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Just how accurate is a field sobriety test?

Field sobriety tests are often used to see just how impaired someone is when they’re not inside their vehicle. This involves a walk-and-turn test, for example, that checks how well someone can balance and follow directions. Another test, the horizontal gaze nystagmus (HGN) looks at the eyes and how they dilate and move in response to light and motion. The third, a one-leg stand, is again looking at balance and making sure that a person can follow directions.

These, usually used in collaboration with a Breathalyzer test, are used to show that a person is too impaired to drive or that they’re fine to continue on. However, failing these tests doesn’t necessarily mean that a person is impaired or intoxicated at all. Perfectly healthy individuals can, and do, fail.

How accurate is a field sobriety test?

The abovementioned tests are not 100% accurate, even when used together. Their accuracy is approximately:

  • 77% when using the horizontal gaze nystagmus test
  • 65% when using the one-leg stand test
  • 68% when using the walk-and-turn test

Even when all three tests are performed together, the accuracy only stands at 82%. That’s a long way from 100%, showing just how imperfect field sobriety tests are.

Remember, too, that even when these tests do indicate that someone may be impaired, they don’t actually show the person’s ability to drive a vehicle or their exact blood alcohol concentration.

Should you challenge field sobriety test results?

Yes. There are ways to challenge these test results, and you should. For example, if you have a neurological condition, such as a seizure disorder or a traumatic head injury, you may fail the HGN despite not having a drop to drink. Similarly, if you have vertigo or poor balance, you may not be able to perform the walk-and-turn or one-leg stand tests well.

Failing these field sobriety tests doesn’t mean that you should accept a charge for driving under the influence. You have an opportunity to defend yourself, and you should use it. These tests are not as accurate as they should be, and there is plenty of room for error.

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