If a police officer says you are suspected of DUI, it means they have seen enough to pull you over and investigate further, but not yet enough to arrest you. The legal threshold for that initial stop is called reasonable suspicion, and it sits well below the probable cause needed for an arrest. Everything that happens between those two points, including the questions you answer, the tests you agree to, and the observations the officer records, determines whether suspicion hardens into an arrest and a prosecutable case.
Why an Officer Suspects You in the First Place
Before the stop, officers are trained to watch for specific driving patterns. Problems maintaining lane position, including weaving, straddling a lane line, swerving, and drifting, are among the strongest indicators. So are speed and braking problems: stopping too abruptly or too far from an intersection, varying speed for no reason, and driving ten or more miles per hour below the limit.1National Highway Traffic Safety Administration. Visual Detection of DWI Motorists
Once the officer approaches your window, a second set of observations begins. They are looking for the smell of alcohol, slurred speech, bloodshot or watery eyes, fumbling with a license or registration, and difficulty following simple instructions. At a blood alcohol concentration around 0.10%, most people show noticeably slurred speech, poor coordination, and slowed reaction time.2National Highway Traffic Safety Administration. Drunk Driving Officers also watch for subtler signs at lower levels, and drug impairment produces its own distinct cues that may not overlap with alcohol.
Officers do not always witness the driving themselves. A 911 tip can establish reasonable suspicion when it carries enough markers of reliability, such as an eyewitness basis for the report, a short gap between the behavior and the call, and the accountability built into the 911 system. The Supreme Court in Navarette v. California held that reporting another car being run off the road strongly suggests impairment, and officers did not need to personally observe additional suspicious behavior before pulling the driver over.3Justia US Supreme Court. Navarette v. California, 572 U.S. 393 (2014)
Sobriety checkpoints are the one setting where the officer does not need individualized suspicion to make initial contact, so long as a neutral, predetermined method selects which vehicles are stopped. Once the officer begins talking to you, the usual rules return: they need reasonable suspicion of impairment to request field sobriety tests and probable cause to arrest.
Reasonable Suspicion and Probable Cause
Reasonable suspicion is the threshold the Supreme Court set in Terry v. Ohio: the officer must point to specific, articulable facts that would lead a reasonable person to believe criminal activity may be occurring.4Constitution Annotated. Amdt4.6.5.1 Terry Stop and Frisks Doctrine and Practice It demands more than a gut feeling but less than the certainty needed for an arrest.5Legal Information Institute. Reasonable Suspicion
An officer who watches you drift across the center line twice and then brake erratically has reasonable suspicion. An officer who simply does not like the look of your car at 1 a.m. does not. The distinction matters because if a court later finds the stop lacked reasonable suspicion, any evidence gathered during it can be thrown out under the exclusionary rule, a Fourth Amendment principle reinforced in Mapp v. Ohio and designed to deter unconstitutional searches by removing any benefit from them.6Constitution Annotated. Amdt4.7.2 Adoption of Exclusionary Rule
Probable cause is the higher standard the officer must reach before arresting you. It requires enough evidence that a reasonable person would believe a crime has been committed.7Legal Information Institute. Probable Cause In a DUI stop, probable cause usually builds from a combination of factors: the driving that prompted the stop, what the officer observes at the window, performance on field sobriety tests, and any preliminary breath result. The smell of alcohol alone gives the officer reasonable suspicion to investigate further but probably not probable cause to arrest. Add slurred speech, a poor walk-and-turn, and a roadside breath reading above 0.08%, and the calculus changes.
What the Officer Will Ask You to Do
A DUI investigation moves through phases. Each one serves a purpose for the officer, and understanding that purpose helps you see where you have choices.
Initial Questioning
After pulling you over, the officer asks for your license, registration, and proof of insurance, and makes conversation. This is not small talk. How quickly you locate your documents, whether your speech is clear, and whether any odor is present all feed the investigation. The officer may ask where you are coming from or whether you have had anything to drink. These questions are designed to produce evidence, and your answers are admissible in court even without Miranda warnings at this stage.
Field Sobriety Tests
If initial observations suggest impairment, the officer will usually ask you to perform standardized field sobriety tests. NHTSA validates three:
- Horizontal Gaze Nystagmus, in which the officer moves a stimulus across your field of vision and watches for involuntary jerking of the eyes.
- Walk-and-Turn, which asks you to take nine heel-to-toe steps along a line, turn, and walk back while following specific instructions.
- One-Leg Stand, which asks you to stand on one foot for roughly 30 seconds while counting aloud.
