Standardized field sobriety tests — Horizontal Gaze Nystagmus (HGN), Walk-and-Turn, and One-Leg Stand — are used by police as observational tools to help establish probable cause for impairment during traffic stops. However, their use is characterized by subjectivity and vulnerability to procedural error, raising significant questions about their reliability and role in determining intoxication. Motorists in most jurisdictions are not required to take these tests. However, if you refuse, the officer will likely obtain a warrant for blood or breath testing.
Key Takeaways
- Field sobriety tests are subjective tools used to establish probable cause, not guilt.
- HGN, Walk-and-Turn, and One-Leg Stand tests are susceptible to officer error and environmental factors.
- Poor performance on field sobriety tests does not measure BAC and does not determine the outcome of a DWI case.
What are Standard Field Sobriety Tests (SFSTs)?
Standardized Field Sobriety Tests (SFSTs) are roadside observational tests used by law enforcement to determine whether probable cause exists for a DWI arrest. They are not scientific measurements and do not measure blood alcohol concentration (BAC).
SFSTs rely on the officer's subjective observation and interpretation, rather than objective data. For example, an officer subjectively assesses whether a driver can walk in a straight line or smoothly track a moving object; this assessment is not guided by standardized metrics but by the officer's own perceptions and judgments. This reliance on personal interpretation introduces variability and potential bias into the evaluation process, highlighting the test's inherent subjectivity.
There are three standardized tests recognized nationwide:
- Horizontal Gaze Nystagmus (HGN)
- Walk-and-Turn
- One-Leg Stand
If these tests are not administered exactly as trained, their reliability drops significantly.
Who Created SFSTs and Why Police Use Them
SFSTs were standardized through national law enforcement training programs to promote consistency in roadside evaluations. Officers use these tests as investigative tools to help establish probable cause, not as proof of intoxication.
They are administrative tools rather than medical or scientific examinations.
Where SFSTs Fit in a DWI Investigation
SFSTs are one step in a larger investigative process:
- Traffic stop or initial contact
- Officer observations
- Field sobriety tests
- Arrest decision
- Breath or blood testing
SFST performance alone does not determine guilt.
Horizontal Gaze Nystagmus (HGN) Test
What the HGN Test Measures
The HGN test assesses involuntary nystagmus while tracking a stimulus. Alcohol may exaggerate this movement, but many non-alcohol-related factors can also cause it.
How the Test Is Supposed to Be Conducted
The officer must:
- Check for equal pupil size and smooth tracking
- Move the stimulus at a specific speed
- Hold the stimulus at maximum deviation
- Observe each eye independently
Common HGN “Clues”
- Lack of smooth pursuit
- Distinct jerking at maximum deviation
- Onset of jerking before 45 degrees
Common HGN Problems
- Improper stimulus speed
- Poor lighting or visual distractions
- Medical conditions affecting eye movement
- Failure to follow standardized procedures
Takeaway
HGN is highly technical, and minor errors can undermine its reliability. For example, in State v. Lasworth (1998), the New Mexico Court of Appeals held that improper administration of the HGN test, such as incorrect stimulus speed or poor adherence to standardized procedures, rendered the results inadmissible as evidence.
This ruling demonstrates that when officers fail to conduct the HGN test in accordance with established protocols, the resulting evidence may be excluded from court proceedings, potentially weakening the prosecution’s case. Thus, procedural errors not only raise questions regarding test reliability but may also impede the admissibility of critical evidence and impact the overall outcome of a DWI prosecution.
Walk-and-Turn Test
What the Walk-and-Turn Monitors
This test evaluates divided attention—the ability to listen, follow instructions, and perform physical actions simultaneously.
Proper Instructions Matter
The officer must clearly explain and demonstrate:
- Heel-to-toe steps
- A straight line
- A specific turning method
- A fixed number of steps
Common Walk-and-Turn “Clues”
- Losing balance during instructions
- Starting too soon
- Stepping off the line
- Improper turn
- Incorrect number of steps
Why This Test Is Often Flawed
- Uneven or sloped surfaces
- Footwear
- Age, weight, or injuries
- Nervousness or confusion
Takeaway
The Walk-and-Turn test is highly subjective and environment-dependent.
One-Leg Stand Test
What the One-Leg Stand Measures
This test evaluates balance and divided attention, not intoxication itself.
