The Exact SFST “Clues” Prosecutors Point To (And How They Frame Them As Impairment)
Horizontal Gaze Nystagmus (HGN): The “6 Clues” That Get Presented As Science
In many DUI prosecutions, the HGN test is treated like the most “scientific” of the Standardized Field Sobriety Tests (SFSTs), largely because it focuses on eye movement rather than balance. Officers are trained to look for six total clues—three in each eye: lack of smooth pursuit (the eye “jerks” instead of tracking smoothly), distinct and sustained nystagmus at maximum deviation (involuntary jerking when the eye is held to the side), and onset of nystagmus prior to 45 degrees (jerking starts before the eye reaches a roughly 45-degree angle). In court, prosecutors often use those terms—“distinct and sustained,” “smooth pursuit,” “45 degrees”—to make the observation sound precise and objective, and they may imply that more clues equals a higher likelihood of impairment. That framing can be persuasive to a judge or jury, especially if the officer testifies confidently and uses NHTSA-style phrasing that sounds like a checklist rather than a subjective roadside impression.
What gets lost, though, is that HGN still depends heavily on proper setup and administration. The officer needs appropriate positioning, lighting, correct timing, and a stimulus held at the right distance; they also need to screen for issues that can interfere with results. And even when an officer claims all six clues, that does not automatically mean there’s a reliable BAC correlation in the real-world setting of a stop. In contested cases, the defense often focuses on whether the officer’s testimony jumps from “I saw nystagmus” to “therefore the person was intoxicated,” without laying a careful foundation for scientific reliability or acknowledging other causes of nystagmus. The more the testimony overreaches—especially implying a specific BAC number from HGN—the more opportunities there are to challenge what the test can actually prove.
Walk-And-Turn (WAT) and One-Leg Stand (OLS): Divided Attention, “Clue Counting,” and Over-Interpretation
The Walk-and-Turn test is built around “divided attention”—the idea that you must balance while also following a sequence of instructions. Prosecutors commonly highlight the 8 clues officers score: can’t keep balance during instructions, starts too soon, stops while walking, doesn’t touch heel-to-toe, steps off the line, uses arms for balance, improper turn, and incorrect number of steps. The One-Leg Stand is similar—also divided attention—and is typically scored on 4 clues: sways, uses arms for balance, hops, or puts the foot down during the 30-second count. In court, these tests often get condensed into a simple conclusion—“failed”—and prosecutors use that conclusion to argue the driver lacked coordination and mental focus consistent with safe driving.
But “clue counting” can become misleading when the conditions are stacked against the person from the start. Roadside surfaces are rarely ideal, footwear isn’t always practical for heel-to-toe steps, nerves can affect performance, and instructions may be given quickly or inconsistently. Even small deviations—like a turn that isn’t exactly as demonstrated—can be written up as “improper turn,” even if it doesn’t reflect impairment. A key point for readers to understand is that these tests are supposed to be standardized, meaning the instructions and scoring language matter. When the instructions are muddled, the demonstration is unclear, or the officer blends standardized clues with personal interpretation, it becomes easier to argue that what looks like “impairment” in a report may actually be ordinary human variability under stressful, imperfect conditions.
How Field Sobriety Tests Become Evidence In Court (Narratives, Paperwork, Video, And Probable Cause)
From Roadside To Report: The Officer’s “Impairment Story” And Common Report Language
SFSTs don’t exist in a vacuum; they usually appear as the centerpiece of an officer’s broader “impairment story.” DUI police reports and probable cause narratives often follow a familiar pattern: a reason for the stop, then a string of “objective symptoms” such as odor of alcohol, bloodshot watery eyes, slurred speech, admitted to drinking, and unsteady gait. SFSTs are then positioned as the moment the officer “confirmed” what they suspected. That sequencing matters because it tends to read like a logical progression: observation → confirmation → arrest. By the time the report gets to phrases like “failed the walk and turn” or “displayed 6/6 HGN clues,” many readers (including jurors) feel like the conclusion is already baked in.
Defense work often starts by slowing that story down and checking whether it’s supported by detail rather than template language. For example, “unsteady gait” can be a conclusion—what does the video actually show? “Slurred speech” can be subjective—was there wind noise, poor audio, or a language barrier? And “admitted to drinking” may be ambiguous—one drink hours ago is not the same as impairment at the time of driving. SFST write-ups are especially vulnerable to exaggeration when the report uses broad labels instead of describing what happened step-by-step (what the instruction was, how it was demonstrated, what the person did, and whether the officer corrected misunderstandings). In court, those missing specifics can become leverage: if the report can’t show careful adherence to standardized instructions, the reliability of the “clues” becomes a fair question.
