For more than three decades, I have worked in North Carolina courtrooms handling DUI cases where law, science, and technology collide.
When an officer testifies about roadside tests or a LCA – Licensed Chemical Analyst explains “breathalyzer” machine results, the evidence is often presented with the weight of scientific certainty. But those of us who defend DWI charges know that level of certitude is sometimes a bit overstated.
The reality is that DUI cases are a perfect storm of pharmocokenetics, engineering, and criminal procedure.
A defense lawyer may be cross-examining a trooper one day about the proper administration of standardized field sobriety tests, and the next day parsing laboratory records of blood draws, the inner workings of the AlcoSensor FST GC – gas chromatography. In order to be effective, you cannot simply rely on knowing the statutes or the latest case law. It helps to also develop fluency in the science that prosecutors present to juries (and judges) as fact.
That is what this series, Breath, Blood, and Bull: Sorting Real Science from Junk in DUI Cases, is designed to do.
The purpose is not to overwhelm you with jargon or push fringe theories. It is to offer a set of tools, legal, scientific, and practical tips that you can carry into the courtroom.
Some of these tools will affirm your instincts. Others may challenge what you thought you knew. A few may simply spark curiosity or serve as a reminder that scientific claims should be tested, not assumed. Indeed, that’s the basis of the scientific method.
What follows is not intended as a law review article or a comprehensive manual. It is a working set of materials, grounded in trial practice and years of teaching police officers, fellow defense lawyers, and legal professionals.
Some of the content reflects settled principles. Some touches on older, even discredited theories, because knowing the history helps us understand how we arrived where we are today. Reasonable lawyers will disagree on strategy, and the science itself is always evolving.
The point is not agreement but engagement.
If you take away even one idea that helps you cross-examine an officer more effectively, challenge a breath test with more precision, or advise a client with greater confidence, then my goal has been achieved.
For simplicity, and because many jurisdictions and the public commonly use the term “DUI,” we will generally adopt “DUI” throughout, even though various acronyms exist for impaired driving, including “DWI” or what some assume is “drunk driving.”
Studying DUI Defense – Panoply of Perspectives
Know this about these materials:
- Varied Complexity & Audience:
Recognizing that legal practitioners come from different backgrounds, some with extensive scientific or legal knowledge, others with limited experience, these materials present concepts at varying levels of depth and complexity. If even a single idea or “nugget” of information proves helpful in a particular case, the goal of these materials has been met. - Intent & Perspective:
Nothing herein is intended to be controversial or unkind, and reasonable minds can and do differ on many of these topics. Certain theories or approaches mentioned may be considered questionable, outdated, or “junk science” by some (including this author). They are included to spark critical thinking and conversation, not to endorse any particular viewpoint. The author may personally disagree with some of the ideas presented, but they are offered here to broaden the reader’s understanding of potential arguments. - Accuracy, Scientific Support & Historical Content:The information in these materials is provided for educational purposes only. Legal practices, scientific methods, and technologies vary and change over time. Some topics are provided for historical background.
- Sources:
Portions of these materials are adapted directly from various sources. Although formal footnotes or quotations are not included, readers should be aware that certain sections are drawn from previously published works. These materials are intended to serve as a teaching tool rather than a comprehensive law review article or published materials. - Jurisdiction & Verification:
Local preferences, protocols, and court rulings differ among the different jurisdictions in which lawyers operate. Always confirm any legal points, scientific references, or technological details with reliable, up-to-date sources. Readers should consult with experts to verify the accuracy and relevance of any information before relying on it in a real-world legal context.
Syllabus: Science and Technology in DUI Enforcement
- Introduction
- Defending DUI: Law Meets Forensic Science
- Purpose and Scope of These Materials
- Disclaimer: Accuracy, Jurisdictional Differences & Sources
- Standardized Field Sobriety Tests (SFSTs)
- Overview of SFSTs and Their Purpose
- The Three Validated Tests: HGN, Walk-and-Turn, and One-Leg Stand
- Science and Validity of SFSTs
- Common Errors in SFST Administration
- Challenging SFST Evidence in Court
- Breath Testing Technology
- How Modern “Breathalyzers” Work – The AlcoSensor FST & Intoximeter EC/IR II
- Calibration Requirements and Potential Malfunctions
- Common Sources of Error in Breath Testing (Mouth Alcohol, GERD, etc.)
