Field Sobriety Tests (FSTs): A DUI Investigation Tool

Learn about the field sobriety tests (FSTs) that police use during DUI stops, and how a driver’s FST performance might come into play in a DUI case.

An officer who pulls someone over and suspects drunk driving will often ask the person to perform several “field sobriety tests” (FSTs). FSTs are physical tests that are meant to gauge a person’s level of impairment.

The Three “Standardized” Field Sobriety Tests

Officers use lots of different FSTs, but the most common are known as the three “standardized” FSTs:

  • the horizontal gaze nystagmus (HGN)
  • the walk and turn, and
  • the one-leg stand.

Based on studies conducted in the 1970s, the National Highway Traffic Safety Administration (NHTSA) concluded that these three FSTs were reliable indicators of alcohol impairment. Together, the three FSTs are sometimes referred to as the “standardized FST battery.”

During a DUI investigation, an officer might ask a motorist to do one, two, or all three of the tests.

How FSTs Factor Into DUI Cases

FSTs have two basic purposes: for police to determine whether there’s probable cause for a DUI arrest and to obtain evidence of driver impairment for later use in court.

Establishing Probable Cause

An arrest is lawful only if it’s based on probable cause. Probable cause is established if the facts support an objective belief that the person to be arrested has committed a crime. So, for a DUI arrest to be legal, police must have evidence that the motorist was driving under the influence of drugs or alcohol.

In assessing probable cause for a DUI arrest, lots of considerations can come into play—driving pattern, manner of speech, and the driver’s appearance, to name a few. And for many officers, poor FST performance ranks high among the factors that justify a DUI arrest.

Proving Impairment

All states have two types of DUI charge: “per se” and “impairment.” A per se DUI just requires proof that the defendant was driving with a blood alcohol concentration (BAC) of .08% or more. But to prove an impairment DUI, the prosecution must show that the defendant was actually affected by the alcohol or drugs ingested.

Generally, officers can testify in court about observations they made while giving FSTs to a driver. And if the driver had lots of screw ups, the prosecution is likely to argue that it proves impairment. In response, a defense attorney might argue that a defendant’s poor FST performance was due to something other than intoxication (tiredness, for example) or that FSTs are unreliable indicators of impairment altogether. But ultimately, it’ll be up to the judge or jury to decide whether the driver was too drunk or high to be driving.

Can I Refuse to Do a Field Sobriety Test?

FSTs are typically voluntary. So drivers who refuse an officer’s request to take an FST can’t be punished or charged with a crime.

But a refusal can still have consequences. In most states, an officer is allowed to testify in court about a motorist’s refusal to participate in FSTs. This rule puts drivers in an  awkward position: They can either take an FST and risk doing badly or refuse to take the test and risk looking like they’re hiding something.

Courts in a few states—including Massachusetts, Oregon, and Virginia—have found that drivers ought to be able to refuse an FST without having to fear repercussions. In these states, motorists can decline to take an FST without having to worry about it coming back to bite them in court. (Com. v. Brown, 83 Mass. App. Ct. 772 (2013), Farmer v. Com., 10 Va. App. 175 (1990), State v. Fish, 321 Or. 48 (1995).)

Talk to a DUI Lawyer

DUI laws, including those dealing with FSTs, differ by state. If you’ve been arrested for a DUI, get in contact with a DUI attorney in your area. A quality DUI lawyer can tell you how the law applies to the facts of your case and whether you have any good defenses.

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