Field sobriety tests are a familiar part of DUI investigations, but they are often misunderstood.

Most people have seen some version of them on television: an officer asks a driver to follow a pen with their eyes, walk heel-to-toe along a line, or stand on one leg. What is less obvious is that the standardized tests are supposed to be administered in a particular way, officers are trained to look for particular observations, and a person's performance can be affected by circumstances that have nothing to do with alcohol or drug impairment.

Field sobriety tests also do not measure a person's blood alcohol concentration. They are investigative tools. Their significance in an Idaho DUI case depends on what led to the investigation, how the tests were administered, what the driver actually did, what the officer documented, and how the roadside testing fits with the rest of the evidence.

This guide explains the three standardized field sobriety tests used in Idaho DUI investigations, additional tests an officer may use, common administration and interpretation issues, medical and environmental factors that can affect performance, the role of body-camera footage, and how field sobriety evidence may be used in an Idaho DUI case.

What Are Standardized Field Sobriety Tests?

The National Highway Traffic Safety Administration, commonly known as NHTSA, developed a standardized battery of three field sobriety tests:

  • Horizontal Gaze Nystagmus (HGN);

  • Walk-and-Turn (WAT); and

  • One-Leg Stand (OLS).

Idaho State Police procedure likewise identifies these three tests as the standardized field sobriety test battery.

The word standardized is important.

The tests are not intended to be improvised balance exercises. Officers receive training concerning how the tests should be explained, demonstrated, administered, observed, and documented.

The three tests also examine different things.

HGN involves observations of eye movement. The Walk-and-Turn and One-Leg Stand are divided-attention exercises requiring a person to listen to instructions while simultaneously performing physical and mental tasks.

An officer may consider the results when deciding whether there is probable cause for a DUI arrest. If a case is prosecuted, observations from the tests may later become part of the evidence considered by a prosecutor, judge, or jury.

Horizontal Gaze Nystagmus

Horizontal Gaze Nystagmus is usually performed while the driver stands still and follows a stimulus with their eyes.

The stimulus might be a pen, fingertip, penlight, or similar object.

The officer generally instructs the person to keep their head still and follow the stimulus using only their eyes.

What Is Nystagmus?

Nystagmus is an involuntary jerking movement of the eyes.

Certain forms of nystagmus may become more pronounced after alcohol consumption. Officers trained in HGN are taught to look for particular eye movements that NHTSA has identified as relevant to alcohol impairment.

That does not mean that every instance of nystagmus is caused by alcohol. Nystagmus can have other causes, which is one reason HGN should be evaluated in context rather than treated as a stand-alone measurement of intoxication.

Preliminary Eye Checks

Before looking for the standardized HGN clues, an officer should perform preliminary checks.

These generally include observations concerning:

  • Equal pupil size;

  • Resting nystagmus; and

  • Whether the eyes track the stimulus together.

The purpose of these checks is partly to identify circumstances that may warrant additional attention before the officer interprets the eye movements as alcohol-related.

The Six HGN Clues

Officers are trained to look for three standardized clues in each eye:

  1. Lack of smooth pursuit;

  2. Distinct and sustained nystagmus at maximum deviation; and

  3. Onset of nystagmus prior to approximately 45 degrees.

Because each clue can be observed in each eye, an officer can identify as many as six standardized HGN clues.

Why Administration Matters

The way the stimulus is moved matters.

Different portions of the HGN examination involve different movements and timing. The officer's distance from the subject, the position of the stimulus, the speed of the movement, and how long the stimulus is held at certain points can all become relevant when evaluating the test.

Potential issues can include moving the stimulus too quickly, failing to complete the preliminary checks, improper positioning, interruptions during the test, or significant visual distractions.

HGN is also different from the two physical tests in an important legal respect. Idaho courts have recognized that HGN relies on scientific principles in a way that observations such as losing balance or stepping off a line do not.

HGN evidence may support an inference of alcohol impairment when an appropriate foundation exists, but it does not establish a person's specific blood alcohol concentration.

The Walk-and-Turn Test

The Walk-and-Turn is a divided-attention test.

