Police officers rely on field sobriety tests (FSTs) as a primary tool to check driver impairment. The tests check balance, coordination, and cognitive ability to create grounds for a DUI arrest. Despite being used extensively, scientists often doubt the validity of field sobriety tests. Medical issues, environmental elements, and inconsistent officer decisions affect how people perform their tests, reducing the certainty of FST results.

A thorough understanding of the scientific principles of the field sobriety test enables a proper assessment of their precision and dependability. The article examines research around FSTs and looks at reasons why sober individuals fail these tests.

The Three Standardized Field Sobriety Tests

During DUI stops, law enforcement officers employ three standardized field sobriety tests, which have received approval from the National Highway Traffic Safety Administration (NHTSA). The research-based development of standardized impairment detection tests occurred in the 1970s and 1980s.

1. Horizontal Gaze Nystagmus (HGN) Test

During the HGN test, law enforcement officers evaluate involuntary eye movement, known as nystagmus, because this movement becomes more noticeable in individuals under the influence of alcohol. During the testing procedure, the officer requests drivers to track a moving object like a pen or flashlight with their eyes. The officer examines three specific indicators in each eye during the test:

• Lack of smooth pursuit: The eyes jerk while tracking the object.
• Distinct nystagmus at maximum deviation: The eye involuntarily jerks when looking as far to the side as possible.
• Onset of nystagmus before 45 degrees: The jerking occurs before the eye reaches a 45-degree angle.

NHTSA studies from 1998 demonstrate that the HGN test identifies BAC levels above 0.08% with 88% accuracy. However, false positives occurred in 37% of cases. 

2. Walk-and-Turn Test

The walk-and-turn test assesses a person's capacity to execute balance and coordination-based tasks under direction. During the assessment, the driver performs nine heel-to-toe steps in a straight line, followed by a single-foot turn and return to the original position. At the same time, the officer observes signs of impairment, including:

• Losing balance or stepping off the line
• Stopping mid-test
• Failing to walk heel-to-toe
• Using arms for balance
• Starting too soon or failing to follow directions

The 1998 NHTSA research indicates that the walk-and-turn test successfully detects impairment above 0.08% BAC in 79% of cases. However, false positives occur 52% of the time. 

3. One-Leg Stand Test

During this test, drivers must stand on one foot with the other foot elevated six inches above the ground while performing verbal counting. During the observation, the officer looks for evidence of impairment through various indicators:

• Swaying or hopping
• Using arms for balance
• Putting the raised foot down prematurely

The 1998 NHTSA research indicates that the one-leg stand test successfully detects impairment above 0.08% BAC in 83% of cases. However, false positives occur 41% of the time. 

The Reliability of Field Sobriety Tests

Field sobriety tests remain popular, yet scientists continue to question their scientific reliability. Scientific research by NHTSA demonstrates reasonable test accuracy for SFSTs, yet real-life testing environments present challenges to achieving consistent results.

1. Officer Subjectivity

The performance of field sobriety tests depends entirely on police officers making observations, which creates subjective results. Standardized testing procedures do not eliminate the possibility that personal opinions and mistakes during administration will affect the results. The combination of nervousness or fatigue creates symptoms that an officer might interpret as intoxication signs, which leads to incorrect drunk driving accusations.

2. External Factors That Affect Performance

Several external conditions can influence FST performance, including:

• Weather conditions: Rain, snow, or extreme heat can impact balance and coordination.
• Road conditions: Driveways with rough pavement or gravel can make drivers appear unsteady.
• Footwear: High heels, boots, and sandals can create difficulties for drivers when attempting to walk in a straight line.
• Age and physical condition: Older adults and people who have mobility issues may find it challenging to perform tests regardless of their sobriety status.

3. False Positives Due to Medical Conditions

Several medical and neurological conditions prevent drivers from performing field sobriety tests effectively, including:

• Vertigo or inner ear disorders affecting balance.
• Diabetes causing dizziness or confusion.
• Neurological conditions (such as multiple sclerosis or Parkinson’s disease) impairing coordination.
• Anxiety or panic attacks, leading to physical trembling or disorientation.

An individual can fail field sobriety tests even when sober due to various causes, which makes it essential to challenge such results in court when appropriate.

Field Sobriety Tests vs. Chemical Tests

Field sobriety tests help create probable cause but cannot directly measure BAC levels. After failing an FST, the police will ask drivers to take breath, blood, or urine tests to determine if they are intoxicated.

Which is more reliable?

• Field Sobriety Tests: Subjective, prone to errors, affected by external factors.
• Breathalyzers: The tests deliver precise results, but their calibration status may affect the measurements.
• Blood Tests: These are considered the most accurate method for measuring BAC but require laboratory analysis.

Although breath and blood tests provide more definitive results, FSTs are often used in court as supporting evidence, especially when chemical tests are refused or unavailable.

The Role of Field Sobriety Tests in DUI Cases

Field sobriety test results alone cannot lead to conviction but help validate other evidence during arrests. The limitations of these tests make it possible to dispute their results during court proceedings.

Defense attorneys frequently argue that:

• The test was administered incorrectly.
• The driver had a medical condition that affected performance.
• Environmental conditions led to unfair results.
• The officer’s judgment was flawed or biased.

The poor reliability of field sobriety tests can serve as a basis for defense attorneys to undermine prosecution evidence, resulting in decreased charges or complete case dismissal.

Bottom Line

Field sobriety tests are a standard law enforcement tool for assessing impairment, although they are scientifically imperfect. These tests give officers a first look at the driver's condition, but results can be affected by various external elements, human judgment, and medical situations.

Understanding the science behind these tests highlights their limitations and underscores why they should never be viewed as absolute proof of intoxication. If you have been arrested for DUI based on field sobriety test results, consulting an experienced attorney is essential to challenge the evidence and protect your rights.