Most DUI stops in Missoula follow a pattern. An officer sees something about the driving, makes contact, notices an odor or slurred speech or watery eyes, and then asks the driver to step out for field sobriety tests. What happens in the next few minutes on the shoulder of the road can shape the whole case. I am certified in NHTSA Standardized Field Sobriety Testing, and I have worked inside a local prosecutor's office, so I have seen how these tests are used to build a case and where they tend to come apart.
The Three NHTSA Standardized Field Sobriety Tests
The National Highway Traffic Safety Administration (NHTSA) developed a standardized battery of three roadside tests. "Standardized" is the important word. Each test is supposed to be explained, demonstrated, given and scored the same way every time, under suitable conditions. The training treats those steps as part of what makes the results meaningful. When an officer skips a step, changes the instructions or gives a test in poor conditions, the result is harder to rely on.
| Test | What you are asked to do | What I look at |
|---|---|---|
| Horizontal gaze nystagmus (HGN) | The officer watches your eyes follow a stimulus, such as a pen or light, from side to side. | Whether the officer screened for medical and eye conditions, the lighting, flashing lights or passing traffic nearby, and how the stimulus was held and moved. |
| Walk-and-turn | You take heel-to-toe steps along a line, turn in a set way, and walk back while following spoken instructions. | The surface, slope and footing, footwear, weather, how clearly the instructions were given, and any injury, weight or balance issue. |
| One-leg stand | You stand on one foot with the other raised and count aloud for a set time. | Cold, wind, uneven or icy ground, footwear, age, back, leg or inner-ear problems, and nervousness at the roadside. |
Officers sometimes add other exercises, such as reciting the alphabet or counting backward. Those are not part of the three-test standardized battery, and I treat them differently when I review a case.
Conditions That Affect Reliability in Western Montana
Roadside tests are balance and attention tasks given to someone who has just been pulled over, often at night, often in front of flashing lights and passing traffic. In western Montana the conditions can be hard on anyone. Common problems I look for include:
- Weather. Cold, wind, rain and snow affect balance and the ability to hear instructions.
- Surface. Gravel shoulders, ice, slush, slopes and cracked pavement make walking a straight line and standing on one foot harder.
- Footwear. Boots, heels, sandals or bare feet change how a person moves.
- Lighting. Emergency lights, headlights and darkness can affect an eye test and a person's footing.
- Medical issues. Injuries, weight, age, inner-ear and eye conditions, and some medications can affect performance with no alcohol involved.
- Nerves. Being stopped by the police is stressful, and stress affects coordination and memory.
Officer Training and Administration Errors
The reliability of a field sobriety test depends on the person giving it. In a case review I compare the report and any body camera or dash camera video against how the tests are taught. I look at whether the officer asked about injuries and medical conditions before starting, whether the instructions were complete and clear, whether the officer demonstrated the test, whether the location was suitable, and whether the report matches what the video shows. Reports are often written later from memory, and small differences between the paper and the recording can matter.
On cross-examination, my certification lets me ask precise questions: what the training says about a given step, whether the officer followed it here, and what that means for the result. The goal is not to argue that roadside tests never mean anything. It is to show the judge or jury exactly how much weight these particular results can carry.
The Roadside Breath Screen Is a Different Test
Many people remember blowing into a small handheld device at the roadside and assume that was "the breath test." Montana law treats it separately. Under MCA § 61-8-1016(1), implied consent covers a preliminary alcohol screening test (PAST), and taking a PAST does not satisfy the obligation to take the evidentiary test that follows. The PAST must be given by an officer and on an instrument certified by the department. If a person refuses both a PAST and another test during the same incident, the department may not count each as a separate refusal for suspension (§ 61-8-1016(2)(e)). What a refusal costs is covered on our test refusal and implied consent page.
Why Roadside Tests Matter Most in an Impairment DUI
Montana has more than one way to charge DUI. A per se charge, such as driving with an alcohol concentration of 0.08 or more under MCA § 61-8-1002(1)(b), rests mainly on a test number. An impairment charge under § 61-8-1002(1)(a) is different. It asks whether alcohol, drugs or both diminished the person's ability to drive safely (the definition of "under the influence" in § 61-1-101(83)). There may be no number at all, or a number below 0.08. In those cases the officer's observations and the field sobriety tests often carry much of the State's case.
The test result still matters when there is one. Under § 61-8-1002(2), a result of 0.04 or less allows an inference that the person was not under the influence. Between 0.04 and 0.08 there is no inference either way, and at 0.08 or more the inference is that the person was under the influence, though that inference is rebuttable.
Drug DUI and the Other Competent Evidence Rule
Drug cases put even more weight on roadside evidence. Under MCA § 61-8-1018(1)(a), a person may not be convicted of an impairment DUI under § 61-8-1002(1)(a) based on the presence of a drug unless some other competent evidence exists that tends to establish the person was under the influence of a drug. A lab result alone is not enough for that charge. The State usually points to driving behavior, observations and roadside tests to fill the gap, which is why the quality of those tests can decide a drug-based case. Our THC and marijuana DUI page explains the separate per se limit for THC.
What This Means for Your Case
Field sobriety tests are one part of a DUI case, not the whole of it. I review them alongside the reason for the stop, any test results, the paperwork and the deadlines, including the 30-day window to petition to challenge a license suspension under MCA § 61-8-1017. Depending on the facts, problems with the roadside tests may affect probable cause for the arrest, the strength of an impairment charge, or the options for resolving the case. If this is your first arrest, our first-offense DUI page covers the penalties, and our main Missoula DUI defense page explains how the whole process works.
This page explains Montana law in general under the 2025 Montana Code Annotated and is not advice on a specific case.