THC, the psychoactive chemical in marijuana, can affect people very differently. So measuring THC has not been proven to be a reliable measurement of someone’s driving ability.
California’s subjective DUI laws
On average, marijuana does impair driving skills for most people, especially 20 to 40 minutes after smoking. But measuring THC isn’t a good way to tell how much worse.
The lack of scientific evidence didn’t stop Washington and Colorado from setting a THC limit for drivers. But California went the other way. Here, there’s no numerical limit on THC. Instead, the law says you can’t drive under the influence of any drug, period.
In California, a marijuana DUI conviction depends on things an officer observes, such as the smell of marijuana in the car, or a physical sobriety test like those given to drivers who may be drunk.
Some officers are trained to identify specific signs of marijuana use, like dilated pupils. Sgt. Jennifer Tate, a drug recognition expert with the Berkeley Police Department, says the physical tests have gotten better over the years.
“We don’t need a better test,” says Tate. “If people are impaired, we can tell.”
But Andrea Roth, a law professor at UC Berkeley, says those physical tests are relatively subjective, relying on the judgment of the officer. Because of that, she says there’s potential for racial bias in how they’re enforced. Cops already pull over black and Hispanic drivers at higher rates. Roth says an objective test could help prevent innocent drivers from getting DUIs.