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How law enforcement determines driving impairment

On Behalf of | Sep 29, 2022 | DUI

How drunk is too drunk to drive? It is difficult to make this determination on your own because alcohol affects your judgment.

It can be difficult for authorities to assess your impairment as well. They use assessments that have gone through testing for accuracy, but there is a margin for error.

How does alcohol affect your driving ability?

Most states, including Virginia, have laws that make it illegal to drive with a blood alcohol concentration of 0.08% or greater. Depending on factors such as your body weight and the kind of alcohol you are drinking, you may reach this benchmark after consuming four drinks.

However, according to AAA, alcohol can start to impair your driving ability at a BAC of 0.02%. You may reach this benchmark after one drink.

How does law enforcement determine driving impairment?

If authorities pull you over on suspicion of impaired driving, they may perform a breath test to find out your BAC and whether it is over the legal limit of 0.08%. If so, the law presumes that you are unable to drive safely.

However, because impairment can occur at a BAC below the legal limit, law enforcement can use other assessments to determine whether or not you are safe to drive. There are three types of Standardized Field Sobriety Tests:

  1. One-Leg Stand Test
  2. Walk-and-Turn Test
  3. Horizontal Gaze Nystagmus Test

The first two are more or less what they sound like. The Horizontal Gaze Nystagmus Test involves an officer asking you to follow the motion of a pen or finger with your eyes. While you do this, he or she is looking for an involuntary ocular tic that becomes more pronounced when you are under the influence of alcohol.

Research has shown that taken together, these three tests are 90% effective at determining your driving impairment. Individually, however, none of these tests are anywhere near that effective.


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