Before an officer can pull you over on suspicion of impaired driving, they need reasonable suspicion for a DUI stop: specific, observable facts pointing to possible impairment, not a hunch or a guess. That standard comes from the Fourth Amendment, and it governs the entire encounter. If the officer cannot articulate what they actually saw before the lights went on, the evidence collected afterward can be thrown out of court.
What the Standard Actually Requires
The Supreme Court set the rule in Terry v. Ohio. An officer “must be able to point to specific and articulable facts which, taken together with rational inferences from those facts, reasonably warrant that intrusion.”1Justia US Supreme Court Center. Terry v Ohio 392 US 1 (1968) Driving late at night, leaving a bar parking lot, or fitting some vague profile does not meet that bar. The officer needs something concrete.
Reasonable suspicion sits below probable cause. Probable cause is what officers need to arrest you or get a warrant. Reasonable suspicion is lower, but it still requires more than a feeling. A traffic stop counts as a Fourth Amendment “seizure,” so these protections attach the moment the emergency lights come on.2Legal Information Institute. Traffic Stop
Courts judge whether the standard was met using the totality of the circumstances: everything the officer knew or observed at the moment of the stop. One factor in isolation might not be enough. Several together often are. If you later challenge the stop, a judge works through that record piece by piece, which is why departments train officers to document what they saw in real time.
The Driving Cues Officers Watch For
The National Highway Traffic Safety Administration publishes a guide identifying visual cues that predict impaired driving, and officers across the country train on it. NHTSA groups the cues into four categories: lane position problems, speed and braking issues, vigilance failures, and judgment errors.3National Highway Traffic Safety Administration. The Visual Detection of DWI Motorists
Lane position problems are what most people picture: weaving within a lane, drifting across lane lines, straddling the center line, nearly striking another vehicle or a fixed object, or taking a wide turn that swings into the far lane. These suggest the driver is struggling with spatial awareness.
Speed and braking cues are less obvious but carry weight. Driving ten or more miles per hour below the speed limit, accelerating and decelerating for no apparent reason, and stopping too far from or too short of an intersection all raise flags. Jerky stops in particular suggest delayed reaction times.
Vigilance problems include driving the wrong way, responding slowly to traffic signals, failing to turn on headlights at night, and stopping in a travel lane for no reason. Judgment problems cover following too closely, making unsafe lane changes, and behaving unusually after being signaled by an officer.3National Highway Traffic Safety Administration. The Visual Detection of DWI Motorists
When an officer spots several of these cues together, the suspicion argument becomes strong. One weave across a lane line at 2 a.m. is borderline. That weave combined with a jerky stop at a red light and a failure to signal gives the officer plenty to point to in a report.
Minor Traffic Violations as the Real Trigger
Many DUI arrests do not start with impaired-driving cues at all. A broken taillight, an expired registration, a failure to signal: any of these gives an officer lawful grounds to pull you over. Once they approach the window and notice an odor of alcohol, bloodshot eyes, or slurred speech, the stop shifts from routine traffic enforcement into a DUI investigation.
The law is firmly on the officer’s side here. In Whren v. United States, the Supreme Court held that “the temporary detention of a motorist upon probable cause to believe that he has violated the traffic laws does not violate the Fourth Amendment’s prohibition against unreasonable seizures, even if a reasonable officer would not have stopped the motorist absent some additional law enforcement objective.”4Justia US Supreme Court Center. Whren v United States 517 US 806 (1996) If you actually committed a traffic violation, it does not matter that the officer’s real motive was to check whether you had been drinking. The stop is legal.
The practical effect is that an officer who suspects impairment but has not seen enough impaired-driving cues can follow a vehicle and wait for any minor infraction. Touching a lane line, a burned-out license plate light, rolling through a stop: all qualify. Defense attorneys sometimes call these fishing expeditions, but the Court has made the officer’s subjective intent irrelevant so long as the objective violation exists.
Stops Based on a 911 Call
Officers do not always have to witness the suspicious driving themselves. In Navarette v. California, the Supreme Court held that an anonymous 911 call reporting a specific dangerous driving incident can supply reasonable suspicion for a stop, even if the responding officers never personally observe erratic driving.5Justia US Supreme Court Center. Navarette v California 572 US 393 (2014)
The caller in that case reported being run off the road by a specific truck and gave the make, model, color, license plate, and direction of travel. The Court pointed to three things that made the tip reliable: the caller claimed eyewitness knowledge of dangerous driving, the report was made shortly after the incident, and the 911 system itself made the caller traceable, which discourages fabrication.
Not every tip clears that bar. A vague report that someone “looks drunk” without describing a specific driving behavior usually falls short. The caller has to describe something they actually saw: a near-collision, a run red light, a swerve into oncoming traffic. And the responding officers have to find the vehicle quickly enough that the tip stays fresh. A report from hours earlier carries far less weight than one made in the moment.
Checkpoints Are the Exception
Sobriety checkpoints are the one major carve-out from the requirement that an officer have individualized suspicion before stopping you. In Michigan Dept. of State Police v. Sitz, the Supreme Court upheld them, finding the state’s interest in preventing drunk driving outweighed the brief intrusion on motorists stopped at a checkpoint.6Legal Information Institute. Michigan Dept of State Police v Sitz 496 US 444 (1990) At a lawful checkpoint, the officer does not need to have seen you do anything wrong.
Not every state allows them. Thirteen states do not run sobriety checkpoints. In ten of those (Idaho, Michigan, Minnesota, Montana, Oregon, Rhode Island, Texas, Washington, Wisconsin, and Wyoming), checkpoints are prohibited by state law, state constitution, or judicial interpretation. Missouri authorizes them by statute but prohibits funding checkpoint operations through its state budget.7National Highway Traffic Safety Administration. Publicized Sobriety Checkpoints
Where checkpoints are allowed, the selection of which vehicles to stop must follow a neutral, predetermined formula set by supervisors rather than left to the discretion of officers on the ground. The checkpoint in Sitz stopped every approaching vehicle; others use sequential formulas like every third or fifth car.6Legal Information Institute. Michigan Dept of State Police v Sitz 496 US 444 (1990)
Challenging a Stop That Lacked Reasonable Suspicion
If the officer did not have reasonable suspicion when they pulled you over, a defense attorney can file a motion to suppress. That is a pretrial request asking the judge to exclude evidence collected during the stop. At the hearing, the officer testifies about what they observed before activating the lights, and the judge decides whether those observations actually added up to reasonable suspicion. If the ruling goes your way, breathalyzer results, field sobriety test performance, and any statements you made can all be kept out of trial.
The underlying doctrine is the exclusionary rule, extended through what courts call “fruit of the poisonous tree.” If the initial stop was unconstitutional, everything flowing from it is tainted and cannot be used against you.8Legal Information Institute. Fruit of the Poisonous Tree Losing the breath test alone is often enough to gut a prosecution, which can mean reduced charges or outright dismissal.
There are limits. Evidence from an independent source, evidence that would have been inevitably discovered anyway, and evidence tied to a voluntary statement by the defendant can survive even a bad stop. Courts also recognize a good-faith exception when officers reasonably relied on information that later turned out to be flawed. A successful suppression motion needs more than a questionable stop; the defense has to show the evidence is genuinely tied to the constitutional violation with no independent path to its discovery.
This is where most DUI defense strategies begin. An attorney will pull the dashcam footage, the officer’s written report, and any dispatch audio to see whether the stated reason for the stop holds up against what the video actually shows. Officers sometimes describe cues in boilerplate language copied from training materials rather than specific observations from the encounter in front of them, and that gap can be enough to break the suspicion finding.