Carolina Criminal & DUI Lawyer BlogWhen a law enforcement officer in North Carolina suspects impaired driving, they may conduct Standarized Field Sobriety Tests 
(SFSTs) to gauge whether enough evidence exists for an arrest or further chemical testing. Roadside dexterity tests—commonly the Horizontal Gaze Nystagmus (HGN) test, the Walk-and-Turn test, and the One-Leg Stand test—remain a subject of debate. Questions arise about whether these tests are truly “standardized,” whether they reliably they measure impairment or are overly subjective, and how courts treat SFSTs as evidence.
This post explains some of the history of SFSTs, what each test entails, and why a DUI defense lawyer might challenge how an officer administered them. If you have been asked to perform SFST or want legal advice on DWI defenses, call or text the Powers Law Firm at 704-342-4357, or email Bill Powers at Bill@CarolinaAttorneys.com. An informed understanding of roadside tests may help in deciding how to respond during a traffic stop and in developing a defense strategy tailored to the specifics of your individual DUI charges.
Field Sobriety Tests: Table of Contents
- Introduction to Field Sobriety Tests
- The Science (and Skepticism) Behind SFSTs
- NHTSA Standards and Officer Training
- How Courts Treat SFST Evidence
- Defenses to SFST Results
- Practical Advice for Drivers and Officers
- Powers Law Firm: Legal Guidance on SFSTs
1. Introduction to Field Sobriety Tests
Field Sobriety Tests are roadside tasks that an officer requests when a driver shows signs of potential impairment—like weaving, falling asleep at an intersection, strong odor of alcohol, and positive PBT results. “Drunk Driving” dexterity tests date back to the 1970s, when the National Highway Traffic Safety Administration (NHTSA) began researching a standardized approach to detecting impaired driving. Initially, local police used ad hoc tests such as reciting the alphabet or touching the tip of your nose (the “finger to nose test”), which varied widely in reliability.
NHTSA’s studies led to three specific standardized tests:
- Horizontal Gaze Nystagmus (HGN): Observing a subject’s eyes as they follow a stimulus side to side.
- Walk-and-Turn (WAT): A divided-attention task requiring heel-to-toe steps in a straight line, a pivot, and a return (sometimes called the Heel to Toe test).
- One-Leg Stand (OLS): Having the subject stand on one foot and count aloud until told to stop.
Officers who adhere to NHTSA’s protocols are better able to detect appreciable impairment. Defense lawyers focus on the administration of SFSTs and whether external factors like health or environment are fully considered in determining probable cause and the arrest decision. Want more information? Check out our DUI FAQs here.
2. The Science (and Skepticism) Behind SFSTs
Horizontal Gaze Nystagmus (HGN)
HGN checks for involuntary jerking of the eyes that intensifies with alcohol consumption. The officer typically holds a pen or small flashlight 12–15 inches away from the subject’s face, moving it slowly from side to side. Theoretically, as alcohol impairs the brain’s ability to control eye muscles, the jerking becomes more pronounced.
Potential Issues: Conditions like fatigue, certain medications, or natural nystagmus can complicate HGN results. If an officer moves the stimulus too quickly or at the wrong angle, the reliability dips. Critics argue that failing to follow exact procedures can lead to false positives or “clues” that do not actually indicate impairment.
Walk-and-Turn (WAT)
This test has the subject walk nine heel-to-toe steps along a straight line, pivot using a series of small steps, and return the same way. Officers watch for “clues,” including stepping off the line, failing to touch heel-to-toe, using arms for balance, or taking an incorrect number of steps.
Potential Issues: Balance problems, foot pain, or sloped pavement may cause missteps unrelated to alcohol. If an officer neglects to demonstrate the test carefully or fails to confirm the subject is wearing stable footwear, the subject might perform poorly for reasons unrelated to intoxication.
One-Leg Stand (OLS)
The subject lifts one foot six inches off the ground, keeping arms at the sides while counting aloud—often to 30 seconds. The officer looks for swaying, hopping, or lowering the foot.
Potential Issues: Knee injuries, obesity, or advanced age may hinder balance. Stress and anxiety about a police encounter can create instability, as can wearing high-heeled shoes. Failing to mention these factors or skipping any required instructions may raise questions about the one leg stand test and its fairness.
3. NHTSA Standards and Officer Training
The NHTSA guidelines lay out detailed instructions, including how to position the subject, what instructions to give, and how to score each “clue.” For instance, HGN requires checking three distinct clues per eye at certain angles, WAT dictates precise demonstration and pivot steps, and OLS includes a recommended counting duration.
