A Montana officer cannot stop a vehicle on a hunch. The officer needs particularized suspicion: specific facts that point to an offense by someone in that vehicle. A traffic or equipment violation the officer saw almost always meets the standard. A vague feeling that a driver "looked wrong" does not. This page explains the standard and the reasons officers most often give. It is part of our Montana DUI Guide.
The Standard: Particularized Suspicion
The rule is in the investigative stop statute. A peace officer may stop a 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)).
The Montana Supreme Court restated the test in Brown v. State, 2009 MT 64, 349 Mont. 408. The officer must have:
- objective data and articulable facts from which the officer can make certain reasonable inferences; and
- a resulting suspicion that the person has committed, is committing, or is about to commit an offense.
Whether those two things existed is a question of fact, decided on the totality of the circumstances. It is a lower bar than the probable cause needed for an arrest, but it has to rest on something the officer can point to.
Reasons Officers Commonly Give
Most DUI stops begin with an ordinary traffic or equipment violation. These are some of the reasons that appear most often in reports, with the statute behind each.
| Stated reason | Statute | What the statute says |
|---|---|---|
| Speeding, or driving too fast for conditions | MCA § 61-8-303 | The posted limits are maximums. The statute also requires a reduced, reasonable and prudent speed for traffic, visibility, weather and road conditions. |
| No signal before a turn or lane change | MCA § 61-8-336 | A signal must be given continuously for the last 100 feet before turning in a business, residence or urban district, and the last 300 feet elsewhere. |
| Leaving a marked lane | MCA § 61-8-328 | A vehicle must stay as nearly as practicable within a single lane. Touching the fog line is a separate question, discussed below. |
| Headlights off after dark | MCA § 61-9-201 | Lamps are required from one-half hour after sunset to one-half hour before sunrise, and whenever people and vehicles are not clearly visible at 500 feet. |
| A taillight or plate light out | MCA § 61-9-204 | Most vehicles need two working taillamps, and a white light that makes the rear plate legible from 50 feet. |
| A missing or blocked license plate | MCA § 61-3-301 | Most motor vehicles must display a front and a rear plate, unobstructed and securely fastened. |
The violation does not have to be serious. In State v. Allen, 1998 MT 293, the stop was for a missing front license plate. In Hulse v. State, 1998 MT 108, it was headlights off after dark. Both turned into DUI arrests, and in both the Court treated the original stop as lawful.
Driving That Is Legal but Suspicious
An officer does not have to see a ticketable violation. In Brown, a deputy watched a pickup that was "barely moving" on a public road at 2:51 a.m. suddenly pull over and shut off its lights. Nothing about that was illegal. The Court held that the facts, taken together, could reasonably suggest impaired driving, and it quoted an earlier case: the question is not whether any one driving aberration was itself illegal, but whether the officer could point to specific and articulable facts that reasonably warranted the intrusion.
Hulse gives the classic examples of driving that supports a DUI stop without more: driving all over the road, crossing the center line and the fog line, weaving in and out of traffic, or braking for green lights.
When the Stated Reason Is Not Enough
The standard has teeth. Two Montana Supreme Court decisions ordered DUI cases dismissed because the stop did not meet it.
- State v. Lafferty, 1998 MT 247. A trooper acting on an anonymous report followed a pickup for about a mile on the interstate in the afternoon and saw it cross the fog line twice and drive on it once. The Court held that this was not a violation of the lane statute, which concerns moving from one marked lane to another, and that the trooper had not testified that such driving indicated impairment. As the Court put it, not every kind of driving that is out of the ordinary is necessarily a sign of driving under the influence.
- State v. Lee, 282 Mont. 391 (1997). An anonymous caller said she believed a named driver was drunk and speeding. The officer found the car traveling under the speed limit and saw nothing but the driver slowing down as the patrol car closed in behind him. The Court held there was no objective data to justify the stop.
Both cases involved a tip from the public. Tips follow their own three-part test, covered in anonymous tips and reported drivers.
Stops That Do Not Need Suspicion of a Crime
Two kinds of police contact follow different rules. An officer who reasonably believes someone needs help may check on them under the community caretaker doctrine, explained in welfare checks on parked cars. And a law enforcement agency may set up a temporary roadblock for the purposes listed in MCA § 46-5-502, which our page on roadblocks and open containers covers.
What the Officer May Do After a Lawful Stop
Once you are lawfully stopped, the officer may request your name and present address and an explanation of your actions, and may demand your driver's license, registration, and proof of insurance (MCA § 46-5-401(2)(a)). For a traffic stop, the officer must tell you the reason as promptly as possible, unless there is an emergency or a safety concern.
The stop is supposed to stay tied to its reason. It may not last longer than necessary to carry out its purpose (MCA § 46-5-403). Moving from a taillight to a DUI investigation takes new facts, such as the odor of alcohol. See how long a traffic stop can last and whether you have to answer questions.
How a Stop Is Challenged
The reason for the stop is tested with a motion to suppress. The court hears from the officer, looks at any video, and decides whether the facts the officer actually had add up to particularized suspicion. If they do not, the evidence obtained from the stop is not admissible at trial (MCA § 46-13-302).
These motions turn on details: how far the car drifted, how long the officer followed, what the camera shows compared with what the report says. That is why the video matters so much, and why we ask for it first. Our page on challenging the traffic stop walks through how that review works. Most stops are upheld. Some are not, and you cannot tell which kind you have from the citation alone.
This page explains Montana law in general as of the 2025 Montana Code Annotated and the court decisions cited. It is not legal advice about your specific case.