Criminal Law

What Is Probable Cause or Reasonable Suspicion for a DUI (or DWI) Arrest?

By Richard Stim, Attorney

Reasonable suspicion and probable cause are what the police must have to stop you and arrest you if they suspect you are driving under the influence. Because the U.S. Constitution guarantees everyone freedom of movement, a police officer cannot arbitrarily stop your car (with certain exceptions, like temporary and random sobriety checkpoints). In order to pull you over, a police officer must have "reasonable suspicion" that you’ve violated the law. And to arrest you, the officer needs even more: "probable cause."

What Is Reasonable Suspicion?

Reasonable suspicion requires that, before detaining someone, an officer have an objectively reasonable basis for suspecting criminal activity. In other words, before pulling a driver over, an officer needs to reasonably suspect a violation of law. This violation of law can be just about anything—an officer can enforce a traffic stop for speeding, operating with faulty equipment, erratic driving, or an array of other factors.

Forming Probable Cause

Probable cause is the standard by which judges evaluate arrests. There is probable cause for an arrest if the facts support an objective belief that the suspect has committed a crime. Officers look for such facts after developing a suspicion that a motorist has been driving under the influence.

Your driving may be the first factor that an officer evaluates in deciding whether there’s probable cause for a DUI arrest. But that’s typically only the beginning of the investigation. (Of course, an officer doesn’t need to actually see suspicious driving to form a reasonable belief that a driver is drunk— evidence of an accident or an injury may lead to that conclusion.) And erratic driving isn't always the basis for a drunk driving stop. It could be running a red light, for instance, and only after pulling you over does the officer starts to suspect you were driving under the influence.

Regardless of the reason for the detention, after pulling you over, the officer will look, smell, and listen. The officer will likely consider issues like whether:

  • you act suspiciously while sitting in your car
  • you have trouble getting your registration or license, and
  • there is any evidence of alcohol or drug use—typically smells from your breath or, in the case of marijuana, the smoke.

Every move you make will be carefully observed for possible evidence of impairment and noted for later use against you. These observations will definitely pop up in the police report.

So, what kinds of facts are enough for a finding of probable cause? An officer might have probable cause for a DUI arrest after, for instance, seeing you drive the wrong way on the road in the early morning hours, smelling alcohol on you, observing your glazed and bloodshot eyes, and hearing you admit that you drank more than you should have.

If an officer has probable cause to arrest you for a DUI, the “implied consent” laws of all states require you to submit to a chemical test (usually a choice of blood or breath) for the purpose of determining the amount of alcohol in your body. (See Refusing a DUI Chemical Test to learn about the experiences of drivers who refused chemical testing.)

Arguing Lack of Reasonable Suspicion or Probable Cause

Arguing that a police officer lacked reasonable suspicion or probable cause is often a challenge. But sometimes officers make mistakes, and when they do, the result can be dismissal of charges or a finding of not guilty.

Suppose, for instance, that a driver’s lawyer can establish that the arresting officer didn’t have a valid basis for pulling over the driver. Without reasonable suspicion for the detention, everything that follows that detention is typically inadmissible in court. That means that, even if the officer gathered evidence after the stop that the driver had, for instance, a blood alcohol level over the legal limit, that evidence will be inadmissible if the stop was baseless.

Your Chances of Winning a Probable Cause Argument

We conducted a survey of people charged with driving under the influence, and we focused in particular on the “lack of probable cause” defense. Of those respondents who argued that the police didn’t have probable cause:

  • 42% were convicted of DUI
  • 33% pleaded guilty to DUI
  • 8% pleaded guilty to a lesser charge
  • 8% were found not guilty, and
  • none had the charges dismissed.

From these results it appears that arguing a lack of probable cause doesn't often get one out of a DUI. But while interesting, these survey results are far from the end-all indicator of what will happen if you argue that the police didn’t have a lawful basis to investigate or arrest you. For a much better gauge of your chances fighting a DUI prosecution, consult an experienced lawyer. (For comparisons of the outcomes and costs for drivers that had attorneys versus those that didn’t, see Does a DUI Lawyer Give You a Better Outcome?)

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