The roadside exercises feel like the heart of a DUI case, and for good reason, since they are vivid and easy for a jury to picture. What gets lost is that the standardized exercises were developed and validated for alcohol. Carrying them straight into a drug case, and treating a stumble as proof of drug impairment, asks them to do a job they were never tested for. As an instructor in these exercises, I know both what they can show and where that claim breaks down.
The roadside exercises earned their reputation against alcohol. Carrying them straight onto a drug case asks them to do a job they were never validated to do.
Built and Validated for Alcohol
The standardized field sobriety exercises, the eye test, the walk and turn, and the one leg stand, were studied against alcohol. The research that gave them their credibility measured how well they flagged drivers over the alcohol limit, with performance checked against breath and blood alcohol. Drugs were not the subject of that validation. So when an officer uses the same exercises to support a claim of drug impairment, the science that backs the exercises does not come along for the ride.
The government tested whether they work for cannabis, and they did not
You do not have to take a defense lawyer’s word that these exercises were not built for drugs, because NHTSA checked. In the analysis behind its 2017 report to Congress, researchers looked at whether the roadside exercises could tell whether a driver’s THC was above or below the line some states use, and the exercises could not do it. Only the finger-to-nose exercise showed any difference at all, and even the group as a whole did not meet basic sensitivity for predicting whether someone was impaired. These are alcohol tests being asked to do a job they were never validated for, and when the government measured how well they did that job, the answer was not well. So an officer’s account of a stumble on the shoulder is one person’s impression of a hard task rather than a drug test, and it identifies no substance and proves no impairment by one.
The Many Innocent Reasons to Stumble
Performance on these exercises is affected by far more than intoxication. Nerves, unfamiliar footwear, a sloped or gravel surface, injuries, a medical condition, age, and weight can all produce the kind of imperfect performance an officer may record as a clue. On a dark roadside, with a person frightened and off balance from the situation itself, the exercises are a noisy measure even for alcohol, and noisier still when offered as proof of a drug. Held to what they were validated to do, they describe a short performance, not a diagnosis.
What the Exercises Can Support
This is not a claim that the roadside exercises are worthless. As an instructor I know they have a real, narrow purpose: when administered correctly, they were designed to help an officer estimate whether a driver is over the alcohol limit. The problem is the leap from that purpose to a claim of drug impairment. The studies behind the exercises did not test them against drugs, the scoring was built around alcohol, and a poor performance has many ordinary explanations, including nerves, footwear, the road surface, injuries, age, and weight. Held to what they were validated to do, the exercises can describe how a person performed on a few tasks. They cannot, on their own, prove that a particular drug impaired that person at the wheel.
The Exercises the Evaluation Adds
The drug evaluation does not stop at the roadside exercises. It adds tasks such as the modified Romberg balance test, in which a person stands with eyes closed and estimates the passage of thirty seconds, and the finger to nose test. These are still observational tasks scored by an officer, and they carry the same questions as the rest. The internal clock in the Romberg test varies widely among sober people, so an estimate that runs fast or slow is weak evidence of anything by itself. Balance with the eyes closed is affected by fatigue, footwear, the surface, and ordinary anxiety. Adding more officer-scored exercises does not turn a set of subjective observations into a measurement, and each added task is one more place to ask whether it was administered and scored the way the protocol requires.
I teach the standardized exercises, so I know exactly what they were designed to detect, and it was alcohol, not drugs. When the State tries to turn a wobble on a dark roadside into evidence you were on something, I bring in the same ordinary explanations that undo these exercises in any case, the nerves, the fatigue, the shoes, the uneven ground, and I add the government’s own finding that they do not track cannabis. A hard physical task done imperfectly by a frightened person is not proof of a drug, and I make sure the jury sees the difference.
I started out as an Assistant Public Defender in Florida’s Thirteenth Judicial Circuit, in Tampa, and today I am one of six ACS-CHAL Forensic Lawyer-Scientists in Florida. I work both the science and the procedure in your case the way the State’s own analysts and officers are trained to, and I show a jury the exact point where the evidence does not hold up. Learn more about my background.
Questions About Field Tests in Drug Cases
Were field sobriety tests designed for drugs?
No. The standardized exercises, including the eye test, the walk and turn, and the one leg stand, were developed and validated to help detect alcohol impairment. Their validation studies were built around blood and breath alcohol, not drugs.
Can the exercises still be used in a drug case?
Officers do use them, but using them as proof of drug impairment stretches them past what they were validated to do. The defense can hold the State to that limit and show the jury what the exercises were built to measure.
Why does it matter that you are an SFST instructor?
Because the exercises only mean something when they are administered and scored correctly, and an instructor knows exactly how that is supposed to happen. I can show when the instructions, the demonstration, or the scoring departed from the standard, which drains the result of weight.
What can make someone perform poorly without any drug?
Many ordinary things, including nerves, unfamiliar footwear, an uneven or sloped surface, injuries, a medical condition, age, weight, and the stress of a roadside stop at night. None of these is a drug, yet each can affect performance.
Do the DRE psychophysical tests fix this?
Not really. The evaluation adds exercises such as the Romberg balance test and finger to nose, but they are still observational tasks scored by an officer, and they carry the same questions about administration, scoring, and innocent explanations.
Do field sobriety tests work for drugs?
They were built and validated for alcohol, not drugs. In the analysis behind the 2017 NHTSA report, the roadside exercises could not reliably tell whether a driver's THC was above or below the line states use, and only finger-to-nose showed any difference. A stumble identifies no drug and does not prove impairment by one.
Related pages: drug recognition expert overview, HGN and drugs, attacking DRE reliability, and why there is no per se drug limit.
This page is general information, not legal advice, and it does not create an attorney-client relationship. Drug DUI in Florida is governed by Fla. Stat. 316.193, section 877.111, and chapter 893, and expert testimony is governed by Fla. Stat. 90.702. Procedures and rules change, and every case turns on its own facts. Past results do not guarantee a similar outcome.