Officers typically take multi-hour courses, learning each step to reduce subjective errors. In North Carolina, training may include practice sessions, written exams, and supervised demonstrations. However, if an officer lacks formal certification or deviates from the manual, a defense attorney may argue the evidence is unreliable, both as to the arrest decision and during the guilt/innocence phase of a trial.
Common Pitfalls include:
- Skipping or Rushing the Demonstration: If the driver doesn’t see the proper way to walk heel-to-toe, confusion is likely.
- Ignoring Health Issues: A driver with back problems or knee pain might sway on one leg regardless of alcohol consumption.
- Failing to Check Footwear or Road Conditions: Gravel, a steep shoulder, or high heels can affect balance more than mild impairment.
Even seemingly small omissions can undercut the reliability of SFST results and prompt motions to suppress or reduce their weight at trial.
4. How Courts Treat SFST Evidence
North Carolina courts generally allow Field Sobriety Test testimony if officers can demonstrate they followed recognized standards. The officer may describe each “clue” observed, citing how many times the subject stepped off the line or lost balance.
HGN: Often treated as specialized or scientific evidence, requiring a foundation that the officer is properly trained to detect nystagmus. If the officer misses a step—like incorrectly timing each pass—the defense may raise doubt about the result.
Walk-and-Turn / One-Leg Stand: Typically considered “observational” tests rather than purely scientific. Officers explain the subject’s actions (did they stumble, use arms for balance, fail to follow instructions?). The defense can question the roadside environment or the subject’s medical limitations.
Courts emphasize that these tests are investigative tools, not conclusive proof. Juries often hear about other factors, like driving behavior or the odor of alcohol, to form a bigger picture of whether the person truly was impaired.
5. Defenses to SFST Results
Challenging Officer Compliance
If the officer did not follow NHTSA protocols or complete all steps, the subject’s “failing” performance may be invalid. Demonstrations should be thorough, instructions must be clear, and the officer should note footwear or physical injuries.
Medical or Environmental Factors
Defense lawyers frequently mention knee surgeries, inner ear disorders, obesity, or wearing unstable shoes as reasons for poor balance. Gravel shoulders, heavy traffic noise, or dim lighting may also affect results. Without acknowledging such issues, the officer’s conclusions may appear less reliable.
HGN-Specific Critiques
Nystagmus can be caused by fatigue, bright lights, or neurological conditions. If the officer fails to hold the stimulus at the correct angle or jerk the light too quickly, it might prompt “artificial” clues. Defense attorneys often scrutinize HGN to ensure each step was timed properly.
Observational vs. Scientific
If the court views HGN as scientific, the state must lay more foundation (training, proper procedure) than for WAT or OLS. Any shortfall can weaken the officer’s testimony.
6. Practical Advice for Drivers and Officers
For Drivers:
• Recognize that SFSTs differ from implied-consent breath or blood tests; there is no legal mandate to perform them, though refusal could still lead to an arrest if other impairment signs exist.
• Politely mention knee issues, footwear concerns, or balance problems. If you perform the tests, follow instructions carefully to avoid confusion-based errors.
• Nerves and stress are common. Officers may note if you seem anxious or fidgety, but remember that anxiety alone is not proof of impairment.
For Law Officers enforcing the NC DWI laws:
• Demonstrate each step carefully. Inconsistency with NHTSA guidelines may open the door to motions to suppress or cross-examination challenges.
• Document environmental conditions (road slope, weather, lighting) and ask about the driver’s health or footwear. This detail can affirm you considered potential alternate explanations.
• Keep training updated. Cases often hinge on whether the officer knew each step of HGN or properly scored WAT and OLS.
7. Powers Law Firm: Legal Guidance on SFSTs
Field Sobriety Tests play a significant role in how officers assess impairment. Yet questions persist about their objectivity, especially if injuries, road conditions, or rushed instructions skew the outcome. If you face a DWI charge after allegedly “failing” these roadside tests—or if you want legal insight on whether FSTs were administered properly—reach out to the Powers Law Firm.
Call or text 704-342-4357, or email Bill Powers at Bill@CarolinaAttorneys.com. We regularly investigate FST footage, analyze officer training records, and identify any technical issues that may serve as a strong defense. An informed approach to SFST challenges can make a substantial difference in whether probable cause holds up in court.