Field Sobriety Tests

Field Sobriety Tests: Introduction

If you or a loved one are arrested for suspicion of driving under the influence, then it is likely that you were asked to perform a series of field sobriety tests.  Law enforcement officers are trained to provide several different tests to determine whether they believe you are too impaired to operate a motor vehicle.  All police officers are trained on standardized field sobriety tests, which include the HGN, walk and turn, and one leg stand.  These tests are considered standardized because the National Highway Traffic Safety Administration trains all officers to conduct these tests in the same way.  Therefore, a law enforcement officer in the State of Oregon will be trained exactly the same as an officer in the State of Tennessee.  Additionally, there are non-standardized tests that officers are trained to conduct in each of their respective state training academies. 

 The Horizontal Gaze Nystagmus (HGN)

HGN Explained

The very first standardized test, and arguably the most accurate test, is the 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. Horizontal gaze nystagmus (HGN) refers to a lateral or horizontal jerking when the eye gazes to the side. In the impaired driving context, alcohol consumption or consumption of certain other central nervous system depressants, inhalants or phencyclidine, hinders the ability of the brain to correctly control eye muscles, therefore causing the jerk or bounce associated with HGN. As the degree of impairment becomes greater, the jerking or bouncing, i.e. the nystagmus, becomes more pronounced. This is assessed in the horizontal gaze nystagmus test. 

There are several types of nystagmus. Alcohol causes two types: alcohol gaze nystagmus, which includes HGN, and positional alcohol nystagmus. Although alcohol causes both, alcohol gaze nystagmus and positional alcohol nystagmus are very different and easily distinguishable. Testing for positional alcohol nystagmus is not a part of the standardized field sobriety test battery.

While conducting this test, law enforcement officers are looking for six "clues" of impairment, three in each eye:

  • Lack of Smooth Pursuit: The officer moves the object slowly but steadily from the center of the subject's face towards the left ear. The left eye should smoothly follow the object, but if the eye exhibits nystagmus, the officer notes the clue. The officer then checks the right eye.
  • Distinct Nystagmus at Maximum Deviation: Starting again from the center of the suspect's face, the officer moves the object toward the left ear, bringing the eye as far over as possible, and holds the object there for four seconds. The officer notes the clue if there is a distinct and sustained nystagmus at this point. The officer holds the object at maximum deviation for at least four seconds to ensure that quick movement of the object did not possibly cause the nystagmus. The officer then checks the right eye. This is also referred to as "end-point" nystagmus.
  • 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 edge of the suspect's left shoulder. The officer notes this clue if the point or angle at which the eye begins to display nystagmus is before the object reaches forty-five degrees from the center of the suspect's face. The officer then moves the object towards the suspect's right shoulder. For safety reasons, law enforcement officers usually use no apparatus to estimate the forty-five degree angle. Generally, forty-five degrees from center is at the point where the object is in front of the tip of the subject's shoulder.

The bad news is that this test is very scientific and hard to understand.  The good news is that this test is inadmissible in Tennessee courts, unless the officer is qualified as an expert in optometry.  The other good news is that an officer is rarely qualified as an expert in optometry, which means that the most accurate indicator of impairment is normally not even admissible in court.

HGN Observed

Walk and Turn (WAT)

Walk and Turn Explained

The second standardized field sobriety test is the nine step walk and turn.  Although the explanation of the HGN may be overwhelming, the remainder of the tests are fairly simple.  They are designed to determine whether you can divide your attention between doing multiple tasks.  Driving a motor vehicle requires doing several things at once.  Therefore, law enforcement are trained to determine whether you can also do several things at once.

While conducting this test, law enforcement officers are looking for eight "clues" of impairment:

  • Loss of balance during instruction phase;
  • starting the test before instructions are completed;
  • stops while walking to regain balance;
  • misses heel-to-toe;
  • steps off line;
  • uses arms for balance;
  • makes an improper turn;
  • takes an incorrect number of steps.

NHTSA research indicates that 79% of individuals that show two or more "clues" will have a blood alcohol level of .08% or greater.

Walk and Turn Observed

One Leg Stand (OLS)

One Leg Stand Explained

The last standardized field sobriety test is the one leg stand.  During this test, a person is instructed to stand with one foot approximately six inches off the ground and count out loud by thousands until told to put their foot down.  The officer times the person for 30 seconds but will not tell the person that in the beginning. While conducting the test, law enforcement officers are looking for four "clues" of impairment:

  • swaying while balancing;
  • using arms for balance;
  • hopping to maintain balance;
  • putting foot down.

NHTSA research indicates that 83% of individuals who exhibit two or more "clues" will have a blood alcohol level of .08% or greater.

One Leg Stand Observed

What does all of this mean?

If you have been arrested for driving under the influence and just read all of this content, then your anxiety probably went up.  Don't worry!  An experienced DUI lawyer will know how to conduct these tests.  An experienced DUI lawyer will also be able to ascertain whether the officer instructed you correctly on your field sobriety tests.  Although these tests are very technical, they also require very specific instructions.  An untrained or inexperienced officer can instruct these tests in such a way as to cause you to fail them.  Therefore, if you are arrested for DUI, make sure your lawyer understands the complexities of field sobriety tests.