Field sobriety tests are not designed for failure in the sense of being deliberately rigged, but the scoring is unforgiving enough that a sober person can fail without doing much wrong. Two clues out of eight on the Walk-and-Turn is a failed test. NHTSA’s own validation research found roughly one in three sober people get flagged as impaired on that test alone. The exercises measure balance, coordination, and your ability to follow multi-step instructions under stress on the side of a road, and plenty of things besides alcohol will cost you points.
The Scoring Thresholds Are Lower Than Most People Expect
The three standardized tests developed by the National Highway Traffic Safety Administration all use a clue-counting system. The officer watches for specific behaviors and tallies them. Meet the threshold and you’ve failed.
On the Horizontal Gaze Nystagmus test, four clues out of six leads the officer to classify your BAC as at or above 0.08. On the Walk-and-Turn, the threshold drops to two clues out of eight. On the One-Leg Stand, it’s two clues out of four.1National Highway Traffic Safety Administration. Standardized Field Sobriety Testing Refresher Manual
In practice, that means if you take nine heel-to-toe steps forward and back but miss heel-to-toe contact once and raise your arms slightly for balance, that’s two clues and a failed test. The scoring is binary. There’s no sliding scale, no credit for mostly passing, and no distinction between a clue caused by alcohol and a clue caused by something else. An officer who sees you start walking before the instructions end and then step off the line because the shoulder is uneven has their two clues, whatever the reason.
What NHTSA’s Accuracy Data Actually Shows
A 1998 NHTSA-funded validation study tested the three standardized tests against confirmed BAC levels. The headline accuracy numbers at the 0.08 threshold:
- HGN: 88% accurate overall
- Walk-and-Turn: 79% accurate overall
- One-Leg Stand: 83% accurate overall
Combined, the three tests reached about 91% accuracy.2U.S. Department of Transportation (National Highway Traffic Safety Administration). Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent
Those numbers look reasonable until you pull out the false positive rates buried in the same research. The Walk-and-Turn correctly identified sober people as sober only 69% of the time. That leaves about 31% of sober subjects wrongly classified as impaired. The One-Leg Stand did a little better at 73% correct, meaning a 27% false positive rate.2U.S. Department of Transportation (National Highway Traffic Safety Administration). Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent Roughly one in four sober people failed the One-Leg Stand under controlled research conditions. On an actual roadside at night, the real-world rate is almost certainly higher.
Even combining all three tests produced only a modest correlation with actual BAC (r=0.69).2U.S. Department of Transportation (National Highway Traffic Safety Administration). Validation of the Standardized Field Sobriety Test Battery at BACs Below 0.10 Percent These are screening tools, not precision instruments.
Why Sober People Fail
NHTSA’s own training materials acknowledge that certain people will struggle regardless of sobriety. The agency’s participant manual notes that individuals over 65, people with back, leg, or inner ear problems, and people who are overweight by 50 or more pounds may have difficulty with the One-Leg Stand. Similar warnings apply to the Walk-and-Turn for people over 65 or with back, leg, or inner ear conditions.3National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual
The validation studies themselves had limits. Less than 1.5% of research subjects were over 65, and the studies contained no data on subject weight at all. The population most likely to fail for non-alcohol reasons was barely represented in the research that supposedly validated the tests.
Medical Conditions and Medications
Inner ear disorders, neurological conditions, arthritis, and leg injuries all impair balance independently of alcohol. Diabetes can cause dizziness and confusion when blood sugar drops, mimicking intoxication. Common prescription medications can trigger the exact eye-jerking the HGN test looks for. Anti-seizure medications like carbamazepine, lamotrigine, and phenytoin are known to produce nystagmus as a side effect.4National Center for Biotechnology Information (NCBI). Nystagmus Types – StatPearls Sedatives, lithium, and sedative-hypnotic drugs can do the same. Someone on prescribed anti-seizure medication could show four out of six HGN clues without any alcohol in their system.
Brainstem conditions, cerebellar degeneration, and brain tumors can cause gaze-evoked nystagmus identical to the alcohol-induced type.4National Center for Biotechnology Information (NCBI). Nystagmus Types – StatPearls Officers aren’t trained to tell them apart during a roadside stop.
The Testing Environment
NHTSA’s training manual specifies that the Walk-and-Turn and One-Leg Stand should be conducted on a “reasonably dry, hard, level, non-slippery surface.” If those conditions aren’t available, the manual recommends either moving to a better location or administering only the HGN test.3National Highway Traffic Safety Administration. DWI Detection and Standardized Field Sobriety Testing Participant Manual In practice, these tests are routinely given on gravel shoulders, grassy medians, and sloped pavement. Poor lighting, passing traffic, wind, and rain all make a hard test harder.
Flashing police lights can corrupt the HGN test too. NHTSA trains officers to position the subject facing away from cruiser lights to avoid triggering optokinetic nystagmus, an involuntary eye-jerking caused by focusing on multiple moving objects. When officers skip that step, the strobes can produce the very clue being scored against you.
Footwear and Nerves
High heels, boots, and stiff dress shoes make heel-to-toe walking and one-legged balancing substantially harder. Nervousness and adrenaline from the stop itself impair concentration, cause trembling, and make multi-step instructions harder to follow. None of these indicate impairment. All of them generate scoreable clues.
How Failed Tests Get Used Against You
Field sobriety tests exist primarily to give officers probable cause for an arrest. They are observational screening tools, not direct measurements of blood alcohol content. The officer counts clues, combines that with other observations like the smell of alcohol, slurred speech, or your driving pattern, and decides whether to arrest.
FST results can also be introduced as evidence at trial. Prosecutors use them as circumstantial proof of impairment, and that evidence matters most when chemical test results are borderline, unavailable, or suppressed. Defense attorneys challenge FST evidence by showing the officer didn’t follow NHTSA’s standardized procedures, that environmental conditions undermined reliability, or that a medical condition or medication explained the clues. An officer who gave the Walk-and-Turn on a steep gravel shoulder at 2 a.m. faces tougher cross-examination than one who used a level, well-lit parking lot.
You Can Usually Refuse the Tests
In most states, field sobriety tests are voluntary, and you can decline them without facing an automatic license suspension. Officers generally aren’t required to tell you the tests are optional, and many people perform them without knowing they had a choice. Refusing doesn’t mean the officer will let you leave. If other indicators of impairment are already present, you can still be arrested.5Justia. Refusing a Field Sobriety Test in a DUI Stop and Your Legal Rights
Chemical tests are different, and the distinction matters. Breath tests on an evidential machine at the station, blood draws, and urine tests fall under implied consent laws in every state. By driving on public roads, you’ve already agreed to submit to chemical testing if lawfully arrested for DUI. Refusing a chemical test triggers automatic penalties, most commonly a mandatory license suspension ranging from several months to a year or more, and the refusal itself can be used against you at trial.6Justia. Refusing a Chemical Test in a DUI Stop and Implied Consent Laws The portable roadside breath device, often called a PBT, sits in a gray area that varies by state.
So the tests aren’t rigged, exactly. They’re low-threshold screening tools with real false positive rates, administered under conditions the research didn’t test, scored by an officer who already suspects you. That’s not the same as being designed for failure. It’s close enough that the distinction may not help you much on the roadside.