In every state, DUI alcohol can be proven two ways: by showing the driver had a blood-alcohol concentration of .08 percent or higher (“per se” DUI), or by proving that the driver was “impaired” by alcohol. As part of marijuana legalization, some states have tried to analogize to alcohol, criminalizing driving with a certain THC blood level, such as 5 ng/mL.
Under California’s Adult Use of Marijuana Act (AUMA), however, DUI marijuana can only be proven by showing impairment. Berkeley Law Professor Andrea Roth says California should resist the temptation to follow other states and create a “per se” DUI marijuana law.
Roth notes in a recent paper that unlike with alcohol, there is no predictable relationship between the level of THC in the blood and the level of intoxication in the brain.
Different cannabis strains have different potencies, and the mode of ingestion and user’s history impact THC’s effect, Roth explains in The Uneasy Case for Marijuana as Chemical Impairment Under a Science-Based Jurisprudence of Dangerousness. Unlike alcohol, THC stays in one’s system for hours or days after ingestion, and is not water and fat soluble like alcohol. In short, “just because a person has THC in her blood does not mean that she’s impaired,” Roth says.
The .08 blood-alcohol standard is “based on decades of careful scientific research showing how specific blood alcohol concentration increases relative crash risk,” she explains. Roth notes that morally blameworthy dangerousness— in this case increased crash risk—has always been what the law looks to in deciding how to criminalize impairment from a legal drug.
So far, she notes, relevant studies have not found increased relative crash risk from THC blood levels alone. “By criminalizing DUI marijuana based on THC blood levels absent any relationship to increased relative crash risk, we label and punish as criminals people who have not been demonstrated to be at a morally blameworthy level of chemical impairment,” she says.
Roth believes officer training, body cameras, and further scientific crash-risk studies can combat drugged driving without relying on incoherent numerical standards in criminal laws. AUMA allocates a portion of the tax revenue from recreational sales to the California Highway Patrol for training law enforcement officers in techniques to detect impaired driving—and to establish statewide protocols and standards for identifying impaired drivers.