Field sobriety tests are roadside exercises officers use to investigate possible impairment and decide whether there is probable cause for a DUI arrest. The standardized battery taught by the National Highway Traffic Safety Administration consists of:
- Horizontal gaze nystagmus, or HGN;
- The walk-and-turn; and
- The one-leg stand.
These tests do not produce a blood-alcohol number, and NHTSA’s training materials do not treat them as a literal “pass” or “fail.” Officers look for defined clues and consider them with driving, speech, odor, admissions, appearance, and other circumstances.
The most important evidence is often the video. It can show the actual instructions, conditions, performance, and officer commentary rather than the shortened version later written in a report.
Are Field Sobriety Tests Voluntary in Idaho?
Generic internet advice often says field sobriety tests are always voluntary. That is not a safe description of Idaho law.
Idaho appellate courts have held that field sobriety testing is a search, but it can be a reasonable investigative search when an officer has reasonable suspicion that a driver is under the influence. In State v. Orr, the Idaho Court of Appeals held that a lawfully detained driver had no constitutional right to refuse a lawful order to perform field sobriety tests and upheld an obstruction conviction arising from the refusal. The decision relied in part on State v. Buell.
That is different from Idaho’s implied-consent process for breath, blood, or urine testing. The automatic refusal consequences in the evidentiary-testing statute do not, by their terms, turn an FST refusal into a chemical-test refusal. But refusal can still create other legal issues in Idaho.
At a traffic stop, remain calm and do not physically resist. Afterward, a lawyer can examine whether the officer actually had the reasonable suspicion needed to extend the stop and order the tests. This page is general information, not real-time advice for a particular police encounter.
The Legal Basis for Extending the Stop
Even properly administered tests do not answer whether the officer had a lawful basis to conduct them. A routine traffic stop cannot be prolonged indefinitely on a hunch.
The defense should identify what the officer knew before requesting field sobriety testing. Claimed facts may include:
- Driving behavior or a collision;
- Odor of alcohol or another substance;
- Red or watery eyes;
- Speech or coordination observations;
- Statements about drinking, drugs, or medication;
- Open containers or other visible evidence; and
- Information from a witness or dispatcher.
Body-camera, dash-camera, dispatch, and 911 records may support, contradict, or add context to the report. If reasonable suspicion was missing, the legal challenge can reach evidence developed after the stop was improperly extended.
Horizontal Gaze Nystagmus (HGN)
HGN refers to an involuntary jerking of the eyes as they move toward the side. During the test, an officer typically asks the person to follow a stimulus—often a fingertip, pen, or small light—with the eyes while keeping the head still.
NHTSA’s standardized procedure directs the officer to check for equal pupil size, resting nystagmus, and equal tracking before looking for three clues in each eye:
- Lack of smooth pursuit;
- Distinct and sustained nystagmus at maximum deviation; and
- Onset of nystagmus before 45 degrees.
There are six possible clues, three in each eye.
What Can Affect an HGN Evaluation?
The defense may examine:
- Whether the officer screened for unequal pupils, resting nystagmus, or unequal tracking;
- The stimulus’s distance and height;
- Whether the officer moved it at the standardized speed;
- Whether maximum deviation was held long enough;
- How the 45-degree estimate was made;
- Wind, flashing lights, traffic, or other visual distractions;
- Whether the person moved the head and how the officer responded;
- Contact lenses, eye injury, or a documented medical condition; and
- Whether the video supports the report’s claimed clues.
Nystagmus can have causes other than alcohol. Idaho cases permit HGN evidence as an indicator associated with intoxication when an adequate foundation is established, but not as proof of a particular blood-alcohol concentration. An officer should not use HGN to claim a precise numerical BAC.
The Walk-and-Turn Test
The walk-and-turn is a divided-attention exercise. The officer generally instructs the person to stand heel-to-toe during the instructions, take nine heel-to-toe steps along a line, turn using a series of small steps, and take nine heel-to-toe steps back.
NHTSA’s standardized scoring identifies eight clues:
- Cannot keep balance during the instructions;
- Starts too soon;
- Stops while walking;
- Does not touch heel-to-toe;
- Steps off the line;
- Uses arms for balance;
- Makes an improper turn; and
- Takes the wrong number of steps.
Those labels can hide important details. A six-inch gap is not the same event as a slight separation. Pausing to ask what the officer meant is not necessarily the same as stopping from loss of balance. The video should be compared with each written clue.
