The per se limit — 5 ng/mL active THC
Washington RCW 46.61.502 sets a per se blood limit of 5 nanograms of active (delta-9) THC per milliliter of blood. "Per se" means at or above that number, you’re legally impaired regardless of how you feel or perform on field tests. The number was set when Initiative 502 legalized recreational cannabis in 2012 and has stayed in place since.
The limit measures active THC only, not THC metabolites. Inactive metabolites (THC-COOH) can stay in your system for days or weeks after use; active delta-9 THC declines within hours of consumption. Roadside testing targets the active number.
How enforcement actually works
A traffic stop doesn’t start with a blood draw — it starts the same way an alcohol stop does. An officer observes driving behavior, makes the stop, and looks for indicators of impairment: smell, demeanor, performance on standardized field sobriety tests, statements from the driver. If those indicators add up to probable cause, the officer can request a blood draw — typically by warrant or with consent.
The blood test happens at a state lab. Results come back days to weeks later. By that point the driver is usually already charged with DUI on the basis of the field observations; the blood number confirms or revises the prosecution’s case.
The odor question
Smelling cannabis in or around a vehicle does not on its own give an officer probable cause to search the car in Washington. State v. Ladson and follow-on case law have narrowed the "plain smell" doctrine substantially. Odor can contribute to probable cause when combined with other indicators (admission, visible product, driving behavior), but the smell alone isn’t enough.
Practically: don’t consume in the car, don’t leave open packaging in the cabin, keep purchased cannabis sealed in the exit bag and stored where you’d store a bottle of wine driving home from a winery. The legal framing treats it the same way.
Drug Recognition Experts (DREs)
Washington trains a subset of officers as Drug Recognition Experts — DREs are certified to do a 12-step impairment evaluation that goes beyond standard field sobriety tests. Cannabis cases often go to a DRE for evaluation. The DRE’s observations become part of the probable-cause record alongside the field tests.
Penalties
A first-offense DUI in Washington carries a 90-day license suspension, mandatory ignition interlock for at least a year, fines, possible jail time, and ignition interlock device fees. Penalties escalate with priors and with aggravating factors (kids in the car, accident, high blood number). Cannabis DUI is treated the same as alcohol DUI for sentencing purposes.
When you’ll be okay to drive
Active THC clears the blood within a few hours of inhaled cannabis use, but the exact timing varies by dose, tolerance, frequency of use, and individual metabolism. Edibles last longer in the body and the peak comes later. Conservative practice for occasional users:
- After a small inhaled dose: wait at least 4 hours before driving.
- After a moderate inhaled dose (a few hits of a high-potency strain): wait 6-8 hours.
- After an edible: wait at least 8 hours, longer for larger doses.
- After a heavy session: don’t drive that day.
Frequent users sit in a gray zone because their baseline active-THC level can hover near the 5 ng threshold even hours after the last dose. There’s no roadside breath test for cannabis the way there is for alcohol; if you’re unsure, don’t drive.
A practical note
Washington dispensaries seal every purchase in the opaque exit bag that the state requires. The exit bag is the rough equivalent of an unopened wine bottle from a winery — fine to transport, not fine to open in the car. Keep it sealed until you’re home.