The walk-and-turn and one-leg stand are called divided-attention tests because they force you to handle mental and physical tasks at the same time, something that becomes harder with impairment.8National Highway Traffic Safety Administration. SFST Refresher Participant Manual NHTSA research found that when officers use the standardized battery together, they correctly identified impairment roughly 91 to 93 percent of the time in field conditions.9National Highway Traffic Safety Administration. Horizontal Gaze Nystagmus – The Science and The Law
Preliminary Breath Test
The officer may also ask you to blow into a portable breath-testing device, sometimes called a Preliminary Alcohol Screening device. In most jurisdictions the roadside result is not accurate enough to be admitted as direct evidence in court. Its purpose is to help the officer decide whether probable cause exists to arrest. The evidentiary breath test conducted on a stationary machine after arrest is the one that produces court-admissible results.
Your Rights While You Are Suspected
This is where most people’s understanding gets fuzzy, and where the stakes are highest. You have more rights at some points in the process and fewer at others.
Miranda Does Not Apply at the Roadside
Officers are not required to read you Miranda warnings during a routine DUI investigation at the roadside. In Berkemer v. McCarty, the Supreme Court held that roadside questioning during a traffic stop is not custodial interrogation and does not trigger Miranda protections. A traffic stop limits your freedom, but the Court concluded the pressures involved do not rise to the level of a formal arrest.10Justia US Supreme Court. Berkemer v. McCarty, 468 U.S. 420 (1984)
So every question at the roadside, including “have you been drinking tonight?” and “how many drinks have you had?”, is fair game without warnings, and your answers can be used against you. Miranda rights apply only once you are formally under arrest and the officer keeps asking questions designed to produce incriminating responses. Statements obtained in that posture without warnings can be challenged.
You Can Usually Decline Field Sobriety and Preliminary Breath Tests
Field sobriety tests are voluntary in most of the country. You can decline without triggering the automatic penalties that come with refusing a chemical test after arrest. Declining gives the officer less direct evidence, but it does not prevent an arrest; the officer can still rely on driving behavior, physical observations, and your demeanor to establish probable cause. Portable preliminary breath tests sit in a similar gray area in many jurisdictions, where refusal does not carry the implied-consent penalties that attach to a post-arrest evidentiary test, though the rules vary by state.
Implied Consent Kicks In After Arrest
Once you are placed under arrest, the picture changes sharply. Every state has an implied consent law, meaning that by driving on public roads you have already agreed to submit to a chemical test (breath, blood, or urine) if lawfully arrested for DUI. Refusing that post-arrest test triggers automatic penalties, most commonly a license suspension ranging from six months to a year or more for a first refusal, depending on the state. Repeat refusals carry longer suspensions, and in some states the refusal itself can be used against you in court as evidence of consciousness of guilt.
The Supreme Court drew a constitutional line in Birchfield v. North Dakota. States can require warrantless breath tests as a search incident to a lawful DUI arrest and can impose criminal penalties for refusing a breath test. Blood tests are more invasive, and states cannot criminally punish a driver for refusing a warrantless blood draw. Civil penalties like license suspension remain permissible for refusing either type of test.11Justia US Supreme Court. Birchfield v. North Dakota, 579 U.S. ___ (2016)
How Long an Officer Can Keep You There
There is no fixed minute count for how long a DUI investigation can last. Under Fourth Amendment principles, the detention can last only as long as reasonably necessary to accomplish the purpose of the stop. If the officer’s initial traffic concern is resolved and nothing suggests impairment, the officer has to let you go. If observations during the stop raise DUI concerns, the officer can extend the detention to investigate those concerns, because the impairment indicators themselves supply independent reasonable suspicion.
Where officers get into trouble is extending a stop beyond its purpose without developing new grounds. The Supreme Court emphasized in Rodriguez v. United States that unrelated investigative activities cannot prolong a traffic stop without separate reasonable suspicion. Defense attorneys often challenge DUI stops on this basis, arguing the officer kept the driver waiting without articulating why.
When Suspicion Turns Into an Arrest
If the officer concludes that probable cause exists, you will be placed under arrest, handcuffed, and transported to a station or jail for booking. There you will be given an evidentiary chemical test, typically breath or blood. This is the test covered by implied consent, and refusing it triggers an automatic license suspension regardless of whether you are ultimately convicted of DUI.
Most states run two separate tracks after a DUI arrest: a criminal case and an administrative license action. The administrative suspension often takes effect quickly, sometimes within days of the arrest, and is handled by the state’s motor vehicle agency rather than a court. You typically have a short window to request an administrative hearing to contest the suspension, and missing that deadline usually means the suspension takes effect automatically.
The criminal case proceeds on its own timeline. You will be arraigned, formal charges will be read, and you will enter a plea. Depending on the jurisdiction, arraignment may happen within a day of arrest or within several days. From that point forward, your attorney can begin challenging the evidence, from the legality of the initial stop through the administration of field sobriety tests and the handling of chemical test samples. Reasonable suspicion, probable cause, the exclusionary rule, Miranda, and the limits on implied consent are all tools that come into play during that process.