Probable Cause and Reasonable Suspicion for a DUI Arrest

To lawfully pull someone over, police must have a reasonable suspicion that the person has violated the law. However, it doesn't take much—even a minor traffic violation is enough. (But read about the exception to the rule for DUI checkpoints.)

And for a DUI arrest to be legally justified, it must be supported by probable cause. In other words, there needs to be evidence that could support a reasonable belief that the motorist was driving under the influence.

What Is the "Reasonable Suspicion" Requirement for a DUI Stop?

When attorneys review DUI cases for possible defenses, they often look first to the legalities of the initial traffic stop. To stop a vehicle, police generally need reasonable suspicion of illegal activity. In other words, police can't pull you over without having a valid law enforcement purpose for doing so.

In most DUI cases, police stop the vehicle because of a traffic violation (like running a stop sign or speeding) or indicators of intoxication (like swerving). Either of these observations would typically be enough to justify a traffic stop.

Where police don't have a valid reason for the initial traffic stop, the driver could have a good defense to a DUI charge. An illegal traffic stop will normally prohibit the prosecution from using in court any evidence law enforcement uncovers following the stop.

What Is the "Probable Cause" Requirement for a DUI Arrest?

After analyzing the initial traffic stop, defense attorneys typically look to whether the DUI arrest was supported by probable cause. While reasonable suspicion centers on the legalities of the traffic stop, the probable cause analysis focuses on the evidence supporting the DUI charge.

Probable cause supports a DUI arrest where police have evidence supporting a reasonable belief that the driver was operating or in physical control of a vehicle while under the influence of drugs or alcohol. Every case is different, but below we discuss some common categories of evidence that can provide police with probable cause for a DUI arrest.

Bad Driving Can Give Police Probable Cause for a DUI Arrest

With traffic violations and accident investigations, the officer incidentally discovers evidence of driver impairment. However, it's also common for police to instigate a stop based on a suspicion of drunk driving. Such suspicions are typically related to unusual driving—things like swerving, driving the wrong direction on a roadway, or driving extremely slow.

DUI Probable Cause and Officer Observations of Impairment

In addition to driving patterns, an officer's observations during a traffic stop can supply the necessary probable cause for a DUI arrest. To justify an arrest, officers often cite factors like the odor of alcohol, bloodshot eyes, slurred speech, and odd behavior.

Field Sobriety Tests and Breathalizers Can Give Police Probable Cause for a DUI Arrest

Police often use field sobriety tests (FSTs) and breathalyzers during DUI investigations. Generally, drivers don't have to participate in FSTs or prearrest breath tests, but many drivers take these tests voluntarily (or don't know they have a right to refuse). FST performance and breath-test results can be important components of an officer's probable cause determination.

Illegal DUI Stops and Arrests and the "Exclusionary Rule"

The "exclusionary rule" may come into play when a stop wasn't supported by reasonable suspicion or an arrest lacked probable cause. Generally, the exclusionary rule requires the court to throw out any evidence obtained after illegal police action.

In a DUI case, excluded evidence can lead to a better plea bargain or dismissal of the charges altogether.

Talk to a DUI Attorney

Reasonable suspicion and probable cause always involve fact-specific inquiries. And the law is constantly changing. To find out if you have a viable DUI defense, it's always best to talk with a knowledgeable attorney. A qualified DUI lawyer can tell you how the law applies to the facts of your case.