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Standardized Field Sobriety Tests & HGN (a.k.a. Stupid Human Trick #1)

According to the National Highway Traffic Safety Administration (NHTSA) a Field Sobriety Test (FST) is a psychophysical test that measures impairment in the cognitive and motor skills of a driver. Essentially, these are supposed to test a driver’s ability to pay attention to more than one thing at a time and also test his motor skills and/or balance.

For decades, officers have been administering varying forms of FSTs. These have ranged from reciting the alphabet to picking up the officer’s loose change that he threw on the ground. Beginning in the 1970s, NHTSA sponsored a series of pseudo-scientific research studies on FSTs through a contract with the Southern California Research Institute.

The results of these studies found that the three most reliable FSTs were 1) Horizontal Gaze Nystagmus (HGN), 2) Walk-and-Turn (WAT), and 3) One Leg Stand (OLS). The initial studies found that the combination of these three tests, if properly administered, could predict a blood alcohol content above .10 in approximately 80% of the cases. That means that the police would reach the wrong conclusion with 20 out of every 100 people tested.

In the mid-1990s, there were a series of validation studies completed in varying states. In the San Diego validation study, completed in 1998, the question that was being addressed was whether these three FSTs accurately identified blood alcohol contents between .08 and .10.  Interestingly, the study concluded that “correct arrest decisions” were made in 91% of the cases.

Of course, this rasises two interesting questions. First, what is the difference between a “correct arrest decision” and being over .08. I suppose that a “correct arrest decision” could be someone who was not over .08 at the time of driving, but whose blood alcohol rose after the stop and was over the limit at the time of administration. It could also be someone who doesn’t challenge the results. Or maybe even someone who was under .08, but could be charged with another crime such as impaired driving.

Secondly, what about the 9 out of 100 people who weren’t over .08? I guess NHTSA would have you believe that wrongly accusing 9 out of 100 people is ok.

Horizontal Gaze Nystagmus

Nystagmus is defined as an involuntary jerking of the eye. There are a myriad of factors that can cause someone to exhibit nystagmus such as rotational nystagmus (resulting from spinning in circle), caloric nystagmus (resulting from varying temperatures in each ear), positional alcohol nystagmus (resulting from alcohol altering the blood’s specific gravity in the blood and vestibular systems), optokinetic nystagmus (watching sharply contrasting images; strobe lights), physiological nystagmus (process that keeps the eyes from tiring), and other pathological disorders.

The HGN test is the one commonly known as the “eye test”. To perform this test, the cop will stand in front of you and ask you to look at his finger, pen, or flashlight. He’ll then pass his finger back and forth in front of your face. Each time he moves his finger from side to side, he’s attempting to observe one of the clues below.

Clue #1

The first clue that officers look for is lack of smooth pursuit. Essentially, they move their finger from side to side and ask you to follow it with your eyes only. They’re looking to see whether your eyes are able to follow it smoothly or if they make a jerking motion as they follow it. According to the NHTSA manual “[t]he eyes of an unimpaired person will follow smoothly, i.e., a marble rolling across a smooth pane of glass . . . .”

Clue #2

The second clue that a cop is looking for is nystagmus at maximum deviation. When the officer moves his finger to the side he’ll hold your gaze out at the corner of your eye and wait for approximately 4 seconds. According to the NHTSA manual, unimpaired people will exhibit some nystagmus at maximum deviation, but impaired people will have distinct and sustained nystagmus when held for more than 4 seconds. Essentially, the officer is trying to determine if your eyes are bouncing around more than they normally do — as if he knows what your eyes normally look like.

Clue #3

Finally, the officer is going to attempt to determine if you exhibit nystagmus prior to your eyes being at a 45 degree angle.  According to NHTSA, the onset of nystagmus prior to 45 degrees is an indication that your BAC is over .08. The officer doesn’t carry a protractor with him . . . he guesses at 45 degrees. If he’s off by 10 degrees — which could equal just a few inches — the results of his test may be completely unreliable.

Validity of the test

For this test to have any scientific validity, the officer must perform the test in strict conformance with NHTSA guidelines. This very rarely happens. If you’ve recently been charged with OWI/DUI and have had this test or another FST administered, you should contact me or another qualified DUI attorney immediately.

Joshua Blanchard holds a Juris Doctor from the Thomas M. Cooley Law School and is licensed to practice throughout the state of Michigan and in Federal Court. He provides an aggressive defense to people charged with serious crimes. For more information, or to confidentially discuss your situation, please call him at 877-MICH-DUI or e-mail josh@joshuablanchard.com.


This information may not be appropriate for your particular situation, including but not limited to drivers licensed in other states, drivers under the age of 21, drivers with a commercial driver license, drivers carrying a concealed weapon pursuant to a valid concealed pistol license, drivers on probation or parole, drivers who have been involved in a serious accident, or others. To understand how the law impacts your unique situation, you should always consult with a qualified attorney before taking any action. This is offered for informational purposes only and does not constitute legal advice or an offer to enter into an attorney-client relationship.