The Accuracy of Field Sobriety Tests: Fact or Fallacy?

The Field sobriety tests (FSTs) are a key tool for law enforcement to identify impaired drivers especially those under the influence of alcohol or drugs. These roadside tests include the Horizontal Gaze Nystagmus , Walk-and-Turn, and One-Leg Stand. These test are designed to assess a driver's coordination, balance, and focus. But really how accurate are these tests in determining impairment in DUI and refusal case? The problem is their reliability is influenced by scientific validation, human error, and real-world conditions.

The Science Behind FSTs

In the 1980s, the National Highway Traffic Safety Administration standardized three tests—HGN, WAT, and OLS—after studies indicated they could reliably detect impairment. A 1998 study in San Diego, funded by NHTSA, showed that when all three tests were administered by trained officers, they accurately predicted a blood alcohol concentration (BAC) of 0.08% or higher in about 91% of cases. Individually, these tests have varying accuracy: HGN is typically the most reliable (77-88%), followed by WAT (68-79%) and OLS (65-83%).

While these figures sound promising they are not. The result were derived in controlled settings, where ideal condition, trained officers, cooperative subjects, and clear impairment thresholds are met. Real-world circumstances, such as rain, snow, wind and a host of others issues make these tests much more unpredictable and inaccurate.

Limitations and Challenges

Critics point out that the accuracy of FSTs declines when used outside of the lab. Environmental factors such as uneven pavement, poor lighting, or adverse weather can skew results. Physical conditions, including age, injury, fatigue, or neurological disorders, can mimic impairment and lead to inaccurate results. In addition, anxiety or nervousness, which is common during all traffic stops, can also negatively impact performance, particularly on balance-based tests like the Walk-and-Turn, and One-Leg Stand.

The HGN test, which tracks involuntary eye jerking linked to alcohol consumption, is less sensitive to external conditions but still depends on correct administration. If an officer misinterprets the eye movements or overlooks medical issues (e.g., inner ear problems), the test results may be inaccurate. Often the police do not follow proper procedure in administering the tests.

Moreover, drug impairment case introduces even more uncertainty. While FSTs were designed for alcohol, their effectiveness in detecting drugs like marijuana or opioids is less studied and often contested. A 2017 study by the AAA Foundation found that FSTs alone were not consistently reliable in distinguishing drug-induced impairment from other causes, calling into question their wider applicability.

Human Error and Bias

Even with proper training, officers often still do not properly administer the test.  Subjective interpretation plays a significant role in administering FSTs—deciding if someone “swayed too much” or “failed to follow instructions” is not an exact science. Research, including a 2001 study by the Institute for Traffic Safety Management and Research, suggests error rates in administering FSTs can range from 20-30%, depending on the officer's experience and adherence to protocol. Furthermore, if an officer suspects impairment before conducting the test, confirmation bias may influence their judgment.

Legal and Practical Implications

In court, FSTs are often presented as evidence of impairment, but their accuracy is frequently challenged by top DUI attorneys. Even though breathalyzers and blood tests are more reliable in legal contexts. Unfortunately, Field sobriety tests are still a vital tool in establishing probable cause for further investigation. That is why it is so important to have a DUI attorney that will challenge these results. 

The Bottom Line

Field sobriety tests are not without merit, but they are not infallible either. In optimal conditions, with a trained officer, they can be reasonably accurate in detecting alcohol impairment, reaching over 90% accuracy in controlled settings. However, in real-world situations, their reliability can be affected by human error, environmental factors, and non-alcohol-related conditions. For many drivers, passing or failing may depend as much on circumstances as on actual impairment.

Challenges to the Reliability of Field Sobriety Tests

Experienced DUI attorneys know that you don't have to be intoxicated to fail a field sobriety test. Studies have shown that around one-third of sober individuals fail these tests.

Questioning the Reliability of Field Sobriety Tests

Field sobriety tests are subjective assessments that police officers administer during DUI stops. The three primary tests are the Horizontal Gaze Nystagmus (HGN), Walk-and-Turn (WAT), and One-Leg Stand (OLS). Many DUI attorneys argue that these tests are unreliable indicators of impairment and are often challenged in court to dispute a defendant's DUI charges.

Challenging the Officer's Administration of the Test

One reason for the unreliability of field sobriety tests is the discretion that officers have when interpreting “clues” that suggest impairment. For example, an officer may consider two “clues” on the WAT or HGN test as sufficient to declare a failure. If the officer fails to administer the tests properly, the results can be inaccurate.

Moreover, a variety of physical and medical conditions can affect a person's ability to perform on these tests. Factors such as age, weight, or pre-existing medical conditions can lead to false conclusions. Field sobriety tests are only meant to provide “probable cause” to arrest a driver, but their reliability in determining actual impairment is only about 65-80%. Given this, it is crucial to seek the help of an experienced DUI lawyer if you fail a field sobriety test.

Defenses to Field Sobriety Test Results

Several factors may influence field sobriety test outcomes and can be used in your defense:

• Police intimidation
• Failure to properly demonstrate the test
• Poor test explanation
• Inaccurate grading or administration of the test
• Environmental issues (poor lighting, adverse weather, uneven surfaces)
• Physical injuries (back/leg contusions, conditions like diabetes)
• Inappropriate footwear (e.g., high heels)
• Medical conditions or medications affecting test performance
• Age
• Weight

A skilled DUI defense attorney can use these factors to challenge the validity of your field sobriety test results.

What Are the Standardized Field Sobriety Tests?

The Standardized Field Sobriety Test (SFST) consists of three tests that law enforcement uses to determine if a driver is impaired:

1. Horizontal Gaze Nystagmus (HGN): This test assesses involuntary jerking of the eyes. Officers will instruct you to follow an object (typically a pen) with your eyes while keeping your head still. The officer looks for signs of intoxication, such as jerking beyond certain angles. However, other medical conditions can cause nystagmus, so a skilled DUI defense lawyer may argue that the test is not indicative of impairment.

2. Walk-and-Turn (WAT): This divided attention test requires the driver to walk a straight line, following specific steps, while listening to instructions. Any mistakes during the test (like losing balance or stepping off the line) may be recorded as clues of impairment. Critics argue that the design of the test makes it difficult for many individuals to pass, even if they are sober.

3. One-Leg Stand (OLS): In this test, the driver must stand on one foot and count aloud while keeping the other foot off the ground. Clues of impairment include swaying, hopping, or putting the foot down. Like the WAT, this test is not foolproof and can be influenced by factors like physical condition or anxiety.

Challenging Field Sobriety Tests in Court

A skilled DUI defense attorney will challenge the results of field sobriety tests, as these tests are not scientifically conclusive evidence of intoxication. A defense lawyer can hire expert witnesses to testify about the limitations of these tests and demonstrate how their weaknesses can lead to false conclusions. If you are arrested for DUI based on the results of these tests, it's crucial to consult with an attorney who can help you challenge the evidence and fight for your rights.