How the Test Is Administered
The subject must:
- Raise one foot approximately six inches
- Keep arms at sides
- Look at the raised foot
- Count out loud for 30 seconds
Common One-Leg Stand “Clues”
- Swaying
- Using arms for balance
- Hopping
- Putting the foot down
Limitations of the Test
- Lower-body injuries
- Back or joint problems
- Inner ear or balance disorders
- Fatigue or anxiety
Takeaway
Balance problems are not proof of intoxication.
Key Differences Between the Three Tests
Horizontal Gaze Nystagmus
Measures: Eye movement
Most common weakness: Officer technique
Walk-and-Turn
Measures: Divided attention
Most common weakness: Surface and instructions
One-Leg Stand
Measures: Balance
Most common weakness: Physical limitations
Issues That Field Sobriety Tests Cannot Determine
Field sobriety tests cannot:
- Measure blood alcohol concentration (BAC)
- Determine exact alcohol consumption
- Identify the cause of balance issues
- Distinguish alcohol impairment from medical conditions
- Establish guilt
They are screening tools, not diagnostic tests.
Are Field Sobriety Tests Mandatory?
No. Field sobriety tests are voluntary in most jurisdictions. Refusal is not a criminal offense and is not proof of intoxication.
Are Field Sobriety Tests Reliable?
Field sobriety tests:
- Are not chemical tests
- Do not measure BAC
- Depend heavily on officer judgment
They are used to establish probable cause, not guilt.
How SFSTs are challenged
SFST challenges often focus on:
- Improper administration
- Officer training gaps
- Video evidence contradicting reports
- Medical or physical explanations
- Environmental conditions
When SFST reliability collapses, probable cause may collapse with it.
If–Then
- If SFST instructions are unclear or incomplete, reliability decreases.
- If roadside conditions interfere with balance, the results are not probative.
- When medical conditions are present, the risk of false positives increases.
- If the video contradicts the officer's descriptions, the evidentiary weight declines.
Bottom Line
In summary, standardized field sobriety tests, such as HGN, Walk-and-Turn, and One-Leg Stand, primarily serve as observational tools for officers to establish probable cause during DWI investigations.
Yet, their inability to directly measure intoxication, along with their susceptibility to subjective interpretation and procedural error, limits their evidentiary strength.
This limitation underscores the problems of relying solely on these tests in both legal and administrative contexts, as inaccurate administration or interpretation may lead to wrongful arrest decisions or undermine the integrity of the investigative process.
FAQs about SFSTs
What are field sobriety tests?
Field sobriety tests are roadside observational tests police use to assess possible impairment and decide whether probable cause exists for a DWI arrest.
What are the three standardized field sobriety tests?
The three standardized field sobriety tests are Horizontal Gaze Nystagmus (HGN), Walk-and-Turn, and One-Leg Stand.
Do field sobriety tests measure blood alcohol concentration (BAC)?
No. Field sobriety tests do not measure BAC and are not chemical tests.
Are field sobriety tests mandatory?
No. Field sobriety tests are voluntary in most jurisdictions, and refusing them is not a criminal offense.
Does failing a field sobriety test mean you are intoxicated?
No. Poor performance on a field sobriety test does not prove intoxication or guilt.
What does the HGN test measure?
The HGN test observes involuntary eye movement while tracking a stimulus, which officers associate with possible impairment but which can also be caused by non-alcohol-related factors.What does the Walk-and-Turn test evaluate?
The Walk-and-Turn test assesses divided attention by requiring participants to follow instructions while performing physical movements.
What does the One-Leg Stand test show?
The One-Leg Stand test assesses balance and divided attention, not intoxication per se.
Can medical conditions affect field sobriety test results?
Yes. Eye disorders, inner ear issues, injuries, fatigue, anxiety, and neurological conditions can all affect field sobriety test performance.
Can roadside conditions affect field sobriety tests?
Yes. Uneven pavement, poor lighting, weather, footwear, and distractions can significantly impact test performance.
Sources: Michael & Associates research, What you need to know about SFST, DWI and DUI Glossary, National Highway Traffic Safety Administration (NHTSA), The DWI Detection and Standardized Field Sobriety Testing Participant Manual, Justia, Texas District & County Attorneys Association, Texas Department of Public Safety (DPS).
