The Standard Field Sobriety Test

The Standardized Field Sobriety Test (SFST) is a series of three tests performed during a traffic stop to determine if a driver is impaired and/or over the legal blood alcohol concentration (BAC) limit, which is .08 in every jurisdiction in the United States.

Field Sobriety Test

Developed in the 1970s, these tests are scientifically validated and are admissible as evidence in court. A training program was developed to aid law enforcement to become skillful at detecting impaired drivers.

Some courts require the officer to be qualified as an expert or skilled witness in order to express an opinion about the SFST at trial. Ohio was the first and only state where courts have ruled that only evidence from an SFST, as opposed to a basic field sobriety test, is admissible in court.

The three tests of the SFST are the Horizontal Gaze Nystagmus Test (HGN), Walk-and-Turn (WAT) and One-Leg Stand (OLS).

HGN Test

Horizontal Gaze Nystagmus (HGN) is an involuntary jerking of the eye that occurs naturally as the eyes gaze from side to side. When sober, HGN occurs when the eyes are rotated at high peripheral angles. However, when a person is impaired by alcohol, HGN is exaggerated. An alcohol-impaired person might also have difficulty smoothly tracking a moving object. In the HGN test, the officer observes the eyes of a person as he or she follows a slowly moving object such as a pen, horizontally with his or her eyes. The officer looks for three signs of impairment in each eye:

  1. if the eye cannot follow a moving object smoothly,
  2. if jerking is distinct when the eye is at maximum deviation, and
  3. if the angle of onset of jerking is within 45 degrees of center.

If, between the two eyes, four or more clues appear, the suspect likely has a BAC of 0.08 or greater. Research by the National Highway Traffic Safety Administration (NHTSA) indicates this test allows proper classification of approximately 77 percent of subjects.

Walk and Turn

The Walk-and-Turn test and One-Leg Stand test require a person to listen and follow instructions while performing simple physical movements. Intoxicated people have difficulty with tasks requiring their attention to be divided between mental and physical exercises.

In the Walk-and-Turn test, the tester is directed to take nine steps, heel-to-toe, along a straight line. After taking the steps, the suspect must turn on one foot and return in the same manner in the opposite direction. The officer then looks for the below eight signs of intoxication.

  1. Cannot keep balance while listening to the instructions
  2. Begins before the instructions are finished
  3. Stops while walking to regain balance
  4. Does not touch heel-to-toe
  5. Steps off the line
  6. Uses arms to balance
  7. Makes an improper turn
  8. Takes an incorrect number of steps

NHTSA research shows 68 percent of people who exhibit two or more indicators during the test will have a BAC of 0.10 or greater.

One Leg Stand

In the One-Leg Stand test, the suspect is instructed to stand with one foot about six inches off the ground and count aloud by thousands until told to put the foot down. The officer times the subject for 30 seconds. The officer is looking for the below four signs of impairment:

  1. swaying while balancing
  2. using arms to balance
  3. hopping to maintain balance
  4. putting the foot down

NHTSA research shows 65 percent of people who exhibit two or more signs during the test will have a BAC of 0.10 of greater.

When the component tests of the SFST battery are combined, trained officers are accurate in around 90 percent of cases.

Contact an attorney for more information on standard field sobriety tests.