Conditions Matter
NHTSA describes standardized conditions that include a reasonably straight, dry, hard, level, non-slippery surface with enough room for the steps. Conditions that may affect performance include:
- Uneven pavement, gravel, a slope, or a shoulder near traffic;
- Rain, ice, strong wind, darkness, or flashing emergency lights;
- Footwear or an officer’s instruction to remove it;
- Age, weight, injury, balance limitations, or neurological conditions;
- Language, hearing, or comprehension difficulties; and
- Incomplete, confusing, or rushed instructions.
These facts do not automatically invalidate the exercise. They affect how much confidence should be placed in the officer’s interpretation.
The One-Leg-Stand Test
For the one-leg stand, the officer generally directs the person to raise one foot about six inches, keep both legs straight, look at the raised foot, and count aloud until told to stop. The standardized timing is approximately 30 seconds.
NHTSA identifies four clues:
- Sways while balancing;
- Uses arms for balance;
- Hops; and
- Puts the foot down.
The review should include the officer’s instructions and demonstration, the actual test duration, the height and movement of the foot, the surface, footwear, weather, traffic, and any documented physical limitation.
“Sway” is especially easy to overstate in a written report. Everyone moves to some extent while balancing on one leg. The question is what the video actually shows and whether the officer applied the trained criterion consistently.
Standardized and Nonstandard Tests
Officers sometimes use exercises outside NHTSA’s three-test standardized battery, such as:
- Reciting part of the alphabet;
- Counting backward;
- Finger-to-nose;
- Finger-count exercises; or
- A modified Romberg balance test.
These exercises may become part of the officer’s overall observations, but they should not be described as though they carry the validation or scoring system of the standardized three-test battery. Counsel should ask what training, instructions, scoring criteria, and purpose the officer relied on.
Why “Two Clues Means You Failed” Is Misleading
NHTSA training materials use clue thresholds to help officers make arrest decisions. They do not turn the roadside exercise into an Idaho verdict.
A clue count can be challenged because:
- The officer did not give standardized instructions;
- The officer counted an event that does not fit the defined clue;
- The test conditions were unsuitable;
- A physical or communication issue affected performance;
- The report conflicts with the video; or
- The officer treated a nonstandard exercise as scientifically equivalent.
Conversely, apparently strong performance does not guarantee that an officer will not arrest. Field tests are only one part of the probable-cause decision and one part of the prosecution’s evidence.
How an Idaho DUI Lawyer Reviews Field Sobriety Tests
A disciplined review generally includes:
- Identifying the facts the officer had before extending the stop;
- Obtaining all body-camera and dash-camera angles;
- Comparing every instruction and demonstration with the trained procedure;
- Measuring timing where it matters, including HGN stimulus movement and the one-leg stand;
- Documenting the surface, lighting, weather, traffic, footwear, and available space;
- Comparing the officer’s clue chart and narrative with the video;
- Reviewing the officer’s current training and experience;
- Evaluating genuine medical, injury, hearing, language, or balance evidence; and
- Separating standardized clues from the officer’s general impressions.
The goal is not to argue that coordination exercises can never provide useful information. It is to determine what was actually done, whether it was lawful, and how much weight the result deserves.
For the larger case strategy, read Fighting a DUI Charge in Idaho or visit the DUI Help Center.
Frequently Asked Questions
Can I refuse field sobriety tests in Idaho?
Do not rely on the blanket statement that they are always voluntary. Idaho appellate decisions hold that a driver lawfully detained on reasonable suspicion of DUI has no constitutional right to refuse a lawful order to perform field sobriety tests, and refusal may create an obstruction issue. The legality of the detention and order can still be challenged afterward.
Does HGN prove my exact BAC?
No. Idaho permits properly founded HGN evidence as an indicator associated with intoxication, but not as proof of a particular alcohol concentration.
Does two clues mean I legally failed a test?
No. NHTSA clue thresholds help officers interpret the exercise; they are not a legal pass/fail rule and do not establish guilt by themselves.
Does poor performance prove DUI?
No. Field-test performance is considered with the entire case, including the legality of the stop, driving evidence, video, physical conditions, statements, chemical testing, and other observations.
Can an officer arrest me even if I perform well?
Yes. An officer may rely on the totality of the circumstances. Strong performance can still be valuable evidence for the defense.
Talk With an Idaho DUI Defense Lawyer
If field sobriety testing is part of your case, the video should be preserved and reviewed before it is reduced to a clue count. Attorneys of Idaho offers free, confidential DUI case reviews.
Contact Attorneys of Idaho or call 208-314-8888.
This page provides general information, not legal advice. Do not use it as instructions for a live traffic stop.
Talk With an Idaho DUI Defense Attorney
Deadlines and defense options depend on the facts of your case. Contact Attorneys of Idaho for a free, confidential case review.
