How Often Do Field Sobriety Tests Generate False-Positives?
Field sobriety tests (FSTs) are widely used by police officers during traffic stops when they suspect a driver may be impaired by alcohol or drugs. These tests, however, are not foolproof. In fact, they are known to generate a surprising number of false-positives, which can have serious consequences for sober drivers. Understanding how often false-positives occur, why they happen, and what legal rights you have can help you better protect yourself if you ever find yourself in this situation.
What Are Field Sobriety Tests?
Field sobriety tests are physical and cognitive exercises designed to measure a driver’s balance, coordination, and ability to follow instructions. The three standardized field sobriety tests recognized by the National Highway Traffic Safety Administration (NHTSA) are:
Horizontal Gaze Nystagmus (HGN) – The officer asks the driver to follow a moving object with their eyes to detect involuntary jerking movements associated with intoxication.
Walk-and-Turn (WAT) – The driver must walk heel-to-toe along a straight line, turn, and return in the same manner while following verbal instructions.
One-Leg Stand (OLS) – The driver is asked to stand on one foot while counting out loud for a set period of time.
While these tests are commonly used, they are not always accurate. Even the NHTSA itself acknowledges error rates.
Accuracy of Field Sobriety Tests
According to the NHTSA, the three standardized field sobriety tests have varying levels of accuracy:
Horizontal Gaze Nystagmus (HGN): About 77% accurate.
Walk-and-Turn (WAT): About 68% accurate.
One-Leg Stand (OLS): About 65% accurate.
When combined, officers are said to make correct arrest decisions about 82% of the time. This means that in nearly 1 out of 5 cases, a sober driver may still be misidentified as impaired.
In other words, false-positives happen far more often than most people realize.
Common Causes of False-Positives in Field Sobriety Tests
False-positives occur when a driver who is not intoxicated appears to fail one or more field sobriety tests. Several factors contribute to this problem:
1. Medical Conditions
Certain conditions, like vertigo, inner ear problems, neurological disorders, or even eye muscle imbalances, can cause balance issues or eye movements that mimic intoxication.
2. Physical Limitations
People with injuries, arthritis, weight issues, or disabilities may struggle to perform physical tasks like walking heel-to-toe or standing on one leg, even if they are completely sober.
3. Age and Fitness
Older drivers or those who are not physically fit often have difficulty with balance or coordination, making them more likely to appear impaired.
4. Environmental Factors
Uneven pavement, poor lighting, high heels, or windy conditions can all interfere with test performance.
5. Officer Subjectivity
Field sobriety tests are not scored with precise measurements. Instead, officers make judgment calls about whether someone “passed” or “failed.” This leaves room for bias, human error, or overinterpretation.
6. Nervousness and Stress
Being pulled over by police is stressful. Anxiety alone can cause a sober driver to stumble, forget instructions, or perform poorly.
Studies on False-Positives in Sobriety Tests
Research has consistently shown that false-positives occur at significant rates. For example:
A Southern California Research Institute study found that even with standardized tests, officers made incorrect impairment determinations around 20% of the time.
The American Journal of Medicine has reported that fatigue, certain medications, and natural eye movements can mimic impairment in HGN tests.
Defense attorneys often cite cases where sober individuals were misjudged, emphasizing that the margin of error is far too high for a test that can lead to arrest.
Why False-Positives Matter
A false-positive on a field sobriety test can lead to:
Arrest for DWI/DUI despite sobriety.
Mandatory chemical testing (breath, blood, or urine tests).
Suspension of your driver’s license.
Court dates, fines, and possible criminal records.
Even if later evidence shows you were not impaired, the stress, financial burden, and damage to your reputation can be life-altering.
Field Sobriety Tests vs. Breathalyzers and Blood Tests
While field sobriety tests are often the first step in an investigation, they are generally less reliable than chemical testing. However, even breathalyzers and blood tests can produce false-positives due to:
Calibration errors.
Certain medications.
Mouth alcohol from dental work, burping, or acid reflux.
Improper handling of blood samples.
This highlights a major problem: the entire system for detecting impairment is vulnerable to mistakes.
Legal Defenses Against False-Positive Sobriety Tests
If you were arrested after a field sobriety test but believe you were sober, you still have legal options. Common defense strategies may include:
Challenging the officer’s administration of the test.
Presenting medical records that explain your test performance.
Pointing out environmental conditions that could have interfered.
Questioning the reliability of the officer’s training or judgment.
Suppressing improperly obtained evidence.
An experienced attorney can review the circumstances of your arrest and determine whether your rights were violated or whether the evidence against you is flawed.
Protecting Yourself During a Field Sobriety Test
If you are ever asked to perform a field sobriety test in Texas, keep these points in mind:
You are not legally required to take field sobriety tests. Refusal may have consequences, but these tests are voluntary.
Be respectful and calm. Hostility or resistance can make things worse.
Understand that refusal is different from chemical tests. Texas has an “implied consent” law requiring breath or blood tests in certain situations, but field sobriety tests are optional.
Call an attorney immediately if you are arrested.
FAQ: False-Positives in Field Sobriety Tests
Q: How often do false-positives happen in field sobriety tests?
A: Studies show officers make incorrect impairment determinations in about 20% of cases, meaning false-positives are fairly common.
Q: Can medical conditions cause me to fail a sobriety test?
A: Yes. Conditions like vertigo, inner ear problems, or neurological disorders can mimic impairment.
Q: Are field sobriety tests required by law in Texas?
A: No. You are not required to take field sobriety tests, though refusing may still lead to arrest or further testing.
Q: Are breathalyzers more accurate than field tests?
A: Generally, yes, but breathalyzers can still generate false-positives due to calibration issues, medications, or health conditions.
Q: What should I do if I was wrongly arrested after a sobriety test?
A: Contact an attorney immediately to review your case and fight to have unreliable evidence challenged.
Final Thoughts
Field sobriety tests may be a standard tool for law enforcement, but they are far from perfect. Studies show that false-positives occur in nearly one out of every five cases, meaning a significant number of sober drivers are wrongly accused of being impaired. If you or a loved one has been arrested after failing a field sobriety test, remember that you have rights. A skilled attorney can help you challenge unreliable test results and pursue the best possible outcome for your case.
