Do Field Sobriety Tests Really Prove Intoxication?

The Short Answer

Not reliably.

Field sobriety tests are commonly treated as objective proof of intoxication in Michigan DUI cases. In reality, they are screening tools, not scientific tests, and their results are highly sensitive to how they are administered, interpreted, and recorded.

At the Barone Defense Firm, we examine field sobriety tests for what they actually are: subjective exercises, performed under roadside conditions, and evaluated by officers who already suspect impairment. When these tests are carefully analyzed, they often reveal significant weaknesses.

What Field Sobriety Tests Are Supposed to Measure

Standardized Field Sobriety Tests (SFSTs) were developed by the National Highway Traffic Safety Administration (NHTSA) to help officers decide whether further investigation is warranted.

They were not designed to determine blood alcohol concentration, nor were they intended to prove intoxication beyond a reasonable doubt.

The three standardized tests are:

• Horizontal Gaze Nystagmus (HGN)
• Walk-and-Turn (WAT)
• One-Leg Stand (OLS)

Even when properly administered, these tests estimate the possibility of impairment—not certainty.

The Scientific Limits of Field Sobriety Tests

They Are Not Medical or Forensic Tests

Field sobriety tests do not measure alcohol in the body. They attempt to correlate physical performance with alcohol consumption, based on poorly designed statistical studies conducted under inadequately controlled conditions.

Roadside conditions are rarely controlled.

Factors that commonly affect performance include:

• anxiety and stress
• fatigue
• injuries or medical conditions
• footwear and clothing
• uneven or poorly lit surfaces
• weather and traffic conditions

None of these factors are accounted for in the scoring system.

What the Research Actually Shows

Peer-reviewed research has repeatedly shown that field sobriety tests have limited predictive value, even when administered correctly.

A detailed analysis of this research appears in Do Field Sobriety Tests Reliably Predict Intoxication?, published in the Michigan Bar Journal.

That analysis explains why these tests frequently over-identify impairment and why “false positives” are common.

Horizontal Gaze Nystagmus (HGN): The Most Misunderstood Test

What HGN Is Supposed to Show

HGN testing looks for involuntary eye movements as the eyes track a stimulus. Alcohol can cause nystagmus, but so can many non-alcohol-related conditions.

Common Problems With HGN Testing

• improper stimulus distance or speed
• failure to check for equal pupil size or resting nystagmus
• environmental distractions (lights, traffic)
• officer interpretation errors

Because HGN looks scientific, it is often given undue weight—despite being highly dependent on officer training and conditions.

Walk-and-Turn and One-Leg Stand: Balance, Not Intoxication

What These Tests Measure

WAT and OLS test divided attention and balance. They do not measure alcohol.

A person can fail these tests while sober for many reasons, including:

• joint or back issues
• age or weight
• footwear
• nervousness
• uneven pavement

Scoring Is Subjective

Officers score these tests based on observed “clues,” many of which are loosely defined and inconsistently applied. Minor deviations are often exaggerated in police reports.

Administration Errors Are Common—and Legally Significant

SFSTs must be administered according to strict NHTSA protocols. When those protocols are not followed, the reliability of the test is compromised.

Common deviations include:

• incomplete or incorrect instructions
• demonstrations performed incorrectly
• interruptions during testing
• tests conducted on slopes or gravel
• failure to consider medical limitations

When protocols are violated, test results may be challenged or excluded.

Why Field Sobriety Tests Still Appear Convincing

Field sobriety tests often appear persuasive because they are:

• presented through officer testimony
• described using technical language
• reinforced by video clips taken after arrest decisions were already made

But persuasion is not the same as proof.

Independent defender organizations and appellate litigation have repeatedly recognized that SFSTs are far less reliable than commonly assumed, particularly when used as primary evidence of intoxication.

How Field Sobriety Tests Are Used to Justify Breath Testing

In many Michigan DUI cases, field sobriety tests are used to justify:

• arrest
• chemical testing
• continued detention

This makes them especially important to analyze alongside breath-test evidence. Weaknesses in SFSTs often undermine the foundation for later chemical testing.

This page is intended to be read together with our discussion of breath-test evidence and the Intoxilyzer 9000, where subjective observations are often used to bolster unreliable numbers.

Is the Intoxilyzer 9000 Accurate?

Frequently Asked Questions

Next Steps

If you have been arrested for DUI in Michigan and field sobriety tests were used as evidence, it is important to understand whether those tests were administered and interpreted properly.

At Barone Defense Firm, we evaluate whether field sobriety testing was conducted according to recognized standards and whether the results were overstated.

To speak with an attorney, call (248) 306-9158 or 1-877-ALL-MICH (877-255-6424).