A Montana DUI stop can be challenged when the officer lacked particularized suspicion, meaning specific, objective facts pointing to an offense, at the moment the stop was made. If a court agrees, the evidence gathered because of the stop can be suppressed. This page explains the standard, the Montana Supreme Court decisions that define it, how the challenge is made, and where it runs out. It is the deeper treatment of the first item on our Montana DUI defenses page, and part of our Montana DUI Guide.
The Standard: Particularized Suspicion
The statute says a peace officer may stop any person or vehicle "observed in circumstances that create a particularized suspicion that the person or occupant of the vehicle has committed, is committing, or is about to commit an offense" (MCA § 46-5-401(1)).
In Brown v. State, 2009 MT 64, 349 Mont. 408, the Montana Supreme Court stated the test in two parts. The officer must have (1) objective data and articulable facts from which reasonable inferences can be made, and (2) a resulting suspicion that the person has committed, is committing, or is about to commit an offense (¶ 20). The Court also held that an officer's training and experience may bear on which inferences are reasonable but are not the defining element. Whether particularized suspicion existed is decided on the totality of the circumstances.
Two things follow. A hunch is not enough, because a hunch is not objective data. And the question is what the officer knew before the stop, not what the investigation turned up afterward.
What Montana Courts Have Decided
| Case | Facts | Holding |
|---|---|---|
| State v. Lafferty, 1998 MT 247 | Anonymous report of a drunk driver with no details. The officer saw the pickup cross the fog line twice and drive on it once. | No particularized suspicion. Crossing the fog line was not a lane violation, and the officer did not testify that it indicated impairment. Reversed. |
| State v. Pratt, 286 Mont. 156 (1997) | A Missoula convenience store night manager, identified by name, reported an intoxicated driver he had watched at the store, with the vehicle, plate, and direction. The officer found the vehicle but saw no bad driving. | Particularized suspicion existed. The tip was reliable under three factors: an identified informant, personal observation, and corroboration. |
| Brown v. State, 2009 MT 64 | A pickup barely moving at 2:51 a.m. suddenly pulled over and shut off its lights. | The officer had objective data to suspect DUI. The test does not depend on how many years the officer has served. |
| Hulse v. State, 1998 MT 108 | A license suspension case about what an officer may do after a stop. | Field sobriety tests are a search and need their own particularized suspicion of impairment. |
Read together, these cases show how fact-bound the question is. In Lafferty the Court pointed out that the officer never testified that his training or experience led him to infer impairment from the fog line crossings, and that the driver had been watching a patrol car come up fast behind her. In Brown the Court gave its own example of a stop that would be justified even for a rookie: a vehicle driving slowly at 2:00 a.m. without lights and weaving across the center and fog lines.
Stops Based on a Caller's Report
Many DUI stops begin with a phone call. Under State v. Pratt, a court weighs three things: whether the informant identified themselves, whether the report rested on the informant's personal observation, and whether the officer's own observations corroborated it. In Pratt, a named night manager who had watched the driver at his store was enough, even though the officer saw no bad driving before the stop.
Lafferty shows the other side. The caller was anonymous and gave no basis for the opinion that the driver was drunk, so the officer had to personally observe driving that was illegal or indicated impairment (¶ 12). Finding the described truck heading the reported direction was not enough. In a tip case, the dispatch recording and call log are as important as the patrol video. See stops, roadblocks, and open containers for the other ways a stop can begin.
After the Stop: A Second Question
A lawful stop does not automatically justify a DUI investigation. In Hulse v. State, 1998 MT 108, 289 Mont. 1, the Court held that field sobriety tests are a search under the Fourth Amendment and the Montana Constitution and must themselves be based on particularized suspicion (¶¶ 33, 38). Erratic driving can supply that suspicion for both the stop and the tests. But the Court gave this example: an officer who stops a car only for a broken taillight and sees no signs of intoxication on contact may not go on to administer field sobriety tests (¶ 40).
A stop also may not last longer than necessary to carry out its purpose (MCA § 46-5-403). So the review asks three questions in order: was the stop justified, was there a separate basis to investigate for DUI, and did the stop stay within its purpose until there was. See challenging field sobriety tests.
How the Challenge Is Made
- Get the record. The officer's report, patrol and body camera video, dispatch audio and logs, and any caller information, through discovery (MCA § 46-15-322).
- Compare. What the report says the officer saw before turning on the lights, against what the video shows.
- File a written motion to suppress. It must state facts that, if true, would show the evidence should be suppressed, and the court then hears it at the omnibus hearing or a later date (MCA § 46-13-302(2)).
- File it on time. Defenses and objections that can be decided before trial must be raised at or before the omnibus hearing, or they are waived unless the court grants relief (MCA § 46-13-101).
- Hold the hearing. The officer testifies and is cross-examined, and the judge decides.
If the motion is granted, the evidence is not admissible at trial (MCA § 46-13-302(3)). If it is denied, a defendant may, with the court's approval and the prosecutor's consent, plead guilty while reserving the right to appeal that ruling (MCA § 46-12-204(3)). That is what the defendant in Lafferty did.
The Same Issue in the License Case
If you refused a test, the district court hearing on the suspension asks whether the officer had a basis for requesting the test under the implied consent law and whether you refused (MCA § 61-8-1017(4)). Both Hulse and Brown were license cases of this kind. The petition is due within 30 days, well before most suppression hearings in the criminal case, so it is often the first place the officer's reasons are tested under oath. See test refusal and implied consent.
Honest Limits
Many stops hold up. An observed violation such as speeding, a failure to signal, or a burned-out light ordinarily gives an officer the objective data the test requires. The stops worth litigating tend to share a feature: the stated reason is thin, or the video does not show what the report describes. You cannot tell from the citation which kind you have. Our page on what a DUI lawyer does explains how the records are obtained.
This page explains Montana law in general as of the 2025 Montana Code Annotated and the decisions cited. Every case turns on its own facts, and past results do not predict future outcomes. It is not legal advice about your specific case.