SACRAMENTO — Under the age of 21 and thinking of driving high? Think again.

A new bill from state Sen. Jerry Hill, D-San Mateo, would create a zero-tolerance policy for any driver under the age of 21 who is caught operating a motor vehicle under the influence of marijuana, similar to the state’s existing zero-tolerance policy for alcohol.

“It’s illegal for anyone under 21 to ingest marijuana,” Hill said. “And, if you’re going to ingest marijuana, you shouldn’t be driving a car.”

It’s already illegal for anyone to smoke, eat or drink marijuana while driving, but Hill said there aren’t any laws prohibiting traces of marijuana from being in someone’s body while driving — though it is illegal to consume any substance that impairs driving. But, unlike alcohol, where there is a readily agreed-upon national standard constituting impairment — a blood alcohol content of .08 percent or greater — there is no such standard for marijuana.

And, that’s a problem in Hill’s bill, said Dale Gieringer, the state coordinator of California NORML, a nonprofit focused on decriminalizing marijuana in the Golden State. Blood and urine tests are considered effective at determining past use of marijuana, but aren’t effective in determining recent use, according to a 2017 report from the National Highway Traffic Safety Administration (NHTSA) on marijuana-impaired driving.

Skin and saliva tests are better at determining recent use, but, the report’s authors note, “point-of-arrest screening devices have not been shown to be completely accurate and reliable,” and “there are issues associated with distinguishing use versus environmental exposure, that have not been fully addressed.”

The bill calls for saliva or skin tests but also allows officers to request the suspect submit to a “blood, breath or urine” test if a preliminary marijuana screening test is not available.

“(These tests don’t) show this person was using marijuana at the time of driving, unlike alcohol” Gieringer said, adding that marijuana, which bonds to fat cells in the body and can be released slowly over time, can stay in someone’s system for hours, days or even weeks — well after the high wears away.

Nor is it clear how marijuana affects driving. A 2017 investigation by the Denver Post revealed a staggering uptick in the number of fatal crashes involving drivers who tested positive for marijuana following legalization of recreational marijuana — from 47 in 2013 to 115 in 2016, a 145 percent increase. Comparatively, all fatal crashes increased 40 percent in the same time frame and fatal crashes involving alcohol climbed 17 percent, from 126 in 2013 to 151 in 2015; figures for 2016 were not available.

At the same time, the NHTSA report cites multiple studies demonstrating marijuana alone may not be to blame. Doped drivers “typically drive slower, follow other cars at greater distances, and take fewer risks than when sober,” the report found.

“In contrast, subjects dosed with alcohol typically drive faster, follow at closer distances, and take greater risks,” the authors said.

One thing is clear, traffic stops disproportionately impact people of color, said Zachary Norris, the executive director of the Ella Baker Center for Human Rights. A 2016 study by the East Bay Community Law Center found black residents in Oakland made up 70 percent of all traffic stops but only 26.5 percent of the population. In Berkeley, black residents were stopped 30.5 percent of the time but only represent 8.4 percent of the population.

The passage of Prop 64, which reduced penalties for many drug-related offenses, indicated California residents were moving away from a criminal justice response to drug use and were moving toward a public health response, Norris said. This bill moves the state in the opposite direction, he said.

“These kinds of zero-tolerance policies make zero sense,” Norris said. “They have exacerbated racial and economic disparities in terms of our justice system and they ignore the will of California voters in passing Prop 64.”

Under the bill, a person under 21 with any detectable amount of marijuana at the time of driving will face immediate suspension of their driver’s license for one year, along with any applicable fees.