Probable Cause and Video Evidence: Where DUI Cases Often Turn
From a legal perspective, SFSTs are frequently used as a bridge between reasonable suspicion and probable cause. Once an officer believes they have enough to expand the encounter into a DUI investigation, they may ask the driver to perform SFSTs, and the results often become the key justification for an arrest and a request for chemical testing under implied consent rules. Prosecutors typically argue “totality of circumstances”—not any single clue, but the accumulation of driving, demeanor, statements, and test performance. That’s why SFSTs matter even in cases where there’s a breath test dispute, a refusal allegation, or no usable BAC result at all: they help the State tell the court, “The officer had enough to arrest, and the driver appeared impaired.”
This is also where dash cam and body cam footage can make or break SFST claims. Video can reveal whether instructions were rushed, whether the suspect was interrupted, whether the surface was uneven or sloped, and whether the alleged clues are even visible from the camera angle. It can also show timing—whether the 30 seconds on One-Leg Stand was truly 30 seconds, or whether the officer cut it short once they believed they had “enough.” Defense attorneys often look for mismatches between what’s written and what’s recorded: “stepped off line” when there is no clear line, “started too soon” when the officer began the test mid-instruction, or “couldn’t keep balance” when the person was standing normally. Sometimes the footage helps the prosecution, too—but either way, the video tends to anchor the case in something more concrete than a narrative summary.
Reliability Problems And Non-Alcohol Explanations (Using Data, Not Just Opinions)
What NHTSA Validation Studies Are Often Cited For (And What They Don’t Prove)
Prosecutors and officers often reference NHTSA validation research as support for the idea that SFSTs are “accurate.” In broad terms, those studies are commonly discussed as showing that, when properly administered and scored, the three-test battery (HGN, Walk-and-Turn, One-Leg Stand) can help officers make decisions about impairment. The key phrase is when properly administered. Even in favorable research, SFSTs are not marketed as a magic truth machine; they’re screening tools designed for a controlled protocol. And “accuracy” in a study setting is not the same thing as proof beyond a reasonable doubt in the messy, noisy, stressful environment of an actual roadside stop.
Real-world reliability issues are exactly where cases often live: inconsistent training, imperfect conditions, and human bias. SFSTs can generate false positives—people who look “impaired” on these tasks for reasons other than alcohol or drugs. There’s also the problem of confirmation bias: once an officer believes they have a DUI, the scoring can tilt toward interpreting borderline performance as a clue. That doesn’t mean every SFST is “invalid,” but it does mean the defense can reasonably ask: Was the test standardized? Was the officer careful and neutral? Were the conditions appropriate? Were there alternative explanations the officer ignored?
Medical, Neurological, and Environmental Factors That Quietly Skew Results
Many conditions can mimic the very “clues” officers look for. Eye movement issues relevant to nystagmus can stem from medical or neurological causes, migraines, head injuries, certain medications, fatigue, or inner ear/vestibular disorders. Balance and coordination can be affected by diabetes or hypoglycemia, neuropathy, back/knee/ankle injuries, anxiety or panic, and age-related balance changes. Even perfectly sober people can struggle with divided-attention tasks when they’re exhausted, stressed, or trying to process fast instructions while bright lights and passing traffic are in their peripheral vision. When a DUI case relies heavily on SFSTs, these “non-alcohol explanations” can be more than background noise—they can be the difference between a persuasive impairment narrative and a reasonable doubt argument.
Environmental and administrative variables matter just as much. Uneven pavement, gravel, rain, wind, poor lighting, tight clothing, and inappropriate footwear (boots, heels, slick soles) can easily produce swaying, hopping, or stepping off an imaginary line. Add in distractions—sirens, traffic, multiple officers talking—and performance can degrade quickly. A careful defense review asks whether the officer accommodated obvious issues, whether the person asked questions or seemed confused, and whether the officer’s instructions matched standardized expectations. When those factors stack up, SFST “failure” can become far less meaningful than the report makes it sound.
How Defense Attorneys Challenge Field Sobriety Tests (Motions, Objections, Cross-Examination, Experts)
Pretrial Motions That Can Weaken Or Exclude SFST Evidence
Challenging SFSTs isn’t only about arguing the person “did fine.” In many cases, the strongest leverage comes from pretrial motion work that attacks the legality of how the investigation unfolded. A defense attorney may examine whether the initial stop was lawful, whether the detention was improperly prolonged, and whether there was adequate justification to escalate into SFSTs in the first place. If a court finds a constitutional violation, it can limit or suppress certain observations that flowed from it—sometimes including the SFST evidence that later became the backbone of probable cause.
Even when suppression isn’t available, motions can narrow what the State can claim. For example, if the SFSTs were not administered in a standardized way, the defense may argue the evidence should be limited, or that the officer should not be allowed to present certain conclusions as if they are scientific measurements. These issues are often litigated through hearings where the officer’s training, memory, and compliance are tested under oath—long before a jury ever hears the story.