It asks a person to remember several instructions while maintaining balance, walking in a precise manner, counting aloud, and performing a particular turn.

That combination is intentional. The officer is observing more than whether the person can simply walk from one place to another.

How the Walk-and-Turn Is Performed

The officer will generally instruct the person to begin in a heel-to-toe stance, with one foot directly in front of the other.

The person is then instructed to remain in that position until the officer finishes explaining the test.

During the walking portion, the person is generally instructed to:

  • Take nine heel-to-toe steps along a real or imaginary line;

  • Count each step aloud;

  • Watch their feet;

  • Keep their arms at their sides;

  • Make a prescribed turn;

  • Take nine heel-to-toe steps back; and

  • Continue until the test is completed.

The turn is not simply an ordinary pivot.

The person is generally instructed to leave the front foot on the line and use the other foot to take several small steps around it before returning in the opposite direction.

Because that is not how most people naturally turn around, the officer's explanation and demonstration can be important.

The Eight Walk-and-Turn Clues

NHTSA identifies eight standardized clues:

  1. Cannot maintain balance during the instructional stage;

  2. Starts before being instructed to begin;

  3. Stops while walking;

  4. Does not touch heel-to-toe;

  5. Steps off the line;

  6. Uses arms for balance;

  7. Makes an improper turn; and

  8. Takes an incorrect number of steps.

These are defined observations, not simply general impressions.

For example, an officer should distinguish between ordinary movement and actually breaking the instructional stance. Likewise, a minor imperfection in foot placement is not necessarily the same thing as the standardized heel-to-toe clue.

That distinction can become important when the language in a police report is compared with the actual roadside video.

Diagram showing the standardized Walk-and-Turn field sobriety test, including nine heel-to-toe steps and the prescribed turn

The One-Leg Stand Test

The One-Leg Stand is another divided-attention exercise.

The person must maintain balance while listening to and remembering instructions and counting aloud.

How the One-Leg Stand Is Performed

The officer will generally instruct the person to stand with their feet together and arms at their sides.

The person is then directed to:

  • Raise one foot approximately six inches off the ground;

  • Keep the raised foot parallel to the ground;

  • Keep both legs straight;

  • Look at the raised foot;

  • Keep their arms at their sides; and

  • Count aloud in the manner instructed until told to stop.

The officer generally times the exercise for approximately 30 seconds.

The Four One-Leg Stand Clues

Officers are trained to look for four standardized observations:

  1. Swaying while balancing;

  2. Using the arms for balance;

  3. Hopping; and

  4. Putting the raised foot down.

As with the Walk-and-Turn, the definitions matter.

A small natural arm movement, for example, is not necessarily the same as using the arms for balance under the standardized criteria.

If the person puts the raised foot down, the officer's training also addresses how the person should be instructed to resume the test.

Do Field Sobriety Tests Measure Blood Alcohol Concentration?

No.

Field sobriety tests do not tell an officer whether someone's alcohol concentration is .08, .10, .20, or any other specific number.

That requires chemical testing.

The distinction is particularly important with HGN. Although there is a scientific basis for considering certain eye movements in an alcohol investigation, Idaho courts have held that HGN should not be used to establish a specific blood alcohol level.

The Walk-and-Turn and One-Leg Stand are even more straightforward in this respect. They produce observations of physical performance, not a chemical measurement.

A driver who performs poorly on a balance test has not thereby produced a measurable BAC.

Likewise, a person who performs well has not established that their BAC was below a particular number.

Are Field Sobriety Tests Pass-or-Fail Tests?

People often describe field sobriety tests by saying that someone “passed” or “failed.”

That shorthand can be misleading.

Officers are trained to observe and document particular clues. The number and nature of those observations may contribute to the officer's overall assessment, but the roadside investigation does not operate like a driver's license examination where a particular score automatically determines guilt.

A more useful analysis asks:

  • What instructions were given?

  • What did the person actually do?

  • Which standardized clues were observed?

  • Were those clues scored according to the officer's training?

  • Were there other reasonable explanations for the performance?

