Field Sobriety Tests and Why It’s Impossible to Pass Them


One of the most common statements I hear from clients who have been charged with a DUI, but maintain their innocence, is that they “passed” their Field Sobriety Tests. When I ask my clients why they think they passed, they usually respond that the officer told them so. Some believe they aced the tests. In fact, no one can pass the Field Sobriety Tests, because they are not designed to be passed.

Let me repeat that. No one can pass the Field Sobriety Tests.  Everyone fails them because they are more akin to a medical examination than the kind of test a teacher gives a student.

You may be thinking, “I know someone who took the Field Sobriety Tests and was allowed to drive home without getting a DUI. Doesn’t that mean they passed?” Well, no. It just means that the cop determined there was insufficient evidence to make an arrest, i.e. they couldn’t document enough symptoms to determine an underlying cause. And this helps clarify another important reason why Field Sobriety Tests are not like school tests: there is no universal pass/fail standard that applies to everyone equally.

What are Field Sobriety Tests exactly and why do cops use them?

The phrase “Field Sobriety Tests” (commonly shortened to FSTs) encompasses several roadside evaluations given by law enforcement officers. During the FSTs, the officer evaluates  the suspect’s balance, coordination, ability to follow rules, behavior, and other physiological responses that might indicate intoxication.

Of the six or seven roadside tests used in California, National Highway Traffic Safety Administration (NHTSA) has scientifically validated only three for use by officers: the Horizontal Gaze Nystagmus test (HGN), the Walk and Turn test (WAT), and the One Leg Stand test (OLS). Moreover, the officer must strictly follow NHTSA’s guidelines when administering FSTs. All other FSTs–including the Romberg Test, the Finger to Nose Test, the Preliminary Alcohol Screening, and the Alphabet test–have never been scientifically validated for use by officers, even though courts frequently admit the results as evidence.

How scientific are FSTs?

According to a handful of studies conducted during the 1990s, the HGN, WAT, and OLS tests show some usefulness in DUI investigations. When all three tests are administered, officers successfully predicted, with accuracy ranging between 86%-95%, whether the person they are evaluating has a blood alcohol concentration (BAC) at or above a .10%.[1]

The basic framework of these scientific validation studies is the same. Researchers take a group of officers, train them on administering FSTs, then they take another group of participants, have them consume alcohol with some random variation where some subjects are below, and some above .10% BAC. The researchers have the officers administer the FSTs to the other participants, while the officers mark down specific intoxication “clues”, and then based on those clues, guess as to whether the person they are evaluating is at or above .10% BAC. The results are written down and later evaluated. .

If you’re like me the first time I heard what it meant for a study to be “scientifically validated”, I was rather underwhelmed by what I heard, and I had a lot of questions, like:

  • How controlled were these studies?
  • How did the officers document clues prior to making their guesses?
  • Did all participants drink the same amount?
  • Were all participants close to .10% BAC or were some virtually sober while others closer  to .20% BAC?
  • What does any of this have to do with drugs?
  • If these tests are validated, and others are not, what does it say about those other tests, and why are they used at all or allowed as evidence?
  • What is the minimum success rate required to validate a test?

Fundamentally, FSTs are simply coordination and concentration exercises intended to trick people into making mistakes. The type of mistake that is being marked as an intoxication clue is not always what the participant thinks it is, and cops do not tell the person they are evaluating exactly what mistakes they are marking as clues. For instance, maybe someone did not step off the line or take the wrong number of steps in the WAT test, but did they start too early? Did they miss the heel to toe by a fraction of an inch or did they slam their heel into their toe while stepping? Did they use their arms to balance? When they turned, did they pivot on the correct foot and in the right direction? All of these more subtle mistakes will be counted against someone even if they succeed at walking and turning without stepping off the line. Even if someone performs well on one test, they can still perform poorly on other tests and fail the tests as a whole. There is no specific minimum number of clues across all tests that is considered passing the FSTs, and some would argue that the tests are designed to make people fail.