- Strategies for Suppressing or Excluding Breath Test Results
- Blood Testing and Toxicology
- Forensic Blood Alcohol Testing (Headspace GC and GC/MS)
- Chain-of-Custody and Laboratory Procedures
- Errors in Blood Sample Handling: Contamination, Fermentation, and Storage Issues
- Drug Testing and Impairment Thresholds
- Challenges of Drug DUI
- Toxicology Techniques for Drug Detection
- Per Se vs. Impairment-Based Statutes
- Role of Drug Recognition Experts (DREs)
- Retrograde Extrapolation and Alcohol Metabolism
- Science of Alcohol Absorption, Distribution, and Elimination
- How Prosecutors Use Retrograde Extrapolation
- Defense Strategies to Challenge Retrograde Extrapolation
- Factors Affecting BAC Readings and Extrapolation Accuracy
- Conclusion and Final Thoughts
- The Importance of Continued Education
- Resources for Further Study
- Key Takeaways for the Defense Attorney
Standardized Field Sobriety Tests (SFSTs)
Overview of SFSTs and Their Purpose
One of the first hurdles in a DUI case is often the Standardized Field Sobriety Tests (SFSTs) conducted roadside.
Developed through research sponsored by the National Highway Traffic Safety Administration (NHTSA), SFSTs are a trio of “validated” psychophysical tests designed to help officers detect impairment.
The three standardized tests are:
- Horizontal Gaze Nystagmus (HGN)
- Walk-and-Turn (WAT)
- One-Leg Stand (OLS).
Each test assesses a subject’s balance, coordination, divided attention, and other faculties that alcohol (or drugs) can impair.
They are intended to be administered and evaluated in a standardized manner so that results are probative of impairment. NHTSA’s research purports that when these three tests are properly administered as a battery, officers can make reasonably reliable arrest decisions for drivers at or above 0.08 blood alcohol concentration (BAC).
The individual statistics, sometimes cited by prosecutors, highlight he intended reliability of SFSTs under ideal (laboratory) conditions.
However, as defense counsel know well, conditions in the field are rarely ideal, and the validity of SFST results can be highly questionable in practice.
The Three Validated DUI Tests: HGN, Walk-and-Turn, and One-Leg Stand
Horizontal Gaze Nystagmus (HGN)
The HGN test is a neuro-ophthalmologic exam wherein the officer observes the suspect’s eyes as they track a moving stimulus (like a pen or extended finger) horizontally.
Alcohol consumption can cause involuntary jerking of the eyes, known as nystagmus, which becomes more pronounced at high BAC levels.
In the HGN test, the officer looks for three clues in each eye:
- (1) inability to pursue the object smoothly
- (2) distinct nystagmus (jerking) at the eye’s maximum deviation to the side, and
- (3) onset of nystagmus before the eye reaches a 45-degree angle from center. Each eye is checked, yielding a total of six possible clues
NHTSA training materials instruct that observing four or more clues is indicative of a BAC at or above 0.08.
HGN has been traditionally considered the most reliable of the SFSTs; studies have found HGN alone to be around 77% accurate in identifying impaired subjects, higher than the other field tests
The scientific basis is that alcohol impairs the central nervous system, including the vestibular system controlling eye movements, leading to gaze nystagmus that is subtle in sober persons but exaggerated under the influence.
However, important caveats exist. Certain medical conditions (inner ear disorders, neurological issues) or even flashing police cruiser lights can produce nystagmus unrelated to alcohol.
Moreover, because HGN results derive from physiological responses, HGN evidence is scientific in nature.
While admissible as a test without further authentication under Rule 702 in North Carolina, the proponent must otherwise comply with testing protocols and qualify as an expert.
That means the officer should be properly trained and the test must be performed correctly, according to established protocols, to be admissible. That’s consistent with both the law and recent appellate level opinions in North Carolina. (See North Carolina v Moore)
Defense attorneys should be prepared to question whether the officer followed proper HGN protocol (positioning the stimulus 12–15 inches from the suspect’s nose, moving it at the correct speed, etc.) and whether the officer ruled out other causes of nystagmus.
In cross-examination, an officer who cannot articulate the scientific principles of HGN or who deviated from training could significantly undercut the weight of this evidence.
Expert witnesses (such as an optometrist or neurologist) may also be able to educate the jury on the limitations of HGN. For example, there are other causes of nystagmus that exist aside from alcohol.
The goal is to encourage the jury (and courts) from treating HGN as foolproof.
HGN is a neuro-ophthalmic sign that NHTSA selected and validated as an indicator correlated with impairment and with BAC under standardized conditions.
Its probative value in any given case depends on proper training, strict protocol, and ruling out alternate causes of nystagmus.
Walk-and-Turn (WAT)
The walk-and-turn test (the “heel to toe test”) is a divided attention psychomotor test. It requires the suspect to listen to instructions and remember them while performing a physical task, very much like the multitasking required for driving.
In the WAT, the subject is instructed to stand heel-to-toe on a straight line (real or imaginary), take nine heel-to-toe steps along the line, pivot in a prescribed manner, and return nine steps back, all while counting the steps out loud and keeping arms at the sides. Officers are trained to look for eight distinct clues of impairment during this test
- Cannot keep balance during instructions: Steps out of the heel-to-toe stance or sways noticeably before beginning the walk.
- Starts too soon: Begins walking before instructed to do so (indicating possible difficulty following instructions).
