Marijuana DUI cases work differently from alcohol arrests. You can’t just blow into a device and get a reading. No roadside test immediately tells an officer whether you’re impaired by cannabis right now. Washington law sets a limit of 5.0 nanograms of THC per milliliter of blood, but getting to that number requires several steps, and that number doesn’t tell the whole story. Police build these cases through observation, testing, and interpretation. Sometimes those interpretations are wrong.
What Officers Look For During Traffic Stops
It usually starts with a traffic violation or driving pattern that catches an officer’s attention. From the moment those lights go on, they’re documenting everything. They’re watching for physical signs:
- Bloodshot or red eyes
- Slow responses when you’re answering questions
- The smell of marijuana coming from your car
- Marijuana or paraphernalia in plain view
- Dilated pupils or unusual pupil reactions
- Dry mouth, repeated swallowing
These observations create probable cause for further investigation. But here’s something important to understand. A Bellevue DWI lawyer can challenge whether what the officer saw actually proves impairment, or whether it just shows you used marijuana sometime recently. Those are two very different things.
Field Sobriety Tests Don’t Work Well For Cannabis
Officers typically give you the same standardized field sobriety tests they’d use for alcohol. The problem? These tests weren’t designed to detect marijuana impairment. Take the horizontal gaze nystagmus test. It’s effective for alcohol because it measures involuntary eye movements that alcohol causes. Cannabis doesn’t create the same effect. You could pass this test completely and still have THC in your system. The walk-and-turn and one-leg stand tests measure balance and coordination. Sounds reasonable, right? Except lots of things affect your performance that have nothing to do with impairment. Anxiety makes people shaky. Medical conditions affect balance. The roadside isn’t level. The lighting’s poor. Many completely sober people fail these tests.
When Drug Recognition Experts Get Involved
If an officer suspects marijuana impairment, they often call in a Drug Recognition Expert. Eastside DUI attorneys see these evaluations regularly in DUID cases. A DRE conducts a 12-step evaluation:
- Breath test to rule out alcohol
- Eye examinations under varying light conditions
- Divided attention tests
- Vital signs like pulse, blood pressure, and temperature
- Muscle tone checks
- Looking for injection sites
- Interviewing you about drug use
This takes 30 to 45 minutes. At the end, the DRE forms an opinion about what category of drugs you’ve used based on what they observed, but it’s subjective. Training varies between officers. Experience levels differ. And many signs they attribute to marijuana use can be explained by other factors. Nervousness. Fatigue. Medical conditions. The fact that you’ve been standing on the side of the road for an hour.
Blood Testing And The THC Problem
Washington measures THC through blood tests, and this is where things get complicated. Alcohol metabolizes predictably. Your blood alcohol level drops at a fairly steady rate. THC doesn’t work that way. According to the National Highway Traffic Safety Administration, THC concentrations in blood don’t correlate reliably with how impaired someone actually is.
If you use cannabis regularly, you can have detectable THC in your blood days after the last time you used it. The 5.0 nanogram threshold doesn’t account for tolerance. It doesn’t consider that different people metabolize THC at different rates. Blood draws have to follow specific procedures. Contamination happens. Storage protocols get violated. Testing gets delayed. Chain of custody breaks down. A Bellevue DWI lawyer examines every single step of the testing process, looking for problems. Sometimes we find them.
Building Your Defense
DUID cases rely heavily on what officers say they saw and how they interpreted it. That’s subjective evidence, and it can be challenged. Dashcam and body camera footage sometimes contradict what ended up in the police report. Medical records can explain physical symptoms that officers attributed to drug use. Toxicology experts can testify about how THC actually works in the human body and why blood concentration doesn’t equal impairment.
The state has to prove you were impaired while you were driving. Not that you used marijuana yesterday. Not that you have THC in your system. They need to prove actual impairment at the specific time you were operating the vehicle. Timing matters tremendously in these cases. If you’re facing marijuana DUI charges in Washington, understanding how prosecutors construct these cases gives you a better foundation for your defense. The evidence they’ve gathered might not be nearly as solid as it looks on paper.