  • What does the video show?

  • How does the testing fit with the rest of the investigation?

That is much more informative than simply saying the driver “failed the field sobriety tests.”

Optional and Non-Standardized Tests Used in Idaho

Officers may use additional roadside exercises beyond HGN, Walk-and-Turn, and One-Leg Stand.

Current Idaho State Police procedure identifies several optional tests, including:

  • Alphabet recitation;

  • Counting;

  • Finger dexterity;

  • Modified Romberg balance; and

  • Lack of Convergence.

Troopers who have completed Advanced Roadside Impaired Driving Enforcement, or ARIDE, training may also use a Finger-to-Nose evaluation.

These tests should not be confused with the standardized three-test NHTSA battery.

Alphabet and Counting Exercises

An officer may ask someone to recite part of the alphabet or count between designated numbers.

The officer may be observing whether the person understands the directions, remembers the starting and stopping points, maintains the correct sequence, or becomes confused.

Mistakes can have causes unrelated to impairment, including anxiety, language differences, hearing problems, learning disabilities, distractions, or misunderstanding the instructions.

Finger Dexterity

A finger-dexterity exercise generally requires the person to touch the thumb to different fingertips in a particular order while counting.

The officer may observe coordination, sequence, counting, and whether the person follows the directions.

Hand injuries, arthritis, neurological conditions, and other physical limitations may affect performance.

Modified Romberg Balance

A Modified Romberg exercise generally involves standing still, closing the eyes, positioning the head as directed, and estimating the passage of a specified period of time.

Depending upon the circumstances, the officer may observe balance, body sway, the person's time estimate, and ability to follow instructions.

Lack of Convergence

A Lack of Convergence evaluation involves moving a stimulus toward the subject's nose and observing whether the eyes converge toward it.

This may be used as an additional observation, particularly in some drug-impairment investigations.

Finger-to-Nose

The Finger-to-Nose evaluation may involve closing the eyes, tilting the head back, extending the arms, and touching the tip of the nose with a specified finger when instructed.

It is more commonly associated with advanced drug-impairment investigation than the basic standardized SFST battery.

Common Problems in Field Sobriety Testing

Not every deviation from NHTSA training makes a field sobriety test invalid.

That would be too simplistic.

The better question is whether a particular departure from the standardized procedure could reasonably affect the person's performance or the officer's interpretation of it.

Incomplete or Confusing Instructions

Field sobriety tests involve unfamiliar instructions.

If the officer does not clearly explain what the person is supposed to do, a mistake can become difficult to interpret.

For example, the Walk-and-Turn clue commonly described as “starts too soon” presupposes that the person was told not to begin until instructed.

Similarly, an “improper turn” deserves closer examination if the officer never adequately explained or demonstrated the unusual turn the test requires.

Body-camera footage can be particularly useful for determining what was actually said.

Demonstration Problems

The officer's demonstration can matter as much as the verbal instruction.

If the officer demonstrates a movement differently from the way the driver is later expected to perform it, that inconsistency may provide important context.

The One-Leg Stand is a good example. The raised leg is supposed to remain straight, with the foot approximately six inches above and parallel to the ground.

A demonstration that materially differs from those instructions can create confusion.

Scoring Ordinary Movement as a Standardized Clue

Not every movement is a clue.

A person can move slightly while maintaining an instructional stance. A heel and toe can be very close without the step necessarily satisfying the standardized missed heel-to-toe criterion. A person's hands may move somewhat without necessarily meeting the standardized “uses arms for balance” clue.

A report that simply says “failed heel-to-toe” or “used arms for balance” may therefore warrant comparison with the video and the actual scoring criteria.

Counting One Type of Error Repeatedly

The standardized tests use categories of clues.

If someone misses heel-to-toe more than once, that may be important when describing the overall performance, but it does not create a new category of standardized clue every time it occurs.

The distinction between the number of individual mistakes and the number of standardized clues can be important when a written description makes the performance sound worse than the standardized scoring would suggest.

HGN Timing and Technique

Because HGN involves subtle eye movement, administration technique deserves particular scrutiny.

