Why Washington’s Marijuana DUI Laws Don’t Match the Science

If you’re facing a marijuana-related DUI in Tacoma, Lakewood, or elsewhere in Pierce County, you may wonder: do Washington’s THC rules actually reflect real impairment?

The short answer is no. Washington’s law treats THC like alcohol — assuming a specific number in your blood equals intoxication — even though every major scientific body says the two substances behave very differently.

Below, our Tacoma DUI attorneys explain why Washington’s 5 ng/mL rule doesn’t match the science, what the courts have said about it, and what it means for drivers across the state.

For immediate help with your case, call the Law Offices of Smith & White at (253) 203-1645.

Washington’s Marijuana DUI Rule: A Quick Overview

Washington law (RCW 46.61.502) makes it illegal to drive while “under the influence of or affected by” alcohol, cannabis, or any drug.
Alongside that behavioral standard, the statute also sets a numerical threshold for THC:

If a blood test within two hours of driving shows 5.00 nanograms per milliliter (ng/mL) of THC in whole blood, that number automatically meets the per-se DUI standard under RCW 46.61.502(1)(b).

Drivers under 21 face an even stricter standard. Under RCW 46.61.503, any THC above 0.00 and below 5.00 ng/mL within two hours can support a “zero-tolerance” DUI charge, while 5.00 ng/mL or more can be prosecuted under the adult per-se rule.

Washington became one of the first states to adopt a numeric THC limit when voters passed Initiative 502 in 2012.
Since then, numerous studies and policy reviews have shown that the 5 ng figure lacks biological precision.

Still, the Washington Supreme Court reaffirmed the law in State v. Fraser, 2022 WL 16729585 (Wash. 2022) — upholding it as constitutional even while recognizing that the science does not clearly support it.

Why the 5 ng/mL Rule Fails Scientifically

THC Doesn’t Behave Like Alcohol

Blood-alcohol content rises and falls predictably; THC does not.
THC is fat-soluble, leaving the bloodstream quickly and storing in fatty tissue, where it slowly releases for hours or even days.

After smoking, blood THC peaks within minutes and declines rapidly during the first hour, even though psychoactive effects may persist.

Regular users — including medical patients — often maintain residual THC long after impairment has ended.
Someone may test above 5 ng/mL the next morning and be entirely sober.

Two drivers with identical THC readings can show drastically different levels of impairment — or none at all.

No Reliable Link Between THC Numbers and Unsafe Driving

Major traffic-safety agencies agree: blood-THC concentration does not reliably predict impairment or crash risk.

The National Highway Traffic Safety Administration, “Marijuana-Impaired Driving: Report to Congress” (2017) concluded that “no scientifically valid method currently exists to correlate a specific THC concentration with impairment.”
The AAA Foundation for Traffic Safety, “An Evaluation of Data from Drivers Arrested for DUI in Relation to Per Se Limits for Cannabis” (2016) found “no reliable threshold at which most drivers are impaired” and warned that per-se limits risk criminalizing unimpaired drivers.
JAMA Psychiatry (2022) — “Cannabis and Driving Performance: A Randomized Clinical Trial” — found that once individual variability is considered, THC levels are poor predictors of driving performance.

Despite this, Washington’s 5 ng rule continues to treat THC like alcohol — even though their chemistry and metabolism are fundamentally different.

The Problem of Biological Inequality

Because THC is stored in fat, people with higher body-fat percentages can retain it longer, testing higher than leaner individuals after identical use.
Frequent or medical users may always test above 5 ng/mL even when sober.

In that sense, Washington’s law punishes biology as much as behavior.
It exposes heavier individuals and daily users to greater risk of prosecution even when no impairment exists.

While courts haven’t yet labeled this an equal-protection violation, the disparity is real — and future challenges could arise as data grows.

Why the Court Upheld an Unscientific Law

If the science is this weak, why does the rule still stand?

In State v. Fraser (Wash. Sup. Ct. 2022), the defendant argued that Washington’s 5 ng/mL limit violates the state constitution because it is arbitrary and unsupported by scientific evidence.