- Stops while walking: Pauses walking in order to regain balance.
- Fails to touch heel-to-toe: Leaves a gap of more than half an inch between heel and toe on any step.
- Steps off the line: Steps entirely off the designated line one or more times.
- Uses arms for balance: Raises one or both arms more than 6 inches from the sides to maintain balance.
- Improper turn: Does not execute the turn as instructed (e.g., loses balance during the turn or spins incorrectly).
- Wrong number of steps: Takes more or fewer than the instructed 9 steps out or back.
According to NHTSA research, two or more clues on the WAT correlate with an increased likelihood (around 68% accuracy by itself) that the subject’s BAC is at or above 0.08.
The science behind the WAT is that impairment affects coordination and information processing.
An intoxicated or impaired person may struggle to maintain balance on a straight line or remember complex instructions, whereas a sober person is better able to perform the task as directed.
However, numerous factors can undermine the WAT’s reliability.
Physical condition plays a role. People with leg, back, or inner ear problems, the elderly, or those with certain footwear (high heels and flip flops, for example) may have difficulty with the WAT even when entirely sober.
Environmental conditions are also important.
Optimally, the test should be done on a dry, level, non-slippery surface with sufficient space and lighting.
If the shoulder of a road is uneven or full of gravel, a perfectly able-bodied person might “fail” the WAT.
Officers sometimes neglect to inquire about a suspect’s medical issues or to note surface conditions, providing possible fertile ground for cross-examination.
A common administration mistake is giving incomplete or incorrect instructions (not specifying how to turn, or failing to demonstrate the heel-to-toe technique). Deviations from the standardized protocol can invalidate the test’s predictive value.
Defense counsel might benefit from reviewing the officer’s training manual or NHTSA guidelines on the WAT and highlight inconsistencies.
Put simply, all SFSTs are only as good as their administration and sloppy administration can very much yield unreliable results.
One-Leg Stand (OLS)
The one-leg stand is another divided attention test, focused on balance and timing. The suspect must stand on one leg (of their choosing) with the other foot raised approximately 6 inches off the ground, leg straight, and toe pointed out. While balancing, they are instructed to keep their arms at their sides and count aloud (usually “one thousand one, one thousand two…”) until told to stop (typically after 30 seconds). During this test, officers look for four clues of impairment:
- Swaying while balancing (noticeable side-to-side or back-and-forth motion of the body).
- Using arms for balance (lifting arms more than 6 inches from sides).
- Hopping to maintain balance on the standing foot.
- Putting the foot down one or more times before the test is over.
NHTSA training teaches that observing two or more of these clues indicates the likelihood of impairment (the OLS alone was found about 65% accurate in identifying impairment.)
Like the WAT, the OLS is premised on the theory that impaired motorists have difficulty dividing attention between a mental task (counting) and a balance task. It appears straightforward, but in practice, many sober people also struggle with the OLS.
Fatigue, age, poor balance, nervousness, or footwear can all affect performance. Moreover, the OLS requires that the suspect stand on one leg for up to 30 seconds; even slight muscle tremors or natural swaying can be misinterpreted as “clues.”
Officer mistakes in OLS administration include failing to time the 30 seconds (some officers terminate the test early or let it run much longer, which can influence performance), not instructing the suspect to keep looking at their raised foot (which has traditionally been part of the standard instructions), or ignoring environmental factors (uneven ground or wind gusts could cause anyone to sway).
On cross-examination, careful questioning might reveal, for example, that the officer never actually timed the test duration or that the suspect’s footwear was inappropriate (e.g., high heels or boots) but the officer didn’t allow removal or make note of it. By emphasizing these issues, a defense attorney could argue that the SFST results have little probative value for impairment.
Field Sobriety Tests and DUI Defense: Separating Fact from Assumption
Impaired driving cases in North Carolina often require more than a surface-level understanding of law and procedure. They deserve a willingness to dig into the science and evidence the State relies upon.
Field sobriety tests, breath testing machines, and blood analyses all carry the appearance of precision, but that appearance is not always reality.
A scientifically informed defense can serve as one of the strongest tools you can bring into the courtroom, and it begins with questioning assumptions.
The next step in this discussion focuses on the Science and Validity of Standardized Field Sobriety Tests.
We will examine how these tests were developed, the limits of their accuracy, and the ways in which they can be challenged when conditions are less than ideal. By understanding both the strengths and weaknesses of SFSTs, you can better equip yourself to address them effectively at trial.
If you are a lawyer seeking guidance on DWI defense strategy, or if you are facing an impaired driving allegation yourself, I welcome you to reach out. I have spent more than thirty years studying the law, science, and technology in North Carolina’s courtrooms, and my goal is to ensure that evidence presented against a client is fair, reliable, and consistent with the law.
Call or text Bill Powers at the Powers Law Firm at 704-342-4357 to learn more.