Potential issues can include:

  • Incorrect stimulus distance;

  • Improper positioning;

  • Moving the stimulus too rapidly;

  • Inadequate holds at required positions;

  • Skipping preliminary eye checks;

  • Failing to properly evaluate both eyes;

  • Interruptions; or

  • Significant visual distractions.

Ordinary body-camera footage often cannot show whether nystagmus itself was present, but it may still reveal significant administration problems.

Medical and Physical Conditions Can Affect Performance

Balance is not unique to alcohol impairment.

A person's ability to perform the Walk-and-Turn or One-Leg Stand may be influenced by injuries, medical conditions, mobility limitations, neurological issues, age, fatigue, or other circumstances.

That does not mean every medical complaint explains poor performance. The relevant question is whether the particular condition reasonably relates to the particular observation.

Knee, Ankle, Hip, Leg, and Back Problems

The Walk-and-Turn and One-Leg Stand place demands on the lower body.

Prior knee surgery, ankle injuries, hip problems, arthritis, joint replacements, back conditions, or limited mobility may make heel-to-toe walking or standing on one leg more difficult.

A person who tells an officer about a significant knee problem before testing provides information that should be considered when later One-Leg Stand performance is evaluated.

Inner-Ear and Balance Conditions

The vestibular system plays an important role in balance.

Vertigo, inner-ear disorders, and other balance conditions may affect a person's ability to perform balance-based tests.

NHTSA training materials themselves recognize that some physical conditions can make these exercises more difficult.

Neurological and Eye Conditions

Neurological conditions can potentially affect gait, coordination, tremors, speech, balance, and eye movement.

Eye injuries, previous eye surgery, neurological eye conditions, or other causes of abnormal eye movement may also be relevant when HGN evidence is important.

The existence of such a condition does not automatically establish a defense. It does mean the observation should not be evaluated in a vacuum.

Fatigue and Anxiety

Many DUI investigations happen late at night.

A driver may have been awake for an extended period, worked a long shift, or simply be exhausted.

The person may also be standing beside traffic with emergency lights flashing while facing the possibility of arrest.

Fatigue and anxiety can affect concentration, communication, balance, and the ability to remember unfamiliar instructions.

Those circumstances may or may not explain a particular mistake. They are nevertheless part of a fair evaluation of what occurred.

Road Conditions, Weather, Lighting, and Footwear

Field sobriety tests happen in the field.

They are rarely administered under controlled laboratory conditions.

NHTSA training generally contemplates conducting the Walk-and-Turn and One-Leg Stand on a reasonably dry, hard, level, non-slippery surface when circumstances allow.

That does not mean a test becomes useless simply because the roadway is imperfect.

It does mean the actual conditions matter.

Relevant circumstances may include:

  • Gravel;

  • Broken or uneven pavement;

  • A sloped shoulder;

  • Snow or ice;

  • Rain or standing water;

  • Loose material;

  • Darkness;

  • Flashing patrol lights;

  • Passing traffic;

  • Wind;

  • Extreme cold; and

  • Roadside distractions.

A slight balance movement on clean, level pavement may deserve a different interpretation from the same movement on loose gravel beside a sloped highway shoulder.

Footwear

Shoes can also matter.

High heels, loose sandals, flip-flops, heavy work boots, snow boots, damaged shoes, and footwear with poor traction may affect balance and heel-to-toe walking.

NHTSA training instructs officers to take unusual footwear into consideration and discusses circumstances in which a subject may be given an opportunity to remove shoes.

Removing shoes is not always a perfect solution. Barefoot testing may create its own problems on cold, hot, wet, sharp, or gravel-covered surfaces.

The sensible question is whether the person had a reasonable opportunity to perform the exercise under suitable conditions.

Why Body-Camera Evidence Matters

Body-worn camera footage has changed the way field sobriety evidence can be evaluated.

Before widespread body-camera use, an attorney frequently had little more than the officer's written description.

Today, video often allows the roadside encounter to be reviewed directly.

Sometimes the recording strongly supports the officer's report.

