What is Probable Cause for Marijuana Testing in Washington?

Officers must have probable cause to arrest you for a marijuana DUI. Before they even pull you over, they must first have reasonable suspicion that you committed a crime, are committing a crime, or are about to commit a crime. Once an officer stops you, they begin an investigation to decide whether probable cause exists for a marijuana or other DUI. If you are facing a marijuana DUI charge in Washington, you should take it seriously and speak with the Tacoma marijuana DUI lawyers at Smith & White.

Understanding Probable Cause and Marijuana DUI Investigations in Washington

Under RCW 46.61.502(1), it is illegal to drive under the influence of a drug such as marijuana. If an officer has probable cause to believe that you committed a marijuana DUI, they can arrest you.

Probable cause means facts and evidence that would lead a reasonable person to believe that a crime already happened, is happening, or is about to happen. Reasonable suspicion is a lower standard. An officer may stop a driver based on reasonable suspicion, but they need probable cause before they make an arrest.

Reasonable Suspicion vs. Probable Cause on the Road

An officer’s first observations of odd or risky driving usually create only reasonable suspicion. Those observations can justify a stop, but they do not, by themselves, create probable cause for a marijuana DUI. Common grounds for reasonable suspicion include:

Swerving or drifting within a lane
Taking turns too wide
Running stop signs or red lights
Speeding or driving far too slowly for conditions
Failing to signal lane changes or turns
Crossing the center line
Throwing items or trash out the window
Obvious equipment or registration problems, such as an expired tab

After the stop, the officer looks for more information. This second layer of facts is what may build probable cause. At this stage the officer will talk with you, watch how you move, and look around the vehicle.

Marijuana Field Sobriety Tests and Drug Recognition

If the officer believes you might be under the influence of marijuana, they may ask you to perform field sobriety tests or to meet with an officer trained in drug recognition. The goal is to decide whether drugs, and not something else, explain what the officer sees.

Standardized field sobriety tests include the walk-and-turn, the one-leg stand, and the horizontal gaze nystagmus test. When an officer thinks that you “failed” these tests, they may treat the results as signs of impairment and as part of the probable cause to arrest. In a 2016 decision, the Washington Supreme Court allowed the State to use a driver’s refusal to take field sobriety tests as evidence in court. A refusal does not prove guilt, but the prosecutor may argue that it shows a guilty conscience. Your attorney will need to answer that claim and explain other reasons why a person might refuse.

Some officers receive additional training as Drug Recognition Experts (DREs). A DRE uses a structured evaluation that includes eye checks, vital signs, balance tests, questions about medical issues, and a review of any lab results. If the officer follows the protocol and the court finds the foundation sufficient, the prosecutor can offer the DRE’s opinion as expert testimony about drug impairment. A defense lawyer can challenge how the tests were given, whether the officer skipped steps, and whether medical or innocent explanations better fit what the officer saw.

If you admit that you were smoking or consuming marijuana, that statement can also help an officer claim probable cause. They will often combine your admission with driving behavior and physical signs to justify an arrest. You are not required to answer incriminating questions. You have the right to remain silent and to ask for an attorney as soon as you want one.

Chemical Testing

If you are arrested for a marijuana DUI, the officer will usually request a chemical test. Because Washington has no standard evidentiary breath test for marijuana, officers often seek a blood test instead. A blood draw may occur with your consent, under a warrant, or under certain limited exceptions.

The laboratory report that shows your THC concentration will typically go to your attorney as part of the initial discovery. When THC levels are high and the test is valid, the State will almost always try to use the result as evidence that you were impaired. The prosecutor may also file a “per se” marijuana DUI charge based on a THC concentration at or above the legal limit.

The State still must prove that the officer had reasonable suspicion for the stop and probable cause for the arrest. If there was no lawful basis for the stop, your lawyer can ask the court to suppress everything that came afterward. If there was no probable cause to arrest you for a marijuana DUI, your attorney may move to suppress evidence obtained after the arrest, including the blood test that shows THC in your system.

Like other criminal charges, a marijuana DUI must be proven beyond a reasonable doubt. When key evidence is suppressed or weak, the prosecutor may not be able to meet that burden. In that situation, they may agree to reduce or dismiss the charge or to negotiate a more favorable plea.

Consult an Experienced DUI Lawyer in Tacoma to Understand Your Options

If you are arrested for a marijuana DUI, you should consult The Law Offices of Smith & White, PLLC, to understand your options and your next steps. We can review the stop, the officer’s observations, any field sobriety tests, and the basis for any chemical testing to see whether probable cause truly existed. Based in the Tacoma area, we represent clients in Pierce, King, Kitsap, and Thurston Counties. Call us at (253) 203-1645 or complete our online form.