The Washington Supreme Court disagreed — not because the science was persuasive, but because the constitutional review it applied was extremely deferential.

The Court used Washington’s “police-power” test, which only asks whether a law is reasonably related to public safety.
Under that lenient standard, lawmakers do not have to prove that a law actually improves safety — only that it might in theory.

As a result, the Court accepted the State’s broad logic that “recent marijuana use can impair some drivers” and that “a numeric limit may deter people from driving too soon after using.”
It did not require proof that 5 ng/mL correlates with unsafe driving, nor proof that the numeric limit actually deters impaired drivers.

In short, the law survived because it was conceivable, not demonstrated — a policy upheld on possibility, not proof.

Other States Are Moving Beyond Washington’s Outdated Rule

Washington now belongs to a shrinking group of states that still enforce a fixed THC number.

A few maintain numeric limits:

Colorado (5 ng/mL) — a permissible inference, not automatic guilt; juries can reject it.
Montana (5 ng/mL) — a true per-se standard, though expert testimony can rebut impairment.
Utah — uses a zero-tolerance “measurable-substance” rule, not a 5 ng limit, with exceptions for medical use.
Illinois (5 ng/mL) and Ohio (2 ng/mL blood or 10 ng/mL urine) — retain older per-se frameworks despite ongoing scientific criticism.

By contrast, most legalization states — including California and Oregon — require prosecutors to prove actual impairment rather than rely on a number.

Nevada’s legislative hearings on Assembly Bill 400 (2021) described numeric THC limits as “unscientific” and “potentially unjust,” citing federal crash data showing no measurable safety benefit.

Washington’s 5 ng/mL rule now stands among the most outdated in the nation — even as nearby states demonstrate more accurate and equitable approaches.

What a Better System Would Look Like

A fair, science-based marijuana-DUI policy would measure driving, not chemistry. Key reforms should include:

Eliminate fixed per-se THC limits. No THC number consistently reflects impairment. Several states already rely on behavioral and scientific evidence instead of arbitrary thresholds.
Require proof of actual impairment. Prosecutors should show the driver was genuinely unsafe — through officer observations, video, and credible expert analysis. Blood results should support that case, not replace it.
Link evidence to unsafe operation. A better system connects driving conduct and observed signs of cannabis influence rather than assuming guilt from a lab result taken hours later.
Expand cannabis-specific officer training. Drug Recognition Expert (DRE) programs and improved field-testing protocols create more accurate and defensible cases than number-based enforcement.

This mirrors how most states handle prescription-drug DUIs: the question isn’t “what’s in your blood,” but “could you drive safely?”

It’s a model grounded in fairness, science, and genuine public safety — one Washington could adopt with ease.

What to Do If You’re Charged with a Marijuana DUI in Pierce County

If you’re accused of a marijuana-related DUI under Washington’s 5 ng/mL rule:

Don’t assume the number is proof. THC levels fluctuate and often reflect past use.
Request an independent lab review. Blood samples degrade quickly and are frequently misinterpreted.
Work with an attorney experienced in scientific defenses. Expert testimony can show your results don’t match actual impairment.
Act fast on your DOL hearing. You have seven days to challenge an automatic license suspension.

At Smith & White, our Tacoma DUI attorneys combine legal and scientific defense strategies to contest questionable THC readings and protect your rights.

Bottom Line

Washington’s marijuana-DUI law endures not because it’s scientifically sound, but because courts have allowed lawmakers to rely on assumption rather than evidence.

THC does not behave like alcohol.
The 5 ng/mL limit doesn’t measure intoxication, doesn’t predict unsafe driving, and can criminalize people for biological differences instead of conduct.

If you’ve been charged with a marijuana-related DUI in Tacoma, Lakewood, Puyallup, or anywhere in Pierce County, don’t face it alone.
Call the Law Offices of Smith & White at (253) 203-1645 for a confidential consultation.

We’ll help you build a defense based on fact — not assumption.