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.

During traffic stops, law enforcement can request chemical tests of blood alcohol concentration (BAC) and drug detection from drivers suspected of driving under the influence. These include breath, blood, and urine tests. While you have the right to decline, refusing chemical testing leads to major legal and administrative penalties because of implied consent statutes in most states.

The guide details the impact of refusing a chemical test, including legal penalties, administrative consequences, and defense strategies.

Understanding Implied Consent Laws

Every U.S. state has implied consent laws, meaning that by obtaining a driver's license, you automatically agree to submit to a chemical test if lawfully arrested for DUI. These laws exist to build driver cooperation and enable law enforcement to obtain evidence against individuals who drive under the influence.

However, implied consent does not mean you are legally required to take a chemical test before an arrest. Any person over 21 years old and not on DUI probation can refuse preliminary tests such as Preliminary Alcohol Screening (PAS) breathalyzer tests without automatic penalties.

Once you are lawfully arrested, refusal to submit to a chemical test triggers automatic penalties, even if you are not ultimately convicted of DUI.

Immediate Consequences of Refusing a Chemical Test

Denying to take chemical tests will not stop the authorities from proceeding with DUI charges. The act of refusal frequently makes the situation worse.

Key consequences include:

• Automatic license suspension: Your driver’s license can be suspended for 6 months to 3 years, depending on prior offenses.
• Increased penalties if convicted of DUI: Refusing the test can lead to harsher penalties, such as longer jail time and higher fines.
• Use as evidence against you in court: During your trial, prosecutors can use your refusal as evidence of your guilt because they will interpret it as an attempt to conceal your BAC level.
• Mandatory installation of an ignition interlock device (IID): A requirement for IID installation exists in certain states for drivers who reject chemical testing.
• Additional fines and fees: Many states impose extra penalties and court costs for refusal.

The penalties for refusing a chemical test will usually match or exceed those from a failed test.

State-by-State Penalties for Chemical Test Refusal

Penalties for refusing a chemical test vary by state but typically increase with prior offenses.

California and Texas automatically enforce penalties against drivers who refuse testing even when they are not convicted of a DUI.

Note that you can still be convicted of a DUI without a chemical test. Prosecutors maintain the ability to charge and convict you of DUI by using various forms of evidence, even if chemical test results are absent. These include:

• Officer observations: Slurred speech, bloodshot eyes, the smell of alcohol, or erratic driving.
• Field sobriety test results: Poor performance on standardized field sobriety tests (SFSTs).
• Witness statements: Statements from passengers, other drivers, or bystanders.
• Dashcam or bodycam footage: Video evidence of impairment or refusal to cooperate.

During "no-refusal weekends,” some states authorize law enforcement to obtain quick warrants enabling them to conduct forced blood draws on individuals who decline to test.

Defenses Against Chemical Test Refusal Charges

There are legal defenses for cases where you refused a chemical test but now face penalties. Legal strategies include:

• Lack of probable cause: A police officer must establish valid reasons for stopping you and reasonable suspicion of DUI before asking you to take a chemical test. The lack of probable cause can invalidate the test refusal.

• Failure to provide proper warnings: Police must inform you of all penalties for refusing a chemical test. The court may challenge your refusal of tests when police officers did not explain the associated penalties adequately.

• Medical conditions or physical inability: The inability to perform a breathalyzer test due to medical issues, including respiratory problems, could be a legitimate defense. Several medical treatments and anxiety conditions can affect test performance.

• Improper testing procedures: Test results may become inadmissible when police make mistakes when issuing test requests through improper paperwork or faulty equipment.

• Coerced or forced refusal: If an officer pressured or threatened you into refusing the test, this could be grounds to challenge the charge.

A DUI attorney with experience can assess the details of your case to decide which defense can help you.

The Bottom Line

Most of the time, it is unwise to decline a chemical test because:

• A refusal of chemical testing will immediately result in automatic license revocation.
• The consequences for test refusal match those of DUI violations.
• The evidence against you will include your refusal to submit to testing.

You should consider test refusal as a strategic move only under specific circumstances—such as when you are sure your BAC is far above the legal limit and you plan to fight the charges. Getting advice from a DUI attorney about this choice is strongly advised.

If you have already refused a chemical test and are facing penalties, a DUI attorney can help you understand your options and build a strong defense.