Standardized Field Sobriety Tests (SFSTs)
During a DUI traffic stop, officers typically administer a series of Standardized Field Sobriety Tests (SFSTs). These tests were developed by the National Highway Traffic Safety Administration (NHTSA) and are the primary tool officers use to establish probable cause for a DUI arrest. Understanding these tests and their limitations is critical to your defense.
Horizontal Gaze Nystagmus (HGN)
The HGN test measures involuntary jerking of the eye as it follows a stimulus (usually a pen or finger) moving horizontally. Officers look for three “clues” in each eye (six total):
- Lack of smooth pursuit — Does the eye follow the stimulus smoothly or jerkily?
- Distinct and sustained nystagmus at maximum deviation — Does the eye jerk when held at the far side?
- Onset of nystagmus prior to 45 degrees — Does the jerking begin before the eye reaches 45 degrees from center?
Limitations: Nystagmus can be caused by many factors besides alcohol, including medications, medical conditions, fatigue, caffeine, nicotine, and even naturally occurring nystagmus. The test must also be administered in strict accordance with NHTSA protocols to be valid.
Walk and Turn (WAT)
This divided attention test requires you to take nine heel-to-toe steps along a straight line, turn in a prescribed manner, and take nine steps back. Officers look for eight possible “clues”:
- Cannot keep balance during instructions
- Starts too soon
- Stops while walking
- Does not touch heel-to-toe
- Steps off the line
- Uses arms for balance
- Improper turn
- Incorrect number of steps
Limitations: This test is significantly affected by age (over 65), weight (50+ pounds overweight), footwear, road conditions, weather, lighting, medical conditions, and physical disabilities.
One Leg Stand (OLS)
This test requires you to stand on one foot with the other raised approximately six inches off the ground while counting for 30 seconds. Officers look for four “clues”:
- Swaying while balancing
- Using arms for balance
- Hopping
- Putting foot down
Limitations: Like the Walk and Turn, this test is affected by age, weight, footwear, road surface, medical conditions, and physical fitness. Many sober individuals cannot perform this test satisfactorily.
22 Factors That Affect Field Sobriety Tests
There are numerous factors that can cause a sober person to fail field sobriety tests:
- Age (over 65)
- Weight (50+ lbs overweight)
- Back, leg, or inner ear problems
- Footwear (heels, boots, sandals)
- Road surface conditions
- Weather conditions
- Lighting conditions
- Traffic passing nearby
- Fatigue or lack of sleep
- Nervousness and anxiety
- Medical conditions (diabetes, etc.)
- Prescription medications
- Eye conditions or contacts
- Previous injuries
- Non-standardized instructions
- Officer intimidation
- Uneven terrain or slope
- Wind or cold temperature
- Flashing police lights
- Natural lack of coordination
- Head trauma or concussion history
- Inner ear infections or vertigo
Non-Standardized Tests
Officers sometimes administer non-standardized tests such as the Finger-to-Nose test, Romberg Balance test, Finger Count, or reciting the alphabet. These tests have not been validated by NHTSA and have no established reliability in determining impairment.
NHTSA’s Own Research
According to NHTSA’s own validation studies, even when administered correctly, the three standardized tests combined are only approximately 91% accurate for BAC levels above .08 — meaning nearly 1 in 10 people could be wrongly classified. Individual test accuracy is even lower: HGN 77%, WAT 68%, OLS 65%.
Source Materials & Research
The following NHTSA studies and manuals form the scientific foundation of field sobriety testing. Understanding these materials is critical for challenging FST evidence.
NHTSA SFST Student Manuals (1984–2018)
NHTSA SFST Validation Studies
ARIDE & DRE Materials
NHTSA Driving Cues & Detection Guides
Why These Materials Matter: Officers must follow the NHTSA-standardized procedures exactly as trained. Deviations from the manual — in how tests are administered, demonstrated, or scored — can invalidate the results. A knowledgeable DUI attorney will compare the officer's conduct against these standards.