Sometimes it does not.

Comparing the Report With What Happened

A police report may describe someone as:

  • Unsteady;

  • Swaying;

  • Confused;

  • Unable to follow instructions;

  • Failing heel-to-toe; or

  • Using their arms for balance.

Video can show what those descriptions actually mean.

“Unsteady” might accurately describe obvious stumbling. In another case, it might refer to one minor movement during an otherwise normal encounter.

That difference matters when evaluating the strength of the evidence.

Correct Performance Matters Too

Police reports understandably focus on facts the officer considered significant.

They may devote less attention to things the person did correctly.

Video may reveal that someone:

  • Understood complicated instructions;

  • Counted correctly;

  • Took the correct number of steps;

  • Remained on the line;

  • Performed the turn properly;

  • Spoke clearly;

  • Retrieved documents normally; or

  • Walked normally before and after testing.

Correct performance does not erase legitimate clues.

It is still part of the evidence.

Instructions and Demonstrations Can Be Reviewed

Video may also resolve disputes about what instructions were given.

It can show whether:

  • The person was told not to start early;

  • The officer demonstrated the turn;

  • The One-Leg Stand was demonstrated correctly;

  • Instructions were interrupted;

  • The person asked a reasonable question;

  • The officer clarified the test; or

  • Road noise and other distractions interfered with communication.

This can be particularly important when the officer characterizes the driver as unable to follow instructions.

HGN Has Video Limitations

HGN is different.

Standard body cameras usually do not provide a close enough view of the eyes to determine whether subtle nystagmus was actually present.

The video can still help evaluate how the officer administered the test, but it generally should not be assumed that an ordinary body-camera recording allows someone to independently score all six HGN clues.

Video Can Hurt the Defense Too

Body-camera evidence should not be approached with the assumption that it will help the person charged.

Sometimes it makes the prosecution's case substantially stronger.

It may show significant balance problems, difficulty understanding ordinary instructions, damaging admissions, poor coordination, or other signs that are more apparent on video than in the written report.

A useful case review requires looking at favorable and unfavorable evidence with the same care.

Are Field Sobriety Tests Required in Idaho?

Idaho law concerning field sobriety tests is more complicated than the common internet advice to simply “refuse the tests.”

Idaho appellate courts have held that an officer with reasonable suspicion of DUI may use field sobriety testing as part of an investigative detention.

In State v. Ferreira, the Idaho Court of Appeals held that reasonable suspicion, rather than probable cause, is the constitutional standard required before an officer may administer field sobriety tests as part of a DUI investigation.

More recent Idaho cases continue to apply that general framework.

Can Someone Refuse to Perform the Tests?

A person can physically decline to participate, but refusal is not necessarily without legal consequences.

Idaho appellate decisions have held that refusal to participate in field sobriety tests may, depending upon the circumstances, be considered when determining whether probable cause existed for an arrest.

In State v. Martinez-Gonzalez, the Idaho Court of Appeals explained that a refusal or evasion of field sobriety testing may support an inference of consciousness of guilt and may contribute to probable cause.

That does not mean refusing an SFST automatically proves DUI.

There may be medical, physical, communication, or other explanations for a person's decision not to participate.

The circumstances surrounding the refusal matter.

Field Sobriety Test Refusal Is Different From Refusing Evidentiary Testing

This distinction is important.

Declining roadside field sobriety exercises is not the same thing as refusing an evidentiary breath or blood test under Idaho's implied-consent laws.

A refusal of evidentiary testing can trigger separate driver's license consequences and legal proceedings.

People often use the term “breath test” broadly, which can create confusion between a preliminary roadside test and an evidentiary test administered later in the investigation.

An attorney reviewing a case should determine exactly what test was requested and what happened when the request was made.

How Field Sobriety Tests Affect Probable Cause

An officer does not need a particular number of SFST clues before making a DUI arrest.

Probable cause is determined from the totality of the circumstances.

