DWI Defenses in Missouri

Challenging the Traffic Stop

Every DWI case begins with a traffic stop — and every traffic stop must be constitutionally justified. Under the Fourth Amendment, law enforcement must have reasonable suspicion of criminal activity or a traffic violation before initiating a stop.¹

If the officer lacked a valid reason — no swerving, no speeding, no equipment violation, no traffic infraction — then the stop itself was illegal. When a stop is unconstitutional, every piece of evidence gathered afterward can be suppressed through a motion to exclude. Without that evidence, the prosecution typically has no case.

We review dashcam footage, bodycam footage, dispatch records, and the officer’s written reports to determine whether reasonable suspicion actually existed. In our experience, this is one of the most productive areas of defense in DWI cases.

DWI Checkpoints

Missouri law permits DWI checkpoints (sobriety checkpoints), but they must follow strict constitutional guidelines.² The checkpoint must be established pursuant to a written plan, the selection criteria for stopping vehicles must be neutral (e.g., every third car), and the operation must be supervised by a commanding officer. Deviations from these requirements can render the checkpoint — and all evidence obtained from it — unconstitutional.

Challenging Field Sobriety Tests

The Standardized Field Sobriety Tests (SFSTs) developed by the National Highway Traffic Safety Administration (NHTSA) are the most commonly used roadside evaluation tools: the Horizontal Gaze Nystagmus (HGN) test, the Walk-and-Turn test, and the One-Leg Stand test.³

These tests are far less reliable than most people assume:

HGN (Horizontal Gaze Nystagmus). Officers look for involuntary eye movement when tracking a stimulus. But nystagmus can be caused by dozens of medical conditions, medications, fatigue, and even caffeine — not just alcohol. NHTSA’s own research shows HGN is approximately 77% accurate under ideal conditions.⁴

Walk-and-Turn. This “divided attention” test requires walking heel-to-toe along a line while following specific instructions. NHTSA research shows approximately 68% accuracy.⁵ Uneven pavement, poor lighting, footwear, physical conditions, age, weight, and anxiety can all produce false indicators of impairment.

One-Leg Stand. Standing on one foot for 30 seconds while counting. NHTSA research shows approximately 65% accuracy.⁶ Inner ear conditions, back or leg injuries, and medications that affect balance can all compromise results.

Beyond the inherent limitations of the tests themselves, officers frequently fail to administer them according to NHTSA protocols. Improper instruction, incorrect scoring, and environmental interference are common — and each deviation further reduces reliability.

We obtain and analyze the officer’s training records, NHTSA certification status, and field sobriety test administration notes to identify every point of vulnerability.

Challenging BAC Test Results

Breathalyzer Challenges

Missouri uses breath testing instruments that must be properly maintained, calibrated, and operated:

Calibration and maintenance records. Breathalyzer machines must be calibrated at regular intervals. If calibration records show missed maintenance windows, expired calibration, or out-of-range results, the test results can be challenged.⁷

Operator certification. The person administering the breath test must be properly certified. Expired certifications or inadequate training can undermine the results.

Observation period. NHTSA protocols and Missouri regulations require a continuous observation period before breath testing — typically 15 to 20 minutes — to ensure the subject doesn’t eat, drink, smoke, or regurgitate. If the observation period was cut short or not properly documented, the results may be unreliable.

Mouth alcohol contamination. Acid reflux (GERD), recent dental work, mouth sores, and certain medications can trap alcohol in the mouth, producing falsely elevated BAC readings.

Blood Test Challenges

Blood tests are generally considered more reliable than breath tests, but they’re far from immune to challenge:

Chain of custody. The blood sample must be properly drawn, labeled, stored, and transported. Any break in the chain of custody raises questions about contamination or mislabeling.

Fermentation. Improperly stored or preserved blood samples can ferment, producing alcohol that wasn’t present at the time of the draw.⁸

Drawing procedures. Blood must be drawn by qualified personnel using approved methods. The use of alcohol-based swabs at the draw site — while less common now — can contaminate the sample.

Rising BAC Defense

BAC is not static. After a person’s last drink, blood alcohol continues to rise as alcohol is absorbed into the bloodstream — a process that can take 30 to 90 minutes or longer depending on food intake, body weight, and other factors.⁹

If the BAC test was administered well after the traffic stop, the BAC reading at the time of testing may have been higher than the BAC at the time of actual driving. This “rising BAC” defense can be especially powerful when the BAC reading is close to the 0.08% threshold.

