Cannabis Driving Impairment Laws in Washington
Washington's 5 ng/mL per se limit, search warrant rules, and the science-law gap.
This article is general information, not legal advice. Laws change; check current rules with official sources or a lawyer.
Washington has a rigid 5 ng/mL blood THC rule set by voter initiative in 2012. The catch: regular users can stay above that limit for hours or days after use without being impaired. The science says blood THC is a poor marker of driving ability. The law doesn't care. Police will get a warrant for your blood; refusing after the warrant is issued isn't an option.
The 5 ng/mL per se limit
Washington's cannabis DUI rule is codified in RCW 46.61.502 [1]. A driver is guilty of DUI if, within two hours of driving, a blood test shows 5.00 nanograms or more of active (delta-9) THC per milliliter of whole blood [1]. This is a per se limit: prosecutors do not have to prove you were actually impaired, only that your blood concentration met the threshold.
The 5 ng/mL number was set by Initiative 502 in 2012, the same voter initiative that legalized recreational cannabis in Washington. It applies only to delta-9-THC, not to the inactive metabolite THC-COOH (carboxy-THC), which can linger in the body for weeks and does not cause impairment.
The 'affected by' prong—no number required
Even if your blood THC is under 5 ng/mL—or if no blood is drawn at all—you can still be convicted. RCW 46.61.502(1)(c) makes it a DUI to drive while 'under the influence of or affected by' cannabis [1]. Under this prong the state relies on officer observations, field sobriety tests, and often a Drug Recognition Expert (DRE) evaluation [2].
In practice, this means a driver with 2 ng/mL who fails field sobriety tests can be charged, and a driver with 8 ng/mL who was driving perfectly can also be charged. This creates legal uncertainty: the per se limit is not a safe harbor, and sobriety remains open to interpretation.
Search warrants and blood draws
Washington's implied consent statute (RCW 46.20.308) originally covered breath and urine tests for alcohol. Because THC requires a blood draw, and because the U.S. Supreme Court held in Missouri v. McNeely (2013) and Birchfield v. North Dakota (2016) that warrantless blood draws generally violate the Fourth Amendment, Washington officers typically obtain a telephonic search warrant before drawing blood in a suspected cannabis DUI Strong evidence.
Refusing a blood draw after a warrant is issued is not legally possible; the officer may use reasonable force to obtain the sample. Refusing before the warrant is issued may still carry administrative consequences, such as automatic license suspension, but is technically your right. In practice, officers make it clear that a warrant will be obtained regardless Strong evidence.
Drivers under 21 and commercial drivers
Washington has a zero-tolerance rule for drivers under 21: any detectable THC in blood is a violation under RCW 46.61.503 (the 'minor DUI' statute), though the penalties are lower than a standard adult DUI [1]. The threshold for minors is not a fixed number but the presence of any measurable delta-9-THC.
Commercial drivers are governed by federal DOT rules, which prohibit any cannabis use regardless of state legality—a CDL holder can lose their license for a positive test even off-duty, and employment consequences follow [3]. Federal law supersedes state legalization for this purpose.
Penalties and criminal consequences
A first cannabis DUI in Washington is a gross misdemeanor, carrying:
- Up to 364 days in jail
- Up to $5,000 in fines
- Mandatory minimum 1 day jail or 15 days electronic home monitoring (for a first offense with no aggravating factors)
- 90-day license suspension
- Ignition interlock requirements
- Mandatory substance-use evaluation [1][4]
A fourth DUI within 10 years, or a DUI with a prior vehicular homicide or assault, becomes a Class B felony [1]. Felony DUIs carry up to 10 years in prison and up to $25,000 in fines [4].
Beyond criminal penalties, a cannabis DUI conviction triggers insurance increases, employment background check failures, professional license consequences (for healthcare workers, pilots, teachers, etc.), and in some contexts, child custody implications.
Does 5 ng/mL actually measure impairment?
This is where Washington law and the science diverge sharply. THC blood concentration correlates poorly with driving impairment, particularly for regular users [5] Weak / limited. A 2021 systematic review and meta-analysis in Traffic Injury Prevention concluded that THC blood levels are 'a poor and inconsistent indicator of driving impairment,' with high variability among individuals and a weak dose-response relationship [5]. A 2017 NHTSA report to Congress reached a similar conclusion, noting that impairment can occur below 5 ng/mL and that habitual users can exceed 5 ng/mL for many hours after last use without acute impairment [6] Weak / limited.
The reason: unlike alcohol, THC is fat-soluble and is stored in body fat. Regular users develop tolerance and may show high blood levels after minimal recent use. A daily user might register 6–8 ng/mL a full day after their last use, while a naive user might be significantly impaired at 3 ng/mL [5]. Current science offers no simple blood threshold that reliably predicts impairment.
Despite this disconnect, the 5 ng/mL rule remains in force in Washington. Bills to modify or eliminate it have been introduced periodically but had not passed as of 1 November 2026. The practical takeaway: regular cannabis users may exceed the legal limit even when sober, and there is no reliable at-home test to check yourself before driving [5] Weak / limited.
Practical guidance
- Wait several hours before driving. For smoked or vaporized cannabis, most acute impairment resolves within 3–4 hours, but blood THC can remain elevated much longer [5].
- Edibles require longer. THC from edibles is absorbed more slowly and lasts longer; wait at least 8–12 hours before driving, or longer if uncertain.
- Regular daily users should be especially cautious. You may test above 5 ng/mL for many hours or even days after use, even if you feel sober [5].
- Passengers may legally possess cannabis in a vehicle, but open containers of cannabis in the passenger area are prohibited under RCW 46.61.745 [1]. Keep product sealed or in the trunk.
- If stopped, you have the right to remain silent and the right to counsel. Field sobriety tests are technically voluntary in Washington, though refusal can be noted by the officer and used as evidence of impairment in court.
- Do not consent to a warrantless blood draw. You cannot prevent a warrant, but declining a warrantless draw is your legal right. Do not resist or interfere with a lawful warrant.
This article is informational only and is not legal advice. Washington DUI law is technical, fact-dependent, and evolves. If you are facing charges, have been stopped, or have specific questions, consult a licensed Washington criminal defense attorney.
Sources
- Government Washington State Legislature. Revised Code of Washington (RCW) 46.61.502, 46.61.503, 46.61.506, 46.61.745, and 46.20.308.
- Government National Highway Traffic Safety Administration (NHTSA). Drug Recognition Expert (DRE) Program overview and 7-Step DRE evaluation process.
- Government U.S. Department of Transportation, Federal Motor Carrier Safety Administration (FMCSA). 49 CFR Part 382—Controlled Substances and Alcohol Use and Testing.
- Government Washington State Legislature. RCW 46.61.5055—Alcohol and drug violators penalty schedule.
- Peer-reviewed McCartney D, Arkell TR, Irwin C, McGregor IS. Determining the magnitude and duration of acute Δ9-tetrahydrocannabinol (Δ9-THC)-induced driving and cognitive impairment: a systematic and meta-analytic review. Neuroscience & Biobehavioral Reviews. 2021;126:175–193.
- Government National Highway Traffic Safety Administration (NHTSA). Marijuana-Impaired Driving: A Report to Congress. July 2017.
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