LEGAL GUIDE
Written by attorney Elspeth Gray Crawford | Feb 23, 2015

Standardized Field Sobriety Testing in North Carolina

Standardized Field Sobriety Testing (SFST)

The Standardized Field Sobriety Test (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. These tests were developed as a result of research conducted by the National Highway Traffic Safety Administration (NHTSA). Based on the research, NHTSA has developed a training course to help law enforcement officers become more skillful at detecting drivers under the influence of alcohol. An officer who is certified by NHTSA in conducting SFSTs is not only more skilled at detecting DWI suspects, he is better able to describe the behavior of these suspects and in presenting effective testimony in court. The three tests of the SFST are as follows:

Horizontal Gaze Nystagmus (HGN) Walk and Turn (WAT) One Leg Stand (OLS)

These tests are administered systematically and are evaluated according the the measured responses of the suspect. The SFSTs are considered the "gold standard" in psychophysical testing used to detect DWI suspects. It is important not only to know how these tests are conducted, but to understand how these tests are used to collect evidence to use against you in court.

Horizontal Gaze Nystagmus (HGN) Nystagmus is defined as an involuntary jerking of the eyes. Horizontal Gaze Nystagmus (HGN) is an involuntary jerking of the eye that occurs naturally as the eyes are rotated at high peripheral angles. In other words, when you look really hard to the side your eyes involuntarily jerk (or bounce back). Alcohol and certain drugs cause exaggerated HGN and trigger HGN at angles lesser than 45 degrees. A person under the influence of alcohol will also have difficulty smoothly tracking a moving object.

Before conducting the HGN test, an officer needs to check for three things: equal pupil size, resting nystagmus, and equal tracking ability. The presence of any of these three items may indicate a possible medical impairment, such as a traumatic brain injury (TBI) or medical disorder, and negate the results.

During the HGN test, an officer observes the eyes of a person as they slowly follow a small moving stimulus (e.g., a pen, fingertip, or small flashlight) held between 12-15 inches in front of the person's eyes. The examining officer is looking for three indicators of impairment in each eye (for a total of six clues). Each time the officer passes the stimulus in front of one of the eyes, these clues are what he is checking for. The clues are as follows:

  1. Lack of Smooth Pursuit: Left Eye

  2. Lack of Smooth Pursuit: Right Eye

  3. Distinct and Sustained Nystagmus at Maximum Deviation: Left Eye

  4. Distinct and Sustained Nystagmus at Maximum Deviation: Right Eye

  5. Onset of Nystagmus prior to 45 degrees: Left Eye

  6. Onset of Nystagmus prior to 45 degrees: Right Eye

Based on at least three scientific studies, research shows that if four or more clues are evident, it is likely that the person's blood alcohol concentration (BAC) is above 0.10. With four or more clues present, the test is 77% accurate. Although courts consistently reject any attempt to derive a quantitative estimate of BAC from nystagmus, HGN is the most reliable field sobriety test.

Vertical Gaze Nystagmus (VGN) Although not commonly tested for, Vertical Gaze Nystagmus (VGN) is an involuntary jerking of the eyes when they gaze upward at maximum elevation. When a person looks up with their eyes as high as they can, they involuntarily jerk up and down (the eyes bounce down from the top). VGN has proven to be a reliable indicator of high doses of alcohol for that individual. VGN may also indicate the presence of other certain drugs.

​ Walk and Turn (WAT) The Walk and Turn (WAT) and One Leg Stand (OLS) tests both utilize the concept of divided attention: they require the subject to concentrate on two things at once. People under the influence of an impairing substance have difficulty with tasks requiring their attention to be divided between simple mental and physical exercises. Impaired drivers often ignore the less critical tasks of driving in order to focus their impaired attention on the more critical tasks. An impaired driver, for example, may ignore a traffic signal and focus instead on maintaining vehicle speed.

The WAT test consists of two stages: the Instructional Stage and the Walking Stage. It is critical that the officer ensures that the subject is not injured and doesn't have difficulty walking before administering the WAT.

During the Instructional Stage, the subject must stand with their feet in a heel-to-toe position, keep their arms at their sides, and listen to the instructions for the Walking Stage. This divides the subject's attention between a balancing task and an information processing task.

During the Walking Stage, the subject takes nine heel-to-toe steps, makes a prescribed turn using a series of small steps, and takes nine heel-to-toe steps back. The subject must count the steps out loud, watch their feet as they walk, and refrain from using their arms for balance. The Walking Stage divides the subject's attention among a balancing task (walking heel-to-toe and turning), a small muscle control task (counting out loud), and a short-term memory task (recalling the number of steps and the turning instructions).

An officer administering the WAT test is trained to observe the subject's performance for eight clues:

Can't balance during instructions Starts too soon Stops while walking Doesn't touch heel-to-toe Steps off line Uses arms to balance Loses balance on turn or turns incorrectly Takes the wrong number of steps

The inability to complete the WAT test occurs when the subject:

Steps off the line three or more times Is in danger of falling Cannot do the test

Based on the same scientific studies referenced above in the HGN section, if a subject displays two or more of the clues, or is unable to complete the test, the subject's BAC is likely to be above 0.10. These results have proven to be accurate 68% of the time. When the HGN and WAT tests are combined, results have shown to be 80% accurate that BAC is at least 0.10.

​ One Leg Stand (OLS) The One Leg Stand (OLS) is a divided attention consisting of two stages: the Instructions Stage and the Balance and Counting Stage. It is critical that the officer ensures that the subject is not injured and physically able to stand on one leg before administering the OLS.

During the Instructions Stage, the subject must stand with feet together, keep arms at sides, and listen to instructions. This divides the subject's attention between a balancing task and an information processing task.

During the Balancing and Counting Stage, the subject must raise one leg, either leg, with the foot approximately six inches off the ground, keeping the raised foot parallel to the ground. While looking at the elevated foot, the subject must count out loud in the following manner: "one thousand one," "one thousand two," "one thousand three," until told to stop. This divides the subject's attention between balancing and counting.

The officer administers the OLS test for exactly 30 seconds. The timing of the 30 second period by the officer is extremely important. The original research for the SFSTs shows that many impaired subjects are able to stand on one leg for up to 25 seconds, but that few can do so for 30 seconds.

An officer administering the OLS test carefully observes the subject's performance to look for four specific clues:

Sways while balancing Uses arms to balance Hops Puts foot down

Inability to complete the OLS test occurs when the subject:

Puts the foot down three or more times during the 30-second period Cannot do the test

If a subject produces two or more clues or is unable to complete the test, research shows that the subject is likely to have a BAC above 0.10. These results have been found to be accurate 65% of the time.

Combined Measures and Statistics According to a NHTSA study, when the three component tests of the SFST battery (HGN, WAT, and OLS) are combined, officers are accurate in 91% of cases when making the decision to arrest a subject for a BAC of at least 0.08. This number increases 94% if explanations for some of the false positives are accepted.

BAC: 0.08 HGN: 88% WAT: 79% OLS: 83% Combined: 91%

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