DUI Defense: Challenging Field Sobriety Tests and Horizontal Gaze Nystagmus Reliability
April 21, 2026
Facing a DUI charge can feel overwhelming. The flashing lights, the roadside tests, and the uncertainty that follows can leave anyone anxious about what comes next. Many people don’t realize how much subjectivity plays a role in DUI investigations, especially when field sobriety tests are involved.
It’s common to question whether the results truly reflect impairment or if other factors influenced the outcome. What seems like a straightforward process is often filled with gray areas, which is why DUI defense strategies frequently focus on challenging how these tests were conducted and interpreted.
With an office in Brentwood, Tennessee, William L. Pomeroy Law helps individuals accused of DUI in Brentwood, Davidson County, Williamson County, and Rutherford County. If you’re facing charges, reach out to the firm today to discuss your options.
How Field Sobriety Tests Can Be Unreliable
Field sobriety tests are often presented as objective tools to determine impairment. In reality, they rely heavily on an officer’s observations and judgment. This subjectivity opens the door for errors that can significantly impact a DUI case.
Several factors can influence how a person performs during these tests. Nervousness, physical conditions, weather, and even the road surface can all play a role. An individual who is completely sober can still struggle under these circumstances.
Some common issues that affect reliability include:
Environmental conditions: Poor lighting, uneven pavement, or bad weather will affect balance and coordination.
Physical limitations: Injuries, age, weight, or medical conditions will interfere with performance.
Officer instructions: If directions are unclear or rushed, a person will misunderstand what’s required.
Stress and anxiety: Being stopped by law enforcement will make anyone nervous, which will impact performance.
Because of these variables, DUI defense often involves questioning whether the test results truly indicate impairment or if they reflect external influences. A strong defense will highlight these inconsistencies and challenge the assumptions made during the stop.
Horizontal Gaze Nystagmus and Its Limitations
The Horizontal Gaze Nystagmus (HGN) test is one of the most commonly used field sobriety tests. It involves observing the movement of a person’s eyes as they follow an object, such as a pen or flashlight. Officers look for involuntary jerking, which they associate with alcohol consumption.
While the test sounds scientific, it isn’t foolproof. Many factors unrelated to alcohol can cause nystagmus. This makes the test far less reliable than it’s often portrayed.
Some key concerns with HGN testing include:
Medical conditions: Certain neurological or inner-ear issues can cause eye movements similar to those officers look for.
Improper administration: If the officer doesn’t follow proper procedures, the results will be invalid.
Environmental distractions: Flashing lights from patrol cars or passing traffic will interfere with eye tracking.
Lack of medical expertise: Officers aren’t medical professionals, yet they interpret physiological responses.
In DUI defense cases, challenging the validity of the HGN test is a common strategy. An experienced DUI defense lawyer will examine whether the officer administered the test correctly and whether there are alternative explanations for the observed eye movements.
Common Errors Officers Make During DUI Stops
Even well-trained officers will make mistakes during DUI investigations. These errors can significantly weaken the prosecution’s case when properly identified and presented.
A closer look at frequent issues reveals how often procedures aren’t followed correctly:
Improper instructions: Officers will fail to clearly explain how to perform a test, leading to confusion.
Failure to demonstrate: Without a proper demonstration, individuals will perform tasks incorrectly.
Bias in observation: Officers who already suspect impairment will interpret behavior in a way that confirms their belief.
Inadequate documentation: Missing or incomplete reports will create gaps in the case.
Each of these mistakes creates an opportunity for a DUI defense. By identifying inconsistencies and procedural errors, a defense strategy will cast doubt on the reliability of the evidence presented.
It’s important to remember that a DUI charge doesn’t automatically mean a conviction. The details of how the stop and testing were handled will play a major role in the outcome.
How DUI Defense Challenges Test Results
A strong DUI defense focuses on breaking down the evidence piece by piece. Field sobriety tests and HGN results are often central to the prosecution’s case, but they aren’t beyond scrutiny.
Defense strategies will often include:
Questioning test conditions: Highlighting environmental and physical factors that affected performance.
Reviewing officer training: Determining whether the officer followed standardized procedures.
Examining video evidence: Dashcam or bodycam footage will reveal inconsistencies between reports and actual events.
Presenting alternative explanations: Demonstrating that observed behaviors have causes unrelated to alcohol.
By carefully analyzing each element, a DUI defense works to create reasonable doubt. If the reliability of the tests is questionable, the strength of the case against you weakens significantly.
Legal representation plays a critical role in this process. Having someone who understands how to challenge these tests will make a meaningful difference in how a case progresses.
Contact a DUI Defense Attorney Today
A DUI charge can feel like a turning point, but it doesn’t define what comes next. There are ways to challenge the evidence, question the procedures, and protect your future. Field sobriety tests and HGN results aren’t absolute proof of impairment, and they will be successfully challenged with the right approach. If you’re facing a DUI charge, taking action now will make a difference.
Working with William L. Pomeroy Law gives you access to dedicated DUI defense strategies focused on uncovering weaknesses in the case. The firm assists clients throughout Brentwood, Davidson County, Williamson County, and Rutherford County, offering guidance during a difficult time. Contact the firm today to discuss your DUI defense options and start building a path forward.