Field sobriety tests (FSTs) are a cornerstone of traffic enforcement, serving as a tool for law enforcement to build probable cause for driving under the influence (DUI) or driving while impaired (DWI) arrests. While these tests are widely used, their legal foundation is nuanced. State police must navigate a complex intersection of constitutional rights, statutory requirements, and standardized protocols to ensure that evidence gathered from these tests is both admissible in court and protective of individual liberties. Understanding these legal considerations is critical for officers, prosecutors, defense attorneys, and the public at large.

This article examines the key legal elements governing field sobriety testing: the threshold of reasonable suspicion and probable cause, the standardized nature of common tests, evidentiary admissibility, state-specific laws, and common defenses. Each section provides practical insights drawn from case law and NHTSA guidelines, with a focus on how state police can lawfully conduct these procedures while safeguarding the rights of drivers.

Reasonable Suspicion to Initiate Contact

Before administering any field sobriety test, an officer must first have reasonable suspicion that a driver is impaired. This standard, established under Terry v. Ohio, requires specific, articulable facts that would lead a reasonable person to believe criminal activity is afoot. In the context of DUI enforcement, common indicators include:

  • Observing erratic driving behavior (e.g., swerving, weaving within lanes, speeding, or braking improperly).
  • Noticing physical signs of impairment upon vehicle stop (e.g., odor of alcohol, slurred speech, red or watery eyes, fumbling with documents).
  • Admissions from the driver concerning alcohol or drug consumption.
  • Field contacts based on checkpoints, provided the checkpoint follows constitutional guidelines per Michigan Dept. of State Police v. Sitz.

Without these articulable facts, any subsequent request to perform FSTs may be challenged as an unreasonable seizure under the Fourth Amendment. Officers should document the specific observations that gave rise to suspicion before moving to the testing phase.

Probable Cause and the Escalation to Arrest

Field sobriety tests are designed to help officers move from reasonable suspicion to probable cause for arrest. The results of properly administered tests, combined with other observations, can establish that a driver is under the influence to a degree that impairs safe operation of a vehicle. However, probable cause requires a higher quantum of evidence than reasonable suspicion. If FSTs are not conducted according to standardized protocols or if the results are ambiguous, the officer may lack the necessary probable cause, and any arrest could be deemed unlawful.

Additionally, some states require officers to offer a preliminary breath test (PBT) as part of the investigation, while others mandate that the test be requested only after FSTs show indicia of impairment. Officers must be aware of their state’s specific laws governing the sequence of testing procedures.

The National Highway Traffic Safety Administration (NHTSA) has developed three standardized field sobriety tests: the Horizontal Gaze Nystagmus (HGN), the Walk-and-Turn (WAT), and the One-Leg Stand (OLS). Each test has specific scoring criteria and validated thresholds for indicating a blood alcohol concentration (BAC) of 0.08% or higher. However, the legal admissibility of these tests varies across jurisdictions.

Horizontal Gaze Nystagmus (HGN)

The HGN test measures involuntary jerking of the eyes as the gaze moves to the side. It is considered a physiological indicator of impairment, not merely a behavioral one. Courts have debated the scientific reliability of HGN, with some states applying the Frye standard (general acceptance in the scientific community) and others applying the Daubert standard (reliability and relevance). Officers must administer HGN using the NHTSA-approved procedure: checking for lack of smooth pursuit, distinct nystagmus at maximum deviation, and onset of nystagmus prior to 45 degrees. Nolo provides an overview of how HGN is commonly challenged in court.

Walk-and-Turn (WAT) and One-Leg Stand (OLS)

These are divided-attention tests, requiring the driver to process verbal instructions while performing physical tasks. Improper instructions, uneven terrain, poor lighting, or the driver’s physical condition (e.g., age, injury, obesity) can affect performance. The legal validity of these tests hinges on strict adherence to the NHTSA standardized instructions. For example, the officer must instruct the subject to remain in the starting position until told to begin, and to count steps aloud. Failure to follow the script may render the test results inadmissible. Many defense attorneys scrutinize videotapes of SFST administration to highlight deviations from protocol.

Observational Signs and Validated Cues

Beyond SFSTs, officers rely on additional observational cues such as the driver’s admissions, appearance, and behavior. The AAA Foundation for Traffic Safety notes that validated cues significantly improve accuracy when combined with SFSTs. However, relying solely on non-standardized observations (e.g., the “finger-to-nose” test) can weaken probable cause and lead to suppressed evidence. Officers should focus on nationally recognized cues rather than less reliable tests.

Another important factor is the officer’s training and certification. Courts often require that the officer has completed NHTSA-approved SFST training and maintains current certification. Lack of proper training can be a basis for excluding test results.

