alcohol consumption:

  • .01 – Diverted attention
  • .02 – Tiredness
  • .03 – Crash risk increases sharply
  • .04 – Alertness impacted; crash risk increases 18%
  • .05 – Perception and visual functions decrease; crash risk increases 40%
  • .06 – Tracking problems
  • .07 – Cognitive tasks, motor skills and reaction time all impaired
  • .08 – Presumed in all states that the person’s ability to drive normally is poor
  • .10 – Very poor reaction to stimuli and unexpected events

(NSC, Impairment begins with the first drink)

Drunk driving laws prohibit operating over the legal limit and operating under the influence

Alcohol impacts different people differently. In addition, it may impact the same person differently in different circumstances. A person may be unsafe to drive even if their bodily alcohol content is under the legal limit.

In most states, the legal limit is .08% (per 100mL of blood or per 210 Liters of breath). That’s the bodily alcohol content where the law presumes that a person’s ability to drive is impacted. The State of Utah lowered its drunk driving limit even further to .05%. This legal change reduced fatal crashes by more than 5% in Utah. There was no corresponding increase in arrests. (NHTSA, Utah’s .05% law shows promise).

Over the legal limit vs. under the influence

All states prohibit driving over the legal limit. In South Carolina, the law prohibits driving when the driver’s “alcohol concentration is eight one-hundredths of one percent or more.” (S.C. Code § 56-5-2933). However, not only does the law prohibit driving over the legal limit, but it also outlaws driving under the influence of alcohol. South Carolina defines this as being “under the influence of alcohol to the extent that the person’s faculties to drive a motor vehicle are materially and appreciably impaired.” (S.C. Code § 56-5-2930). That may mean different things for different people. Some people may be impaired by alcohol at .08%, while others are impaired at much lower levels. The law anticipates either scenario. The driver is held accountable for driving over the legal limit or while impaired, whichever is lower.

An offender might be charged in the alternative – facing a charge of driving over the legal limit or under the influence. The proof may vary considerably with a charge of driving over the legal limit focusing on the reliability of chemical testing. At the same time, driving under the influence may focus on driving behaviors and field sobriety testing.

Drunk driving laws consider that alcohol affects everyone differently. A person need not be over the legal limit to face a drunk driving charge if the alcohol impacts their ability to drive.

Legal services for drunk driving accident victims

If you are a victim of a drunk driving accident, our law firm can help. We represent people who have been hit by drunk drivers. Contact us to talk about your situation.

Additional sources:

NSC, Impairment Begins with the First Drink

AccessMedicine, The effect of alcohol on neurotransmitters in the brain

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