Field Sobriety Tests


  • Field sobriety tests (FSTs) are OPTIONAL
  • The 3 most common FSTs are the: Horizontal Gaze Nystagmus (HGN), One Leg Stand (OLS), and Walk and Turn (W+T) tests

Field sobriety tests (FSTs) are performed by the police to establish probable cause to arrest a DUI suspect. Field sobriety tests are standardized tests. Therefore, it is crucial that they are administered correctly. If administered correctly, poor performance on the FSTs supposedly corresponds to a BAC of .10 or above.

Completion of FSTs is optional, and it may be to your advantage to decline to perform them. Read below to learn about the 3 most common FSTs, and how to defend against them.


The HGN test is one of three “Scientifically Validated” field sobriety tests. During the HGN test, the officer tests whether or not the individual is able to smoothly track a stimulus with their eyes. The officer uses either their finger or a pen to see if the eyes are able to follow without involuntarily jerking. Like all other field sobriety tests, completion of the HGN test is optional.

Horizontal Gaze Nystagmus Administration

According to the National High Traffic Safety Administration (NHTSA) training manual, an officer must take the following steps when administering the HGN test:

  • Check for eyeglasses (remove them, if applicable).
  • Verbally instruct the individual that their eyes are going to be checked
  • Verbally instruct the individual to keep their head still and follow the stimulus with their eyes only.
  • Verbally instruct the individual to keep following the stimulus with their eyes until told to stop.
  • Position a stimulus 12-15 inches from the individual’s nose and slightly above eye level.
  • Assess for possible medical impairment by checking for equal pupil size, resting Nystagmus.
  • Assess for possible medical impairment by moving the stimulus smoothly across the individual’s entire field of vision and check for equal tracking.
  • Check for lack of smooth pursuit in the individual’s left eye by moving the stimulus to the right at a speed that requires approximately two seconds to bring the individual’s eye as far to the side as it can go and two seconds back.
  • Do the same thing in the opposite direction for the right eye.
  • Check for distinct and sustained nystagmus at maximum deviation beginning with the left eye. To do so, move the stimulus to the individual’s left side until the eye has gone as far to the side as possible and hold it there for a minimum of four seconds. Move the stimulus all the way across the individual’s face to check the right eye, holding once again for four seconds.
  • Check for the onset of nystagmus prior to 45 degrees by moving the stimulus to the right to check the individual’s left eye at a speed that takes four seconds for the stimulus to reach the edge of the individual’s shoulder. Watch the eye for any jerking. If any jerking is seen, stop and verify that the jerking continues. Next, repeat the same procedure for the right eye. Repeat entire procedure.
  • Total the clues.
  • Check for vertical gaze nystagmus (VGN) [1]

Horiztonal Gaze Nystagmus Clues

When performing the HGN test, the officer looks for a total of six clues (three clues in each eye). According to NHTSA, if four out of six clues are present, it is likely the individual’s BAC is greater than .10. The six clues are:

  • Do the eyes move smoothly from side to side, or do they noticeably jerk?
  • When the eye is moved as far to the side as possible and is kept at that position for at least four seconds, does it jerk distinctly?
  • As the eye is moved toward the side, does it jerk prior to a 45-degree angle? [2]

Horizontal Gaze Nystagmus Flaws and Defenses

The HGN field sobriety test has two main flaws. First, it is often administered incorrectly. As you can see, there are many steps in administering this tests which are easy for an officer to overlook or rush through. The second main flaw of the HGN test is that there is a whole laundry-list of alternative explanations for the presence of nystagmus. These include, but are not limited to, lack of sleep and caffeine.


Like all other FSTs, you are not required to perform the One Leg Stand (OLS). There are two OLS administration stages: instruction and balancing/counting.

One Leg Stand Administration

Instructions Stage and Verbal Instructions
The officer begins the OLS by giving the following instructions:

  • “Please stand with your feet together and your arms down at the sides, like this.” (Demonstrate)
  • “Do not start to perform the test until I tell you to do so.”
  • “Do you understand the instructions so far?” (Make sure suspect indicates understanding.)

The instructions stage divides the subject’s attention between a balancing task (maintaining a stance) and an information processing task (listening to and remembering instructions). [3]

Demonstrations and Instructions for the Balancing and Counting Stage
The officer must further explain the test requirements as follows:

  • “When I tell you to start, raise one leg, either leg, with the foot approximately 6 inches off the ground, keeping your raised foot parallel to the ground.” (Demonstrate one leg stance)
  • “You must keep both legs straight, arms at your side.”
  • “While holding that position, count out loud in the following manner: ‘one thousand and one, one thousand and two, one thousand and three, until told to stop.'” (Demonstrate a count as follows: “one thousand and one, one thousand and two, one thousand and three, etc.” Officer should not look at his foot when conducting the demonstration–OFFICER SAFETY.)
  • “Keep your arms at your sides at all times and keep watching the raised foot.”
  • “Do you understand?” (Make sure suspect indicates understanding.)
  • “Go ahead and perform the test.” (Officer should always time the 30 seconds. Test should be discontinued after 30 seconds.)

Observe the suspect from a safe distance. If the suspect puts the foot down, give instructions to pick the foot up again and continue counting from the point at which the foot touched the ground. If the suspect counts very slowly, terminate the test after 30 seconds.

This stage divides the subject’s attention between balancing (standing on one foot) and small muscle control (counting out loud). [4]

One Leg Stand Clues

  • Sways while balancing
  • Uses arm to balance
  • Hops
  • Puts foot down

If a DUI suspect shows two or more of the above clues, or fails to complete the test, the officer assigns a BAC of .10 or more. [5]

One Leg Stand Flaws and Defenses

The main problem with the OLS is that it’s difficult to perform sober. The OLS requires a certain level of balance that many people don’t have due to past injuries or poor coordination. On top of that, performing the OLS on an uneven surface can negatively affect one’s performance on this FST. Even if it’s performed in perfect conditions by an individual who is physically capable, the OLS is only accurate 65% of the time at classify a BAC of .10 or above.


W+T Administration

The W+T test basically requires the suspect to take 9 steps in a straight line, then turn around and take another 9 steps in a straight line in the opposite direction. It is divided into two stages: instruction and walking.

Instructions Stage
The officer begins by having the DUI suspect get into a heel-to-toe stance by giving specific instructions and demonstrating those instructions.

Walking Stage
NHTSA lists the following verbal instructions for the W+T test:

  • “When I tell you to start, take nine heel-to-toe steps, turn, and take nine heel-to-toe steps back.” (Demonstrate)
  • “When you turn, keep the front foot on the line, and turn by taking a series of small steps with the other foot, like this.” (Demonstrate)
  • “While you are walking, keep your arms at your sides, watch your feet at all times, and count your steps out loud.”
  • “Once you start walking, don’t stop until you have completed the test.”
  • “Do you understand the instructions?” (Make sure the suspect understands)
  • “Begin, and count your first step from the heel-to-toe position as ‘One.'”

Walk and Turn Clues

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

If the officer observes two or more clues, there is a 68% chance the suspect has a BAC above .10.

Walk and Turn Flaws and Defenses

The analysis is similar to the above analysis of the OLS. Test conditions can negatively affect one’s performance on this FST. Also, officers are notorious for poorly instructing and demonstrating this FST to suspects.


  • DWI Detection and Standardized Field Sobriety Testing: Student Manual, February 2006 Edition, NHTSA [1]
  • See footnote #1 [2]
  • See footnote #1 [3]
  • See footnote #1 [4]
  • See footnote #1 [5]

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