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Do you know what's really going on in field sobriety tests?

In the unfortunate event you ever find yourself pulled over on suspicion of driving under the influence, there's a good chance that you will be asked to step outside of your vehicle to perform the Standardized Field Sobriety Test after the law enforcement official finishes checking your driver's license and registration.

In reality, the SFST is comprised of three separate tests -- horizontal gaze nystagmus test, the walk-and-turn test and the one-leg stand test -- all of which are designed to help law enforcement officials gauge a person's intoxication levels via observation of physical responses.

What exactly is the horizontal gaze nystagmus test?

The horizontal gaze nystagmus test -- or simply HGN test -- is the test in which a law enforcement official will have a person follow an object (finger, pen, flashlight, etc.) horizontally with their eyes.

What does horizontal gaze nystagmus mean?

Horizontal gaze nystagmusis the involuntarily jerking of the eye that occurs as it is directed to the side.

When a person is sober, this involuntary jerking occurs when the eye is rotated at a high peripheral angle. When they are under the influence of alcohol, however, this involuntary jerking is much more pronounced and occurs when the eye is rotated at a much lower peripheral angle.

What exactly is the law enforcement official looking for in an HGN test?   

The law enforcement official will look for three impairment indictors, including distinct jerking of the eye when it is rotated to a maximum peripheral angle, jerking of the eye at an angle of less than 45 degrees of center, and an inability to follow the moving object slowly and smoothly.

In the event there are at least four indicators between each of a person's two eyes, the standard procedure is to treat the person as likely having a blood alcohol content of .08 or above.

How accurate is the HGN test?

Previous research has found that the HGN test can properly classify 88 percent of all DUI suspects.

We will continue our discussion of the SFST in future posts. As always, remember to consider speaking with an experienced legal professional as soon as possible if you've been arrested for drunk driving, as you may be able to contest the field sobriety test.

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