Field sobriety tests (FSTs) are physical and cognitive exercises police use to assess impairment during DUI investigations. While officers present these tests as reliable indicators of intoxication, they're actually subjective evaluations with significant accuracy limitations—making them fertile ground for defense challenges.
The Standardized Field Sobriety Tests
The National Highway Traffic Safety Administration developed three Standardized Field Sobriety Tests (SFSTs) used nationwide:
Horizontal Gaze Nystagmus (HGN) involves following a stimulus (pen or finger) with your eyes while the officer looks for involuntary jerking. Nystagmus can indicate intoxication but also occurs from medical conditions, fatigue, and medications.
Walk and Turn requires walking heel-to-toe along a line, turning, and returning while following specific instructions. Officers look for losing balance, stepping off the line, using arms for balance, and other "clues."
One Leg Stand requires standing on one foot for 30 seconds while counting. Officers note swaying, hopping, putting the foot down, and using arms for balance.
Accuracy Limitations
NHTSA's own research shows FSTs are far from foolproof. Even when administered correctly, the combined battery is only about 83% accurate—meaning nearly one in five people could fail while sober, or pass while impaired.
Individual test accuracy is lower: HGN alone is roughly 77% accurate, Walk and Turn 68%, and One Leg Stand 65%. These are not the near-certain evidence officers sometimes suggest.
Challenging Test Administration
SFSTs must be administered exactly as trained—deviations reduce already limited reliability. Common administration errors include improper instructions or demonstrations, incorrect stimulus movement in HGN test, uneven or sloped testing surface, inadequate lighting conditions, not accounting for defendant's footwear, and failing to ask about medical conditions.
Body camera or dashcam footage often reveals administration errors that undermine test validity.
Medical and Physical Conditions
Many conditions affect FST performance without any impairment:
Physical conditions include inner ear problems affecting balance, leg, back, or knee injuries, obesity, age over 65, and footwear (heels, boots, sandals).
Medical conditions include diabetes, neurological disorders, eye conditions affecting nystagmus, and medications that affect balance or eye movement.
Environmental factors like cold weather, wind, rain, uneven surfaces, and distracting traffic can all affect performance.
Officer Subjectivity
FST scoring is inherently subjective. What one officer considers "failing" another might pass. Officers who've already decided to arrest may interpret ambiguous performance negatively.
Studies show officer expectation affects scoring—officers told subjects were intoxicated found more "clues" than officers who weren't told, even when reviewing identical performances.
You Can Refuse Field Sobriety Tests
Unlike chemical tests (breath or blood), you generally have the right to refuse field sobriety tests without automatic license suspension. There's often no penalty for declining FSTs, and refusing prevents the creation of subjective evidence against you.
However, refusal may prompt officers to arrest based on other observations. The decision whether to perform FSTs involves weighing potential outcomes—something you must decide quickly without legal consultation.
Non-Standardized Tests
Some officers use non-standardized tests like finger-to-nose, alphabet recitation, or counting backwards. These tests have no scientific validation for detecting impairment and are even more vulnerable to challenge than SFSTs.
Defense Strategies
Challenging FSTs involves analyzing whether tests were properly administered according to NHTSA protocols, identifying medical or physical factors affecting performance, highlighting environmental conditions that compromised tests, demonstrating officer subjectivity in scoring, and presenting expert testimony on FST limitations.
Excluding or undermining FST evidence can significantly weaken DUI prosecutions, potentially leading to dismissal or acquittal when chemical test results are marginal or unavailable.
Getting Legal Help
FST challenges require detailed knowledge of proper administration procedures and scientific limitations. An experienced DUI attorney can review footage, identify errors, and present effective challenges to field sobriety evidence.