Depending on the case, an officer may consider:

  • Driving behavior;

  • A traffic collision;

  • Odor of alcohol;

  • Admissions concerning alcohol or drugs;

  • Speech;

  • Appearance;

  • Coordination;

  • Open containers;

  • Preliminary testing;

  • Field sobriety observations;

  • Refusal to participate in testing;

  • Witness information; and

  • Other circumstances known to the officer.

Good performance on some field sobriety tests does not automatically prevent an arrest if the remaining evidence is sufficiently strong.

Likewise, poor performance on one roadside exercise should not automatically be separated from everything else that occurred.

How Field Sobriety Tests May Be Used in Court

Field sobriety evidence can become relevant at several stages of an Idaho DUI case.

Motions to Suppress

A defense attorney may examine whether the officer had:

  • Lawful grounds for the original stop;

  • Reasonable suspicion to extend the encounter into a DUI investigation;

  • Reasonable suspicion when SFSTs were administered; and

  • Probable cause at the time of arrest.

A poorly administered test does not automatically create a constitutional violation.

The legal question in a suppression motion is usually whether the officer had the required legal justification at the relevant point in the encounter.

Administration problems may nevertheless affect how much weight should be given to the officer's conclusions.

HGN Evidence

Idaho courts treat HGN differently from observations made during the physical tests.

In State v. Garrett and later decisions, Idaho courts recognized the scientific nature of HGN while limiting the conclusions that may be drawn from it.

HGN can be relevant circumstantial evidence of alcohol impairment when an appropriate foundation exists.

It cannot establish a specific BAC without chemical testing.

Walk-and-Turn and One-Leg Stand Evidence

Idaho courts have treated observations from the physical tests differently.

An officer can describe what they observed—such as stepping off a line, losing balance, hopping, or putting a foot down—and the fact-finder can evaluate those observations using ordinary experience.

Those observations can be challenged through the video, cross-examination, medical evidence, testing conditions, and evidence concerning how the test was administered.

Plea Negotiations and Trial

Field sobriety evidence can also affect how prosecutors and defense attorneys evaluate a case before trial.

A video showing unusually good performance, significant administration problems, or clear physical limitations may affect the perceived strength of the prosecution's case.

Video showing obvious impairment may have the opposite effect.

If the case proceeds to trial, the fact finder may have an opportunity to compare the officer's description with the actual recording of the encounter.

How Long Law Office Reviews Field Sobriety Evidence

When field sobriety testing is important to an Idaho DUI case, simply counting the clues listed in the police report is not enough.

Long Law Office reviews the roadside investigation in context.

Depending upon the evidence available, that may include examining:

  • The basis for the traffic stop;

  • The officer's observations before testing;

  • Police reports;

  • Body-worn and dash-camera footage;

  • The instructions actually given;

  • The officer's demonstrations;

  • The administration of HGN;

  • The standardized Walk-and-Turn and One-Leg Stand clues;

  • Medical or physical limitations;

  • Footwear and testing conditions;

  • Statements made by the driver;

  • Preliminary breath testing;

  • Evidentiary breath or blood testing; and

  • The circumstances surrounding the arrest.

The review also considers what the driver did correctly.

Sometimes the video confirms the officer's conclusions. In other cases, the actual performance may look materially different from the description in the report, an alleged clue may not appear to meet the standardized criteria, or physical and environmental circumstances may deserve more attention.

Every case is different. The objective is to understand what the complete evidence actually shows before recommending how the case should be handled.

Related Idaho DUI Resources

Field sobriety testing is only one part of a DUI investigation.

Depending upon the circumstances of the case, additional issues may involve chemical testing, driver's license consequences, refusal proceedings, prior convictions, or the particular level of DUI charge.

For more information, see:

Talk With a Boise DUI Attorney

A field sobriety test result is not the same thing as a conviction.

If you have been arrested for DUI in Boise or elsewhere in southwest Idaho, the roadside tests should be reviewed together with the officer's reports, available video, chemical testing, and the other evidence in the case.

Attorney Cody Long works directly with clients and personally reviews the evidence to identify the factual and legal issues that may affect their case.

To discuss your DUI case and the options that may be available, contact Long Law Office at (208) 287-3303.