There's a number nobody wants to exceed when it comes to drunken driving: 0.08.
Registered on a Breathalyzer, 0.08 means your blood-alcohol content has crossed a legal threshold and you can be convicted of DUI, regardless of how you are driving.
But for marijuana, the line between legal and illegal is hazier.
There is no legal limit in California that defines marijuana intoxication, even though officials say the drug has become nearly as common as alcohol in drivers and presents an increasing safety risk on the road.
A recent crash in Santa Rosa underscored these concerns. In March, a man suspected of being high on marijuana and driving while texting slammed into a car halted in traffic on Highway 12, killing two women.
As more states consider legalizing marijuana and the drug's presence increases, a debate is growing over the best way to prevent stoned driving.
"Marijuana is so prevalent, and it's so easy to get a medical marijuana card, that a huge number of people are using it," said Petaluma Police Officer Matthew "Cap" Capitelli, who is trained to recognize drivers who have been using marijuana or other drugs.
"You stop people and there's a tone of insolence, like, 'Yeah, I've been smoking, but I have my medical marijuana card so I can smoke and drive.' That's not OK for anyone to do," he said.
On a recent Saturday night, Capitelli clocked a man driving 39 mph in a 25-mph zone on Petaluma Boulevard North. Speed is one indicator of intoxication, Capitelli said.
He flipped on his flashing lights and a young driver pulled over. After talking with the man for a moment and shining a flashlight in his face, Capitelli suspected the man was under the influence of drugs and asked him to get out of his truck.
He then began a 12-point evaluation, not unlike a field test for alcohol but much more extensive, to determine whether the man was under the influence of drugs.
Capitelli is a drug-recognition expert, or DRE in police jargon, one of about 1,300 officers statewide who have received specialized training on how to recognize drivers impaired by drugs.
They are in increasing demand thanks to the growing number of drivers under the influence of marijuana since a 1996 initiative legalized pot in California for medical use.
Chris Cochran, a spokesman for the Office of Traffic Safety, said there has been a surge in drivers using marijuana over the past three to four years. He pointed to a 2012 survey of 1,300 nighttime, weekend drivers in which marijuana edged alcohol as the most commonly used substance.
Brad Conners, a Santa Rosa police traffic sergeant, also noted the rise.
"With the increase in medical marijuana usage, and the acceptance of medical marijuana, we've seen an increase in impaired drivers using marijuana," he said.
Nationally, the number of fatal crashes involving marijuana use has tripled over the past decade, according to a February study by the Columbia University Mailman School of Public Health. The study's co-author, Dr. Guohua Li, said marijuana is the most common drug besides alcohol to be detected in injury and fatal crashes.
Some states have responded by implementing a legal limit or a zero-tolerance policy for how much marijuana can be in a driver's system. Such a move is underway in the California Legislature, but for now, prosecutors mainly rely on an officer's assessment of a driver's behavior.