Standardized Field Sobriety Test
Law enforcement authorities have historically used a range of different tests to ascertain the sobriety of a driver who is pulled over for a traffic violation, suspected criminal activity, or as part of a sobriety checkpoint.
Examples of tests that are no longer used in most jurisdictions include the finger-to-nose, backward counting, and reverse-citation-of–the-alphabet tests.
After the accuracy of these tests was successfully challenged in many jurisdictions during the 1960s and 1970s, the National Highway Traffic Safety Administration (NHTSA) developed the Standardized Field Sobriety Test (SFST) in cooperation with the Southern California Research Institute. According to NHTSA, the SFST is a “battery of three tests administered and evaluated in a standardized manner to obtain validated indicators of impairment and establish probable cause for arrest.” The three tests that comprise the SFST battery and that are currently used in both Washington, D.C. and Virginia are described below.
“Nystagmus” is defined as a “congenital or acquired persistent, rapid, involuntary, and oscillatory movement of the eyeball.” “Horizontal gaze” nystagmus is the involuntary jerking when the eyes rotate to the side. Under normal circumstances, nystagmus occurs when the eyes are rotated at “high peripheral angles”; that is, when the eyes are moved far right or left.
Nystagmus can be exaggerated and occur at lesser angles when a person is under the influence of alcohol. An alcohol-impaired person will also have difficulty smoothly tracking a moving object; that is, the jerking may occur while the eyes are being rotated to the side.
In administering the test on a suspect, the police officer will observe the eyes of the suspect as the suspect follows an object, such as a pen or small flashlight, moving slowly horizontally. The examiner looks for three indicators of impairment in each eye for a total of six indicators or clues:
- The suspect’s eye cannot follow a moving object smoothly.
- Jerking is distinct when the suspect’s eye is at maximum deviation. (“Maximum deviation” occurs when the eye is rotated so far to the right or left that no white appears between the pupil and the side of the eye.)
- The angle of onset of jerking is within 45 degrees of the center. (If the pen is held 10 inches from the suspect’s eyes, 45 degrees would be 10 inches to the side.)
If, between the two eyes, four or more clues appear, the suspect likely has a blood alcohol content of 0.08 or greater. NHTSA research suggests that use of this test is accurate 88% of the time in predicting a blood alcohol level of 0.08% or greater for subjects who exhibit 4 or more clues. HGN might also indicate consumption of seizure medications, phenicyclidine, a variety of inhalants, barbituates, and other depressants.
Click here for the six “clues” and specific instructions on administering the test.
The Walk-and-Turn and One-Leg-Stand tests are “divided attention” tests that are easily performed by most people who are sober. The tests are physical dexterity exercises that require a suspect to listen and follow instructions while performing simple movements.
The Walk-and-Turn test requires the suspect to take 9 steps, heel to toe, along a straight line. After the 9 steps, the person must turn on one foot and return in the same manner in the opposite direction. The examiner looks for 8 indicators of impairment:
- The subject cannot keep his or her balance while listening to the instructions.
- The subject begins before the instructions are finished.
- The subject stops while walking to regain balance.
- The subject does not touch heel-to-toe.
- The subject steps off the line.
- The subject uses arms to balance.
- The subject makes an improper turn.
- The subject takes an incorrect number of steps.
NHTSA research indicates that the test is accurate 79 percent of the time in predicting a blood alcohol level of 0.08% or greater for individuals who exhibit two or more of these indicators.
For further information on the eight “clues” and the instructions used to administer the test, please click here.
In the One-Leg Stand, the suspect is instructed to stand with his or her arms together and one foot approximately six inches off the ground while counting by thousands (one thousand one, one thousand two, etc.) until told to stop. The officer times the suspect for 30 seconds. The officer looks for four indicators of impairment:
- The subject sways while balancing.
- The subject uses his/her arms to balance.
- The subject hops to maintain balance.
- The subject puts foot down during the test.
According to NHTSA research, the test is accurate 83 percent of the time in predicting a blood alcohol level of 0.08% or greater for subjects who exhibit two or more clues.
For further information on the four “clues” and the instructions used by the police officer to administer the test, click here.
Source: Appendix A, Standardized Field Sobriety Test, Development of a Standardized Field Sobriety Test, National Highway Traffic Safety Administration, U.S. Department of Transportation.