Should anyone take FSTs?

FSTs are voluntary and no cop may compel you to perform any of them, including the preliminary alcohol screening. If you’re a driver who has been pulled over by a cop, there is really no benefit to performing FSTs, because the tests are not designed to determine whether you can pass or fail. FSTs are tools used by cops to accumulate and document specific pieces of evidence (clues) that they can then later testify to in court.

California Highway Patrol officers see every interaction with a driver as a potential DUI investigation. From the first interaction, they are looking for intoxication clues. If they smell alcohol, notice your eyes are red, or you are slurring your speech, they are going to have you exit the vehicle, an order you cannot lawfully refuse. However, when it comes time to ask you to perform FSTs, the officer must present the choice as voluntary. They may use coercive language or they may imply it is to your benefit to perform FSTs, but they must make it clear that you have the right to refuse. The correct response under these circumstances is to politely tell the officer, “I will not perform any Field Sobriety Tests.” Be direct. Be respectful. Understand that under the law, the officer may still place you under arrest and compel you to take a chemical test. You may find it unjust that you are being arrested on so little evidence, but now is not the time to argue. Feel comforted in knowing that you did your legal best to help your case and live to fight another day.

In reality, it’s not very difficult for the average person, let alone a cop, to accurately guess whether someone has had too much to drink, and with a little bit of training on what a .08% BAC means, it’s not hard to judge when someone is above or below that number, especially if we can look at their eyes, smell their breath, and observe the way they walk and talk. But courtroom evidence requires specific facts to back up opinions, and that is why FSTs are useful. Like doctors need to be able to articulate specific symptoms before making a conclusion as to what someone’s underlying problem is, FSTs are designed to document specific symptoms of intoxication.

Cops are trained how to persuade people to take FSTs by making it seem like the tests will be over quickly, and that they are a great way to prove your innocence. The cop may make it seem like they don’t really think you’re intoxicated, but they need the tests to confirm their hunch before they can let you go. Those same persuasive tactics are used to get you to continue taking more tests. By suggesting to you that you’ve passed a portion of the FSTs, they are simply trying to get you comfortable doing more, so they can document more ways in which you’ve given them clues.

I got arrested for a drug DUI, but I still did the FSTs. What do FSTs have to do with drug DUIs?

FSTs have never been validated for use by cops in determining intoxication by drugs other than alcohol. Nonetheless, they are consistently used in the field and admitted in court in drug DUI cases, because judges assume drug intoxication symptoms are similar to those of alcohol intoxication. Drug FSTs remain a contentious topic in pretrial motions across California, and an area ripe for effective cross-examination. Given how little the average cop knows about the science backing FSTs for alcohol, they are generally even less prepared when cross-examined on drug FSTs.

How a skilled DUI attorney attacks FST results using cross-examination

The most important piece of evidence in any DUI case is the chemical test (breath, blood, or urine). Because there is no way to pass an FST, there is very little benefit to performing them, and if you are even mildly intoxicated, it’s highly likely that you will perform poorly enough to rack up a lot of clues.

The good news for people who have been charged with DUI is that Field Sobriety Tests can be attacked with skillful cross-examination that exposes how weak the science is that validates them and shows how the cop did not administer the FSTs strictly per NHTSA guidelines. Inexperienced or lazy defense attorneys can be intimidated by FST evidence if they don’t know how to attack it, and they will pressure clients to settle a winnable case.

If you’ve been arrested for DUI and you feel you performed well on the FSTs, then the job of your defense attorney is to flip the story of the FSTs in your favor. Because each FST is simply an opportunity for the cop to document clues of your intoxication, every clue that they don’t check is a window of reasonable doubt. In borderline BAC cases or in cases where the chemical test shows a high BAC, the FSTs can tell a story of reasonable doubt, but it’s up to the defense to tell that story in a compelling way.


[1] Not all studies used a .10% standard, but for simplicity in aggregating their findings in this article, I will only refer to .10%