What is the technical reason behind conducting field sobriety tests during a traffic stop?

Posted on behalf of Donahue & Walsh, P.C. | October 26, 2020

In the event as officer suspects you are driving under the influence of alcohol and pulls you over, what should you expect during this traffic stop? The three standardized field sobriety tests are the Horizontal Gaze Nystagmus Test, Walk-and-Turn Test and the One-Leg Stand Test. Let’s take a look at what these tests entail, and the scientific reasoning behind why they are administered.

Horizontal Gaze Nystagmus Test

This test searches for “nystagmus” which is an involuntary jerking or bouncing of the eyeballs. Nystagmus can occur when alcohol impairs the brain’s ability to control the eye muscles, leading to jerking or bouncing motions when eyes gaze to the side. As more alcohol is consumed, this jerking and/or bouncing of the eyes becomes more pronounced.

To administer the Horizontal Gaze Nystagmus Test, an officer holds an object 12-15 inches away from the suspect’s nose while moving the object from side to side. The suspect must follow the object with their eyes, while the officer examines the eyes for jerking or bouncing motions.

Walk-and-Turn Test

When one’s BAC reaches a .1 level or higher, alcohol begins to impact the midbrain, which controls muscle coordination, vision and speech. This test examines if impairment of the midbrain exists by searching for indicators such as weakened coordination and balance abilities.

To administer the walk-and-turn test, the suspect is instructed to take nine heel-to-toe steps in a straight line, before turning around and walking in a straight line again. The suspect must keep their arms by their side and count each step out loud.

Due to the fact that the consumption of too much alcohol impairs the frontal lobe of the brain which controls reasoning and judgement, this test also allows for examination of the ability to divide attention between simple mental and physical exercises. In the case of the walk-and-turn test specifically, these simple mental and physical exercises consist of counting out loud and walking in a straight line.

One-Leg Stand Test

To conduct the one-leg stand test, the suspect is instructed to raise one leg approximately six inches from the ground, while keeping their hands by their side. The officer instructs the suspect to count by thousands (one-one thousand, two-one thousand, three-one thousand etc.) until they are given permission to stop.

This test examines coordination, due to the fact that alcohol impacts the cerebellum, the area of the brain responsible for coordinating movement. Impairment of the cerebellum explains why an excess of alcohol consumption leads to impaired coordination, balance and slower reflexes.

Your Field Sobriety Test Can Be Challenged

According to National Highway Traffic Safety Administration (NHTSA) field sobriety tests are not one hundred percent accurate. With the help of an experienced Donahue & Walsh, P.C. criminal defense attorney, your charges can be forcefully challenged. Our attorneys routinely construct defenses tailored to our clients’ unique circumstances, with the determination to defend your freedom against prosecutors. If you need help dissolving a criminal charge and obtaining the results you deserve, contact us via: 815-472-8743 or fill out our intake form.

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