Miranda and Procedural Violations

Miranda Rights

Statements made during custodial interrogation are inadmissible if Miranda warnings were not properly given.¹⁰ In DWI cases, this most commonly applies to statements made after arrest — at the station, during booking, or during questioning about drinking activity.

Roadside questioning before a formal arrest typically does not trigger Miranda requirements. However, the line between a traffic stop and custodial interrogation is not always clear, and improper questioning can still lead to suppression of incriminating statements.

Implied Consent Advisements (§ 577.041)

Missouri’s implied consent law requires officers to advise individuals of the consequences of refusing a chemical test.¹¹ If the advisement was incomplete, incorrect, or not given at all, it can affect both the criminal DWI case and the administrative license action. Improper implied consent advisement may provide grounds to challenge a test refusal or the resulting 1-year license revocation.

Challenging the “Operation” Element

The prosecution must prove that the person operated the vehicle while intoxicated.¹² “Operate” includes actual physical control — but there are meaningful limits.

Scenarios where this defense applies: sleeping in a parked car, sitting in the driver’s seat with the engine off, being found in a vehicle that was already stopped, or being near (but not in) the vehicle when officers arrived.

Missouri courts consider factors including the location of the keys, whether the engine was running, the position of the driver’s seat, and whether the vehicle was in a location suggesting recent driving. The facts matter — and not every person found in or near a vehicle was “operating” it within the meaning of the statute.

Lack of Intoxication

A BAC below 0.08% does not automatically mean the case is over — the prosecution can still attempt to prove impairment through other evidence. But it significantly weakens their case. And even with a BAC above 0.08%, the defense can present evidence that the person was not actually impaired: coherent speech on bodycam, normal driving behavior on dashcam, no difficulty with non-standardized tasks, and credible testimony about the amount and timing of alcohol consumption.

Negotiation-Based Defenses

Not every DWI case goes to trial. In many cases — particularly first offenses — the most effective defense is skilled negotiation:

Reduction to a non-alcohol traffic offense. This avoids DWI-specific consequences entirely — no SATOP, no SR-22, no DWI on the record.

Suspended Imposition of Sentence (SIS). The court withholds entering a conviction. If probation is completed successfully, no conviction appears on the record.¹³

Diversion programs. Some jurisdictions offer DWI diversion or treatment-based alternatives for first-time offenders.

The strength of the evidence, the circumstances of the offense, and the quality of the defense presentation all influence what the prosecution is willing to offer.

Facing DWI Charges in St. Louis?

Every DWI case has a defense. The question is whether the right team is in place to find it, develop it, and execute it. We’ve challenged unconstitutional stops, exposed faulty breathalyzers, dismantled field sobriety test evidence, and negotiated outcomes that changed the trajectory of cases at every level. The prosecution has resources. The defense needs them too.

Call us 24/7 for a free consultation. The defense starts with a conversation.

References

1. U.S. Const. amend. IV; Terry v. Ohio, 392 U.S. 1 (1968).
2. Michigan Dept. of State Police v. Sitz, 496 U.S. 444 (1990); see also State v. Welch, 755 S.W.2d 624 (Mo. Ct. App. E.D. 1988).
3. National Highway Traffic Safety Administration, DWI Detection and Standardized Field Sobriety Testing (SFST) Manual (2023).
4. NHTSA, SFST Manual (2023) [HGN accuracy].
5. NHTSA, SFST Manual (2023) [Walk-and-Turn accuracy].
6. NHTSA, SFST Manual (2023) [One-Leg Stand accuracy].
7. See § 577.020, RSMo [Chemical testing standards and procedures].
8. See generally forensic literature on blood sample fermentation and preservative protocols.
9. See NHTSA, SFST Manual (2023) [Alcohol absorption and elimination rates].
10. Miranda v. Arizona, 384 U.S. 436 (1966).
11. § 577.041, RSMo [Implied consent — refusal and advisement requirements].
12. § 577.010.1, RSMo; § 577.001.15, RSMo [Definition of “operates”].
13. See § 557.011.2, RSMo [Suspended Imposition of Sentence].