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Texas Field Sobriety Test Lawyers

Field Sobriety Tests in Texas: What the Science Actually Says

The three standardized field sobriety tests (the Horizontal Gaze Nystagmus test, the Walk and Turn, and the One Leg Stand) are the most commonly used tools for building a DWI arrest without a chemical test result. Officers administer them on the roadside and use the results to establish probable cause for arrest and to support the prosecution’s case at trial.

What most people being asked to perform these tests do not know, and what prosecutors rarely explain to juries, is that these tests were validated under controlled laboratory conditions (not roadside conditions) and that the accuracy claims attached to them apply only when the tests are administered exactly as the research protocols specified. When the conditions deviate from those protocols, the validation no longer applies.

Managing Partner Deandra Grant is a trained SFST instructor. She has completed 24 hours of NHTSA standardized field sobriety test training and the NHTSA Field Sobriety Testing Instructor Course, qualifying her to teach these tests to other law enforcement officers and attorneys. She administers and grades the ACS-CHAL Forensic Lawyer-Scientist examination and the DUIDLA Board Certification examination. When she cross-examines an officer about their administration of field sobriety tests in a Texas courtroom, she is examining them on tests she is credentialed to teach.

The Validation Research: What It Actually Showed

The three standardized field sobriety tests were developed and validated through a series of research studies commissioned by the National Highway Traffic Safety Administration (NHTSA) and conducted by the Southern California Research Institute beginning in 1977. The studies produced accuracy estimates for each test when administered under the specific conditions of the research protocol.

Those conditions included a dry, level, well-lit testing surface; a sober, trained administrator; and test subjects who were not experiencing fatigue, illness, injury, or extreme emotional distress. The accuracy figures most frequently cited (77% for the Walk and Turn, 83% for the One Leg Stand, and 88% for the HGN) are flawed to begin with and specific to those controlled conditions. They are not accuracy figures for roadside administration at 2 a.m. on an uneven highway shoulder in headlight glare.

The defense implication is direct: when the conditions under which a test is administered materially deviate from the conditions under which it was validated, the validation research no longer supports the accuracy claim. Officers are trained to administer these tests in a standardized way precisely because any deviation compromises the reliability of the result. When that standardized administration did not happen, the officer’s opinion of impairment is not supported by the science the prosecution is relying on.

The Three Tests: What Each Requires and Where Each Fails

Horizontal Gaze Nystagmus (HGN)

The HGN test asks the subject to track a stimulus (the officer’s pen, finger, or a light) horizontally with their eyes while the officer observes for nystagmus, the involuntary jerking of the eye. The officer looks for six clues across both eyes: lack of smooth pursuit in each eye, distinct and sustained nystagmus at maximum deviation in each eye, and onset of nystagmus prior to 45 degrees in each eye. Four or more clues is the threshold for a positive result.

What the test requires of the officer.  The stimulus must be held 12–15 inches from the subject’s face and moved at a specific pace which is approximately two seconds across the full range of motion for the smooth pursuit passes, and four seconds held at maximum deviation for the nystagmus-at-maximum-deviation evaluation. The officer must be positioned correctly relative to the subject, must account for ambient light conditions, and must be able to clearly observe eye movement. Officers who move the stimulus too quickly, too slowly, or at an angle, or who do not maintain the correct distance, compromise the validity of the clue observations.

Non-alcohol causes of nystagmus.  Nystagmus can be caused by dozens of conditions unrelated to alcohol consumption, including: certain prescription medications (anticonvulsants, sedatives, muscle relaxants); inner ear disorders and vestibular dysfunction; brain injury or neurological conditions; fatigue; and the effects of flashing emergency lights (the very lights present at every traffic stop). An officer who identifies nystagmus without ruling out these alternative causes is offering an opinion that exceeds what the test supports.

The HGN is not admissible to prove a specific BAC.  Texas courts have held that HGN evidence is admissible to support an officer’s opinion of impairment but not to establish a specific blood or breath alcohol concentration. Prosecutors who imply otherwise are misrepresenting what the test shows.

Walk and Turn (WAT)

The Walk and Turn requires the subject to stand heel-to-toe in a defined starting position while receiving instructions, then walk nine heel-to-toe steps along a real or imaginary line, turn in a specific manner, and walk nine steps back. The officer watches for eight clues: cannot keep balance during instructions, starts before instructions are finished, stops while walking, misses heel-to-toe contact, steps off the line, uses arms for balance, turns improperly, or takes the wrong number of steps. Two or more clues is the threshold for a positive result.

Surface and environmental conditions.  The Walk and Turn was validated on a dry, level, hard surface. Gravel, grass, asphalt with a crown or slope, wet pavement, or any surface that is not level introduces an independent variable that affects balance performance in ways unrelated to alcohol. An officer who administers the Walk and Turn on a highway shoulder with a drainage slope and then reports that the subject ‘stepped off the line’ is reporting a result that the validation research does not support.

The instruction phase is itself a scored element.  An officer who does not give the complete standardized instructions, or who gives them in an abbreviated or non-standard way, affects the subject’s ability to perform the test correctly. The instructions include a specific demonstration that the officer is required to perform. Officers who skip the demonstration or modify the instructions are not administering a standardized test.

Medical and physical conditions.  The NHTSA research excluded subjects over 65 years old, subjects with inner ear disorders or back problems, and subjects who were more than 50 pounds overweight from the validation studies for the WAT and One Leg Stand. This means the accuracy figures for those tests were not established for these populations. When an officer administers the WAT to a subject who falls into one of these categories, the accuracy claim does not apply.

One Leg Stand (OLS)

The One Leg Stand requires the subject to stand with one foot raised approximately six inches off the ground, with toes pointed out, and count out loud by thousands until told to stop. The officer times the test for 30 seconds and watches for four clues: swaying, using arms for balance, hopping, and putting the foot down. Two or more clues is the threshold.

Fatigue and the 30-second duration.  A 30-second one-leg stand is physically demanding for many people regardless of sobriety. Research on postural sway has consistently demonstrated that the difficulty of standing on one leg increases significantly after the first 10 to 15 seconds as the stabilizing muscles fatigue. An officer who scores swaying or arm use in the final seconds of a 30-second stand without accounting for the role of muscle fatigue is misattributing a normal physiological response to alcohol impairment.

Footwear.  Subjects wearing heels, boots, sandals, or other footwear that does not provide a stable base are at a significant mechanical disadvantage on the One Leg Stand independent of any alcohol effect. The NHTSA protocol permits subjects to remove footwear before performing the test, but officers are not always consistent in offering this option.

The Negative Scoring System

A fundamental feature of the standardized field sobriety tests that is rarely explained to subjects or juries: the tests use a negative scoring system. A subject receives no credit for doing things correctly. Clues are counted only when something goes wrong. This means a person who walks eight of nine steps perfectly and steps off the line once receives the same score as a person who stumbled on every step. The tests are not designed to demonstrate sobriety. They are designed to identify clues that support a finding of impairment.