Why Field Sobriety Tests May Be Unreliable in DWI Defense
Missouri DWI investigations often include field sobriety tests, and the results are regularly treated as a key part of the officer's decision-making. Those tests happen on the roadside, under pressure, and in conditions that don’t match a controlled setting.
Field sobriety tests are presented as standardized, but the tasks still depend on human instruction, human observation, and real-world surroundings. A careful review looks at what the tests were designed to measure, what they actually measure in practice, and what variables affect performance
At the Jeff Jarrett Law Office, I help individuals in Kansas City, Missouri, evaluate how those tests were used and what they really show in a DWI defense case. I also serve clients in the greater Kansas City Metro area, including Lee’s Summit and Liberty, Missouri. I’m also here to assist clients in Overland Park and Lawrence, Kansas. Contact me today to get started.
What Field Sobriety Tests Are Designed to Measure
Field sobriety tests are a set of roadside tasks used to observe balance, coordination, divided attention, and certain eye movements. Officers use them to document observations during a stop and to decide what steps to take next. These tests don’t measure blood alcohol levels, and they don’t replace chemical testing.
Standardized Tests and What Each One Involves
The standardized field sobriety tests most commonly used in DWI defense cases follow set tasks and defined cues. Each test targets different observations, and each has specific points where the process can break down.
When evaluating a stop, start with the specific test involved and the conditions surrounding it:
Horizontal gaze nystagmus: This involves tracking eye movements as a stimulus is moved in front of the eyes; the evaluation depends on positioning, timing, lighting, and clear instructions.
Walk-and-turn: This requires following multi-step directions while walking heel-to-toe on a line, turning in a specific manner, and maintaining balance during both the instruction and walking phases.
One-leg stand: This involves balance while one foot is raised and a count is performed, and the outcome depends on stability, timing, and how the officer scored small movements.
Separating the tests like this also highlights a bigger point. Even a well-described test becomes harder to evaluate when the roadside setting adds variables that interfere with footing, focus, and communication.
Roadside Conditions That Change What The Tests Show
Field sobriety tests take place in environments designed for traffic, not balance tasks. The shoulder of a road or an uneven sidewalk changes how a person moves and how the officer views that movement.
Since field sobriety tests are performed in real roadside environments, the following conditions can affect footing, focus, and how performance is observed:
Surface and slope: Gravel, broken pavement, steep shoulders, or uneven concrete affect balance and foot placement during heel-to-toe walking and one-leg standing.
Lighting and glare: Headlights, streetlights, patrol lights, and oncoming traffic create glare that affects vision and concentration during instruction and performance.
Weather and temperature: Cold, rain, ice, wind, and heat affect comfort, stability, and the ability to perform slow, controlled movements.
Noise and distractions: Passing vehicles, radios, and roadside activity interfere with hearing instructions and maintaining focus during multi-step tasks.
Once the setting is accounted for, the next layer is the person performing the test. Physical condition and health factors change balance and coordination, and those factors exist regardless of alcohol use.
Health and Body Factors That Affect Test Performance
Field sobriety tests assume a baseline ability to balance, to move heel-to-toe, and to follow directions while standing still. Because balance and coordination can be influenced by ordinary physical and health factors, these issues may affect how someone performs on field sobriety tests:
Balance and inner ear issues: Vertigo, ear infections, and other balance problems affect sway, stepping, and stability during standing and turning tasks.
Injuries and chronic pain: Knee, ankle, hip, back, and foot conditions affect gait and the ability to stand on one leg without visible adjustment.
Vision limitations: Poor vision, glare sensitivity, and contact lens discomfort affect both instruction-following and eye-related observations.
Fatigue and stress response: Sleep loss, anxiety, and stress change reaction time, attention, and steadiness, particularly during divided-attention tasks.
These factors tie directly into the next issue: the tests aren’t only physical. They also require listening, memory, and precise compliance with directions, so instruction quality becomes part of the reliability question.
Instruction and Scoring Issues That Affect Reliability
Field sobriety tests require a person to listen to a sequence of rules, to remember those rules, and to perform them in a precise way. If instructions are rushed, incomplete, or unclear, performance errors reflect communication breakdown as much as physical ability.
Scoring also introduces judgment calls. The officer decides whether a small movement counts as a clue and how much weight to give it, and those decisions end up in the report. At the Jeff Jarrett Law Office, I’ve found that this makes documentation a core issue in DWI defense, which naturally leads to the written narrative and any available recordings of the stop.
Documentation and Video Review Factors
A DWI case record usually includes an officer report, and in some stops it includes video or audio. The value of those materials depends on what they capture and how clearly they show the testing process.
When reviewing documentation, focus on the parts of the record that show conditions, instructions, and what the officer recorded as clues:
Clarity of instructions in the record: The report and audio should show what directions were given, whether they were repeated, and whether the timing matched the standardized method.
What the camera actually shows: Foot placement, the walking line, and the raised foot matter, and the angle and distance determine what is visible.
Consistency between report and recording: The record should align on basic details like location, surface, timing, and the specific cues noted during each task.
Sequence and interruptions: Pauses, restarts, traffic interference, and officer repositioning affect how the task unfolded and how performance was observed.
Looking at documentation this way also helps keep the focus where it belongs. Field sobriety tests are one component of a DWI investigation, and the next step is to assess how they fit with the rest of the evidence in the case.
How Field Sobriety Tests Fit Into The Larger DWI Defense Case
A DWI investigation can include driving observations, physical observations, statements, and chemical testing steps. Field sobriety tests are commonly used to support an officer's narrative, but they don’t stand alone as a scientific measurement of impairment. A defense review looks for internal consistency across the investigation.
Contact My Office Today
Field sobriety tests are often presented as straightforward, but the value of the results depends on the conditions, instructions, and documentation tied to the stop. Looking closely at how the tests were administered and recorded can help keep the focus on what the evidence actually shows in your specific DWI defense case.
At the Jeff Jarrett Law Office, I serve clients in the in Kansas City, Missouri, as well as the greater Kansas City Metro area, including Lee’s Summit and Liberty, Missouri. I’m also here to assist clients in Overland Park and Lawrence, Kansas. Reach out today to get started.