Driving under the influence (DUI), known in Missouri as Driving While Intoxicated (DWI), is a serious offense with potentially lasting consequences. One of the most common tools law enforcement officers use during DWI investigations is the field sobriety test. Understanding field sobriety test laws in Missouri can help drivers better comprehend their rights and responsibilities during a traffic stop.
This article explains how these tests work, what Missouri law says about them, and what drivers should know if they are asked to participate.
What Are Field Sobriety Tests?
Field sobriety tests are roadside exercises used by police officers to assess whether a driver may be impaired by alcohol or drugs. These tests are typically administered after an officer observes behavior suggesting possible intoxication, such as erratic driving, the smell of alcohol, or slurred speech.
Unlike chemical tests (such as breath, blood, or urine tests), field sobriety tests are physical and cognitive exercises. They are intended to help officers evaluate coordination, balance, attention, and the ability to follow instructions.
The Three Standardized Field Sobriety Tests
The National Highway Traffic Safety Administration (NHTSA) has established three standardized field sobriety tests commonly used across the United States, including Missouri:
- Horizontal Gaze Nystagmus (HGN) Test
The officer asks the driver to follow a moving object, such as a pen or flashlight, with their eyes. The officer looks for involuntary jerking of the eyes, which can indicate intoxication. - Walk-and-Turn Test
The driver must take several heel-to-toe steps in a straight line, turn as instructed, and return in the same manner. - One-Leg Stand Test
The driver must stand on one leg while counting aloud for a specified period.
These standardized tests are designed to create consistent procedures for officers and reduce subjective judgment. However, they are not flawless indicators of impairment.
Are Field Sobriety Tests Required Under Missouri Law?
A common question is whether drivers are legally required to take field sobriety tests. Under field sobriety test laws in missouri, these tests are generally voluntary.
Voluntary vs. Mandatory Testing
Missouri has an “implied consent” law, which applies to chemical testing (breath, blood, or urine) after an arrest for suspected DWI. Refusing a chemical test can result in administrative penalties, such as license suspension. You can review Missouri’s implied consent statute in Section 577.020 of the Missouri Revised Statutes through the official Missouri Revisor of Statutes website.
However, field sobriety tests are different. They are typically conducted before an arrest and are not covered by implied consent rules. In most situations, a driver may decline to perform roadside sobriety exercises without facing the automatic penalties that apply to refusing a chemical test.
That said, refusal does not prevent an officer from making an arrest if they believe there is probable cause based on other observations.
How Officers Use Field Sobriety Tests
Field sobriety tests are part of a broader DWI investigation. Officers consider several factors, including:
- Driving behavior (such as swerving or speeding)
- Physical signs (bloodshot eyes, odor of alcohol)
- Statements made by the driver
- Performance on field sobriety tests
No single factor automatically determines guilt. Instead, officers use the totality of the circumstances to decide whether there is probable cause to arrest a driver for DWI.
Limitations of Field Sobriety Tests
It is important to understand that field sobriety tests are not scientific measurements of blood alcohol concentration (BAC). Various factors unrelated to alcohol or drugs may affect performance, including:
- Medical conditions
- Fatigue
- Anxiety or nervousness
- Uneven road surfaces
- Weather conditions
Because of these variables, courts may examine how the test was administered and whether the officer followed standardized procedures.
What Happens After a Field Sobriety Test?
If an officer believes there is probable cause for arrest after conducting field sobriety tests, the driver may be taken into custody and asked to submit to a chemical test. At this stage, Missouri’s implied consent law becomes relevant.
Refusal of a chemical test after arrest can result in administrative license suspension, even if the driver is later found not guilty of DWI. Conversely, submitting to a chemical test that shows a blood alcohol concentration of 0.08% or higher (for most drivers) can serve as strong evidence in court.
For further information about DWI laws in Missouri, the Missouri Department of Revenue provides an overview of alcohol-related enforcement and administrative license actions on its official website.
Legal Standards and Court Considerations
In court, field sobriety test results may be introduced as evidence. Judges and juries may consider:
- Whether the officer was properly trained
- Whether the standardized procedures were followed
- Environmental conditions during testing
- The defendant’s physical or medical limitations
Defense attorneys often scrutinize these details to determine whether the results were reliable. However, even imperfect performance does not automatically result in conviction. Prosecutors must still prove beyond a reasonable doubt that the driver was intoxicated under Missouri law.
Secondary Considerations: Drug-Impaired Driving
Missouri’s DWI laws apply not only to alcohol but also to drugs, including prescription medications and controlled substances. In cases involving suspected drug impairment, officers may rely more heavily on behavioral observations and field sobriety tests, since alcohol-specific breath tests may not detect drugs.
In some situations, specially trained officers known as Drug Recognition Experts (DREs) may conduct additional evaluations to assess impairment.
Key Takeaways About Missouri’s Field Sobriety Test Laws
Understanding Missouri’s Field Sobriety Test Laws can reduce confusion during a stressful roadside stop. Field sobriety tests are commonly used tools in DWI investigations, consisting of physical and observational exercises designed to detect impairment. In Missouri, these roadside tests are generally voluntary and are distinct from chemical tests governed by implied consent laws.
Officers consider multiple factors when deciding whether to make a DWI arrest, and performance on field sobriety tests is only one part of that evaluation. Environmental conditions, medical issues, and procedural errors may all affect the reliability of test results.
