What Counts as Probable Cause for DUI Stops in Phoenix?

If you were arrested for DUI, one of the first questions you might have is whether the officer actually had the legal right to pull you over, test you, and arrest you.

Unlike a conviction for DUI, which requires proof beyond a reasonable doubt, police only need ‘probable cause’ to arrest you. In simple terms, probable cause means the officer had enough facts, based on what they observed, to reasonably believe you were driving while impaired.

Probable cause in a DUI arrest is rarely based on one single behavior or sign. The decision usually happens in stages.

The Officer Must Have a Reason to Stop You

Before anything else, the officer must have a legal reason to pull you over. That can include:

• Swerving or drifting
• Having headlights off at night
• Speeding
• Running a red light or stop sign
• Equipment issues like a broken taillight

Even a minor traffic violation can justify a stop. But a lawful stop is not automatically probable cause for a DUI arrest.

What Happens After the Stop to Establish Probable Cause for a DUI Arrest

Once the vehicle is stopped, the officer begins investigating. Officers look for signs such as:

• Odor of alcohol
• Bloodshot or watery eyes
• Slurred speech
• Difficulty finding documents
• Statements about drinking
• Balance or coordination issues

This is also where officers may ask a driver to perform field sobriety tests or submit to a preliminary breath test.

At this stage, the officer is deciding whether the situation rises to the level of probable cause for arrest.

Is Driving Behavior Alone Enough for an Arrest?

In most cases, no.

Weaving within a lane, failing to signal, making a wide turn, driving well under the speed limit, or rolling through a stop sign may justify the initial stop. But for a DUI arrest, officers typically rely on additional signs of impairment observed during the investigation, such as physical indicators, admissions (like answering, “I had one glass of wine with dinner” when asked about drinking), test results, or coordination issues.

Probable cause is supposed to be based on the totality of the circumstances, not just one small driving error. A single minor mistake on the road does not automatically justify a lawful DUI arrest.

Do You Have to Show Signs of Impairment Before a Breath Test?

This depends on the stage of the investigation.

During a roadside stop, an officer may request a preliminary breath test while still deciding whether to make an arrest.

However, before making a formal DUI arrest and requiring an evidentiary breath or blood test under Arizona’s implied consent law, the officer must have probable cause to believe you were driving under the influence.

In other words, there must be enough observable signs of impairment to justify the arrest itself.

Can You Refuse a Breath Test?

It is common practice for officers to request some form of breath test before arrest, but that is usually a preliminary breath test (PBT), not the formal evidentiary test.

The formal evidentiary breath or blood test under Arizona’s implied consent law happens after arrest.

A roadside breath test requested before arrest does not carry the same automatic license consequences. Refusing an evidentiary breath or blood test after arrest can result in a one-year license suspension under Arizona law, even if you are ultimately not convicted of DUI.

What If There Was No Probable Cause?

Maybe the most common complaint of drivers after a stop is, “I didn’t do anything to justify pulling me over.” Challenging probable cause can be an effective defense strategy if there is evidence that supports your argument that there wasn’t a good reason to stop you in the first place or that probable cause was lacking during the officer’s initial investigation.

When an arrest is challenged, a judge may review:

• The officer’s report
• Body-worn camera or dashcam footage
• The timing of events
• What was observed before the arrest decision

If the court finds that probable cause was not present, it can affect what evidence may be used in the case. That does not automatically mean every arrest without perfect facts is illegal, or that having some evidence thrown out will result in dropped charges or acquittal.

Why This Matters After an Arrest

If you were arrested for DUI in Phoenix and have concerns about whether probable cause existed, or don’t believe there was cause for the initial traffic stop, contact the Law Office of Michael Alarid III.

Michael Alarid III is board certified in criminal law by the State Bar of Arizona and board certified in DUI defense by the National College for DUI Defense (NCDD). He has a deep and proven understanding of law enforcement procedures, traffic stops, probable cause requirements, and DUI testing methods. Discuss your situation and next steps during a free case evaluation by calling (602) 818-3110.