Field sobriety tests are generally considered a search under the Fourth Amendment. While some states treat FSTs as searches requiring probable cause, others view them as lawful with reasonable suspicion because they are less intrusive than a breath test. Most states have implied consent laws that require drivers to submit to chemical testing (blood, breath, or urine) upon arrest, but FSTs are often considered a separate matter. Drivers generally have the right to refuse FSTs, but refusal may be used as evidence of consciousness of impairment in jurisdictions that allow such inference. However, the U.S. Supreme Court in Birchfield v. North Dakota (2016) clarified that breath tests incident to arrest are permissible without a warrant, but blood tests generally require a warrant or exigent circumstances. FSTs are not covered under implied consent in many states, and officers must obtain explicit verbal consent before administering them.

For consent to be valid, it must be given voluntarily and not under duress. Courts examine the totality of circumstances, including whether the driver was told they could refuse (if state law requires such advisement). Some jurisdictions require officers to inform drivers that FSTs are voluntary before asking them to perform the tests. Failure to do so may render the consent invalid. An officer’s tone, display of authority, and language (e.g., “you must take this test”) can also affect voluntariness.

Admissibility of FST Evidence in Court

Even when properly administered, the results of FSTs do not automatically come into evidence. Prosecutors must lay a foundation showing that the tests were conducted in substantial compliance with NHTSA guidelines. Defense attorneys frequently file motions to suppress the results on the following grounds:

  • Lack of reasonable suspicion for the stop.
  • Lack of reasonable suspicion or consent to request FSTs.
  • Administrative errors such as improper instructions, failure to check for medical conditions, or conducting tests on unsafe surfaces.
  • Failure to videotape or adequately document the process.
  • Lack of officer certification or recent training.
  • Scientific unreliability of certain tests (especially HGN).

Courts in states following the Daubert standard may require expert testimony on the scientific basis of SFSTs before admitting the results. Conversely, states that adopt the Frye standard may accept HGN testimony as generally accepted but still require proper administration. The LexisNexis case summary of State v. Doran illustrates how courts can exclude HGN evidence when the officer failed to follow protocol.

State Variations and Precedent

Field sobriety test legality is not uniform across the United States. Some states require officers to inform drivers of their right to refuse FSTs, while others treat the request as part of the investigatory stop. For example, in State v. Reitter, the Washington Supreme Court held that FSTs are a non-testimonial search requiring a warrant or exception. In contrast, the California Supreme Court in People v. McFarland found that FSTs are not a search under the state constitution when based on reasonable suspicion. These differing holdings mean that a procedural error in one state may be legally immaterial in another. Officers across state lines must be trained on their own jurisdiction’s case law and statutes. Prosecutors should also be prepared to cite state-specific precedents when defending FST evidence.

Defense attorneys have developed robust strategies for challenging field sobriety test evidence. Some of the most effective defenses include:

Improper Administration

As discussed, any deviation from the NHTSA standardized script undermines the test’s reliability. Attorneys may obtain the training manual and compare the police video to the official procedures. Missing steps like instructing the subject to keep arms down during the OLS test, or allowing multiple attempts at the WAT without resetting the test, can be fatal to the prosecution’s case.

Medical and Physical Conditions

Conditions such as inner ear disorders, vertigo, back injuries, poor eyesight, and neurological issues can cause a driver to perform poorly on FSTs. Similarly, the effects of fatigue, fear, or anxiety can mimic intoxication. Officers are required to ask about medical conditions before administering tests, but many do not. If a driver has a documented condition, a defense expert can testify that the test results were caused by something other than alcohol or drugs.

Environmental Factors

Uneven pavement, rain, strong winds, poor lighting, or the presence of roadside traffic can affect a driver’s balance and coordination. Courts expect officers to assess whether conditions are suitable for testing. Failure to do so may make the results unreliable.

Miranda and Procedural Rights

While Miranda warnings typically are not required during the field sobriety testing phase because the driver is not in custody, any incriminating statements made after the stop but before testing could be suppressed if the officer engages in coercive questioning. Additionally, the use of body cameras and dash cameras has become standard; if the recording is lost or incomplete, the defense may argue that the missing evidence would have been favorable to the driver, leading to suppression.

Conclusion: Best Practices for State Police

To ensure that field sobriety tests withstand legal scrutiny, state police should adhere to the following best practices:

  1. Document the specific articulable facts that support reasonable suspicion before requesting FSTs.
  2. Follow NHTSA standardized protocols precisely for all three validated SFSTs.
  3. Ask about the driver’s medical conditions and physical limitations before testing.
  4. Administer tests on a flat, dry, well-lit surface free of hazards.
  5. Record the entire encounter via dash camera or body-worn camera, and ensure the recording is preserved.
  6. Obtain explicit consent for FSTs, especially in jurisdictions where they are considered a search.
  7. Do not rely on non-standardized tests (e.g., finger-to-nose, alphabet recitation) as primary evidence of impairment.
  8. Maintain current SFST certification and participate in periodic training updates.

When these steps are followed, field sobriety tests remain a lawful and effective tool for identifying impaired drivers while respecting constitutional protections. The interplay between law enforcement procedures and evolving legal standards requires constant vigilance from officers, prosecutors, and the judiciary. By understanding the legal framework outlined above, professionals can ensure that DUI investigations are conducted fairly and that evidence derived from field sobriety testing meets the high bar required for admissibility in court.