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Police Use Of Field Sobriety Tests In Drunk Driving Stops

Generally, a police officer in a roadside stop investigating a suspected drunk driver will develop probable cause to arrest through the administration of standardized field sobriety tests (SFSTs). SFSTs are a battery of three tests developed by the National Highway Transportation Safety Administration (NHTSA), a division of the United States Department of Transportation. NHTSA SFSTs are designed to be used by officer’s during a roadside stop to assist the officer in determining if a driver is exhibiting signs of being under the influence of alcohol. If the driver fails the SFSTs then the officer will continue his drunk driving investigation – usually by requesting some type of alcohol testing.

In this blog, I will focus on a battery of SFSTs approved for use by law enforcement by the NHTSA. The NHTSA SFTS tests include: (1) the horizontal gaze nystagmus (HGN); (2) the walk and turn; and, (3) the one leg stand. During the admission of the SFTS, officers observe and record clues which are indicators of possible alcohol impairment in the subject being tested.

Horizontal Gaze Nystagmus (HGN):

“Nystagmus” is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculomotor control of the eye. “Nystagmus” is a medical term used to describe a “bouncing” eye motion that is displayed in two ways: (1) pendular nystagmus, where the eye oscillates equally in two directions, and (2) jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a “saccadic” or fast movement. NHTSA Horizontal Gaze Nystagmus: The Science and The Law.

HGN is a specific type of jerk nystagmus with the saccadic movement toward the direction of the gaze. Normally, the eyeball moves smoothly like a marble rolling over a glass plane. Conversely, with HGN jerk nystagmus, the eyeball moves like a marble rolling across sandpaper. Most types of nystagmus, including HGN, are involuntary motions, meaning the person exhibiting the nystagmus cannot control it. An interesting fact is that nystagmus does not effect the subject’s vision.

Alcohol gaze nystagmus (AGN) is gaze nystagmus caused by alcohol. AGN occurs when the eye moves from looking straight ahead (called resting nystagmus), to the side (called HGN), or up (called vertical nystagmus or VGN). This condition is analyzed by the officer at the roadside with the horizontal gaze nystagmus test. Id.

To perform the HGN the officer must administer the test in a way that ensures that the subject’s eyes can be seen clearly. The officer must put the subject in a well-lit area or use a flashlight to observe the subject’s eye movements. The officer must be careful to make sure the subject’s face is not facing blinking lights from the police car or the bright lights of oncoming traffic to avoid optokinetic nystagmus, a condition that would impact the reliability of the HGN.

The HGN can be administered with the subject standing or sitting down. HGN should be tested with the subject’s glasses removed, although contacts can remain. The HGN test requires only an object for the subject to follow with their eyes, such as a pen or the tip of a penlight. The officer places the object approximately twelve to fifteen inches from the subject’s face and slightly higher than eye level. Placing the object above eye level opens the subject’s eyes further and makes their movement easier to observe. The officer instructs the subject to follow the object with the eyes and the eyes only – the head should remain still. Id.

After positioning the object, but before conducting the test, the officer checks for signs of medical impairment. First, the officer checks for “equal tracking” by moving the object quickly across the subject’s entire field of vision to see whether the eyes follow the object simultaneously. The officer then checks for equal pupil size. Lack of equal tracking or equal pupil size may indicate blindness in one eye, a glass eye, a medical disorder or an injury. If the subject exhibits these characteristics, then the officer should discontinue the HGN test and may need to seek medical assistance for the individual if a medical disorder or injury appears to exist. Id.

While conducting the test, the officer looks for six “clues,” three in each eye, that indicate impairment. The left eye is checked for the clue and then the right eye. The clues are: (1) a lack of smooth pursuit where the officer moves the object slowly from the center of the subject’s face but the eye exhibits nystagmus; (2) starting again from the center of the subject’s face, the officer moves the object toward the ear to bring the eye over as far as possible and holds the object for three-four seconds looking for sustained nystagmus; and (3) angle of onset of nystagmus prior to forty-five degrees. In the angle of onset of nystagmus prior to forty-five degrees the officer moves the object at a speed that would take about four seconds for the object to reach the subject’s left shoulder. The officer is looking to see if the eye begins to display nystagmus before the object reaches the forty-five degrees from the center of the suspect’s face.

According to the NHTSA, When the HGN test is properly administered it can accurately predict the presence of a blood alcohol of .10 or higher in the subject eighty percent of the time. National Library of Medicine: Is the driver drunk? Oculomotor sobriety testing.

The Walk And Turn:

The walk and turn requires a person to listen and follow instructions while performing simple physical movements. The walk and turn consists of two stages: (1) the instruction stage; and, (2) the walking stage. NHTSA: Standardized Field Sobriety Tests.

During the instruction stage, the officer instructs the subject to maintain a heel to toe stage with arms at their sides and listen to the instructions. During the walking stage, the subject must take nine heel to toe steps in the prescribed manner, and then take nine heel to toe steps back. During the turn, the subject keeps their front foot on the line, turns in a prescribed manner, and uses the other foot to take several small steps to complete the turn.

The officer watches for the following eight clues:

  • Cannot keep balance while listening to instructions
  • Starts too soon
  • Stops while walking
  • Does not touch heel to toe
  • Steps off the line
  • Uses arms for balance
  • Improper turn
  • Incorrect number of steps

If the subject fails two of these eight clues, then he or she has failed the walk and turn test. Id.

According to the NHTSA, the walk and turn test is sixty-eight percent accurate in predicting blood alcohol levels of at least .10 or higher.

One Leg Stand:

The one leg stand is similar to the walk and turn test. The one leg stand test has two stages: (1) the instruction stage; and, (2) the balancing and counting stage. Id.

During the Instructions Stage, the subject must stand with their feet together, keep their arms by their side, and listen to instructions. In the balancing and counting stage, the subject must raise one foot, either foot, with the raised foot approximately six inches off the ground, with both legs straight and raised foot parallel to the ground. The subject, while looking at the elevated foot, counts out loud in the following manner: “one thousand one”, “one thousand two”, “one thousand three” until told to stop.

The officer watches the subject for the presence of the following four clues:

  • Sways while balancing
  • Uses arms for balance
  • Hopping
  • Puts foot down

If the subject exhibits two of the above clues, then he or she has failed the one leg stand test. Id.

According to the NHTSA, the one leg stand test is sixty-five percent accurate in predicting blood alcohol levels of at least .10 or higher.


The reliability of SFSTs has been a point of contention for defense attorneys challenging drunk driving prosecutions for decades. Central to the problems with SFSTs is that law enforcement officers administering the tests in a roadside stop frequently fail to follow the NHTSA standards for how each test should be used. An officer who fails to properly administer the HGN can often misreport a nystagmus or its origins. Likewise, a subject who is instructed to walk and turn or do the one leg stand can fail the test because of age or an underlying health issue, such as an inner ear problem or obesity. Critical to the use of the SFST is that each must be administered with the strict standards of the NHTSA.

The science of SFSTs has been widely accepted by NHTSA and courts in general. However, the proper administration of SFSTs is not always done by law enforcement. If you have a specific question about an SFST or have been charged with drunk driving, please contact Satawa Law for a no obligation strategy consultation.

Mark Satawa

Mark Satawa is a criminal defense attorney specialized in forensic DNA,
sex crimes, child abuse, shaken baby, medical child abuse, white collar,
and federal crimes.