Admissibility, “Science” Challenges, and Cross-Examination That Exposes Weak Scoring
HGN, in particular, can trigger scientific-evidence concerns depending on the jurisdiction and how the State intends to use it. The core defense themes are usually the same: does the officer have documented training, did they administer the test correctly, did they rule out obvious alternative causes, and are they staying within the limits of what HGN can honestly support? Courts may allow HGN as evidence of possible impairment while restricting testimony that tries to convert HGN clues into a specific BAC. When the State overreaches—presenting HGN like a portable lab result—it can open the door to strong foundational objections and expert-driven rebuttal.
At trial, cross-examination often focuses on the gap between standardized scoring and what actually happened. Common lines of attack include whether the officer read instructions verbatim, gave a clear demonstration, allowed questions, accounted for footwear, ensured a safe and level surface, and timed the tasks correctly. Video impeachment can be especially powerful: if an officer wrote “stepped off line” but the footage shows steady heel-to-toe steps, that discrepancy can undercut credibility across the entire report. And even when some clues appear on video, the defense can highlight how stress, confusion, and roadside conditions can produce the same behavior without proving impairment beyond a reasonable doubt.
Evidence A Defense Team Collects To Rebut SFSTs (And A Publishing Checklist For Accuracy)
The Must-Get Records: Video, CAD Logs, Training Files, and SFST Paperwork
When SFSTs are a major part of the State’s case, the most helpful defense strategy is often building a clean, documented timeline. That starts with collecting the records that show what the officer saw, what they did, and when they did it. A strong evidence request typically targets body cam footage, dash cam video, and any booking-area video, because these can confirm (or contradict) the officer’s claimed clues and the exact wording of instructions. It also includes CAD logs and dispatch records, which can help establish timing—how long the detention lasted, when decisions were made, and whether the encounter was extended without new justification.
Defense teams also look for the “paper trail” behind the SFSTs: SFST worksheets, field notes, and the officer’s training and certification records tied to the NHTSA SFST student manual or related training standards. Those documents matter because they can show whether the officer was refreshed on protocols, whether they used the proper sequence, and whether their scoring language matches what’s expected. When there’s a mismatch—like missing worksheets, vague scoring, or training that’s outdated—SFST evidence can start to look less like a standardized assessment and more like an unverified opinion.
When Experts Matter + Publication Checklist (And Next Steps in Everett, WA)
Not every case needs an expert, but some absolutely benefit from one. A medical expert may help explain vestibular disorders, neurological issues, injuries, or diabetes/hypoglycemia that can mimic impairment clues. A forensic toxicologist may be critical when the allegation involves medications, drug impairment, or when alcohol numbers are being interpreted aggressively. In some situations, an SFST instructor-level reviewer can walk through the administration step-by-step and identify protocol deviations in a way that’s easy for the court to follow. The goal isn’t to “win on technicalities”—it’s to give the judge or jury a grounded explanation of why SFST conclusions may be overstated in that specific case.
If you’re publishing (or relying on) information about SFSTs, credibility matters. A simple accuracy checklist helps: keep the tone informational (not case-specific legal advice), define HGN/WAT/OLS clearly, avoid guaranteeing outcomes, and acknowledge that admissibility rules and procedures can vary. Include practical next steps for readers—request video early, write down medical factors, preserve receipts/timeline, and avoid guessing about what the tests “mean” without the full record. And if you’re dealing with a DUI charge tied to field sobriety tests, it’s worth talking with a defense team that regularly challenges SFST foundation, scoring, and video contradictions. Snohomish Law PLLC works with people facing DUI accusations in Everett, WA, and can help you understand how SFSTs are likely to be used against you—and what can be done to contest them.
If you want a practical, evidence-focused review, contact the office to discuss your situation and the records that should be requested right away.
Frequently Asked Questions:
Are field sobriety tests mandatory?
Often they’re presented as “requests,” but refusing can still have consequences in how an investigation unfolds. Speak with a lawyer about your local rules and your specific facts.
How accurate are field sobriety tests?
NHTSA-related materials discuss accuracy under standardized conditions, but real-world error rates can rise with poor conditions, protocol deviations, and bias.
Can medical conditions cause you to fail?
Yes—balance, coordination, and eye movement can be affected by many medical and physiological factors.
Can HGN be used if the officer isn’t properly trained?
Training and proper foundation are common battlegrounds; admissibility and limits on testimony vary by jurisdiction.
Do officers have to follow NHTSA exactly?
Deviations can undermine reliability and credibility, and sometimes support evidentiary limits—how much that matters depends on the court and the facts.
What if the video doesn’t show the clues in the report?
That discrepancy can be powerful impeachment material and may weaken the entire impairment narrative.
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