Cannabis Driving Impairment Laws in Washington

The 5 ng/mL per se limit

Washington's cannabis DUI rule is codified in RCW 46.61.502. 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 Strong evidence.

The 5 ng/mL number was set by Initiative 502 in 2012, the same voter initiative that legalized recreational cannabis in Washington ^[2]. It applies only to delta-9-THC, not to the inactive metabolite THC-COOH (carboxy-THC), which can linger for weeks.

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 ^[3].

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. Strong evidence

How blood is drawn: implied consent and warrants

Washington's implied consent statute (RCW 46.20.308) originally covered breath 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 ^[4][5]. Refusing a blood draw after a warrant is issued is not an option; refusing before the warrant may still trigger administrative license consequences 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 DUI ^[1]. 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 ^[6].

Penalties

A first cannabis DUI in Washington is a gross misdemeanor, carrying up to 364 days in jail and up to $5,000 in fines, plus mandatory minimums (typically 1 day jail or 15 days electronic home monitoring for a first offense with no aggravators), a 90-day license suspension, ignition interlock requirements, and required substance-use evaluation ^[1][7]. A fourth DUI within 10 years, or a DUI with a prior vehicular homicide, becomes a Class B felony ^[1]. Insurance and employment consequences typically outlast the criminal penalties Strong evidence.

Does 5 ng/mL actually mean impaired?

This is where the law and the science diverge. Unlike blood alcohol, THC blood concentration correlates poorly with driving impairment, particularly for regular users. A 2021 systematic review in Traffic Injury Prevention concluded that THC blood levels are 'a poor and inconsistent indicator of driving impairment' ^[8]. A 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 ^[9] Strong evidence.

Despite this, the 5 ng/mL rule remains in force in Washington. Bills to modify it have been introduced periodically but had not passed as of the last verified date. 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 Strong evidence.

Practical guidance

• Do not drive within several hours of consuming cannabis, and much longer for edibles.
• Regular daily users should be aware they may test above 5 ng/mL for many hours or even days after use ^[8].
• Passengers may legally possess cannabis in a vehicle, but open containers of cannabis in the passenger area are prohibited under RCW 46.61.745 — keep product sealed or in the trunk ^[1].
• 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 used as evidence.

This article is informational only and is not legal advice. Washington DUI law is technical and evolves; if you are facing charges or have specific questions, consult a licensed Washington criminal defense attorney. Information last verified June 15, 2024.

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