How Field Sobriety Tests Work in Idaho DUI Stops

When you're pulled over on suspicion of driving under the influence (DUI) in Idaho, one of the first tools law enforcement may use to assess impairment is the field sobriety test (FST). These tests are a series of physical and cognitive tasks designed to give officers observable clues about whether you're impaired by alcohol or drugs.

While they're widely used in Boise and across the state, many drivers don't fully understand what these tests involve, how reliable they are, and what rights they have during a DUI stop. This article breaks down the details of field sobriety tests in Idaho so you know what to expect and what it all means if you're ever asked to perform them.

What Are Field Sobriety Tests?

Field sobriety tests are standardized assessments developed by the National Highway Traffic Safety Administration (NHTSA) to help police officers evaluate whether a driver may be impaired. These tests are meant to gauge your balance, coordination, and attention—skills that are often impaired when someone has consumed alcohol or drugs.

In Idaho, law enforcement officers typically administer these tests at the roadside during a traffic stop when they have reasonable suspicion of DUI.

The Three Standardized Field Sobriety Tests

There are three main tests that make up the Standardized Field Sobriety Test (SFST) battery approved by NHTSA. They are:

1. Horizontal Gaze Nystagmus (HGN) Test

This test checks for involuntary jerking of the eyes, which is more pronounced when a person is impaired by alcohol. The officer asks you to follow a stimulus—typically a pen or flashlight—with your eyes while keeping your head still.

The officer looks for signs such as:

• Lack of smooth pursuit

• Distinct jerking when the eye is at maximum deviation

• Onset of nystagmus before the eye reaches a 45-degree angle

Each eye is evaluated separately, and multiple “clues” of impairment may be documented.

2. Walk-and-Turn Test

This is a divided attention test requiring both physical and mental focus. You'll be asked to take nine heel-to-toe steps along a straight line, turn on one foot, and return in the same manner.

The officer watches for indicators of impairment such as:

• Starting before the instructions are complete

• Losing balance or stepping off the line

• Not touching heel to toe

• Using arms for balance

• Incorrect turn or wrong number of steps

3. One-Leg Stand Test

In this test, you're instructed to raise one foot six inches off the ground and hold it for 30 seconds while counting aloud.

Officers will observe for:

• Swaying

• Hopping

• Putting your foot down

• Using arms for balance

These tests are intended to be administered in standardized conditions, but roadside settings often vary greatly, affecting how well individuals perform—regardless of sobriety.

Are Field Sobriety Tests Mandatory in Idaho?

No. You are not legally required to take field sobriety tests in Idaho. They are voluntary, but refusing to take them and a breathalyzer may lead to a "refusal" and automatic penalties like license suspension. However, refusing may prompt the officer to proceed with other methods to establish probable cause, such as a breath test or arrest.

That said, many people consent to these tests without fully understanding their rights. Knowing that you can politely decline is important if you're concerned about medical conditions, anxiety, or poor balance that may affect your performance.

How Reliable Are Field Sobriety Tests?

While widely used, field sobriety tests are not foolproof. According to NHTSA data:

• The HGN test is approximately 77% accurate

• The walk-and-turn test is 68% accurate

• The one-leg stand is 65% accurate

This means there is still a significant chance of false positives. Factors that can influence test results include:

• Weather conditions

• Uneven surfaces

• Poor lighting

• Footwear (e.g., high heels or sandals)

• Age, weight, or physical limitations

Because of these variables, performance on these tests can be misinterpreted—even by trained officers.

Can FST Results Be Challenged in Court?

Yes. In DUI cases that proceed to trial, field sobriety test results are often scrutinized by defense attorneys. Possible challenges include:

• Improper administration of the test (non-compliance with NHTSA guidelines)

• Medical or physical conditions affecting performance

• Unfair testing environment (e.g., sloped or icy road)

• Officer bias or subjective interpretation of the results

Body camera footage or dash cam recordings can be vital in evaluating the legitimacy and fairness of the test administration.

What Happens After the FST?

If the officer believes you showed signs of impairment during the tests, the next steps may include:

• A formal arrest for DUI based on probable cause

• An evidentiary breath or blood test. If you refuse to comply with this test, you will end up with a "refusal" and an automatic 1-year license suspension under Idaho's Implied Consent Law

Refusing the post-arrest chemical test can lead to a one-year license suspension and may be used as evidence of guilt in court.

Final Thoughts

Field sobriety tests are a common part of DUI stops in Idaho, but they're far from conclusive. Understanding how they work—and that they are voluntary—can help you make informed decisions if you ever find yourself in a roadside DUI investigation.

Remember: you have the right to remain silent, the right to refuse FSTs, and the right to legal counsel. Field sobriety tests are just one piece of the puzzle, and how you handle that moment can have lasting effects on the outcome of your case. Stay informed, stay calm, and know your rights.