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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.

CHP Sgt. Jarod Primicerio, who coordinates a state drug-recognition training program for police officers, said marijuana can compromise a person's driving performance in many ways, including the ability to multitask and impairing timing and depth perception, among other things.

"Obviously, that's huge," Primicerio said of the problems with depth perception. "We're seeing a lot of rear-enders" resulting from marijuana use.

CHP officers believe that marijuana may have been a factor in a March 15 crash that killed two Santa Rosa women. Sue and Sharon Hufford were in the back seat of a car stopped at a traffic light on Highway 12 near Farmers Lane when a truck rear-ended them. CHP officers say the truck's driver, Nicholas Lee Tognozzi, admitted to looking at his cellphone in the moments before the crash. A drug-recognition expert determined that he also might have been under the influence of marijuana. Prosecutors are waiting for results from a urine test before deciding whether to add DUI to the charges against Tognozzi.

Some advocates insist marijuana doesn't affect driving like other drugs and shouldn't be criminalized in the same way.

Paul Armentano, deputy director of the marijuana advocacy group NORML, acknowledged that marijuana can affect driving but said the effects are relatively small and short-lived compared with alcohol and most other drugs, even prescription drugs.

"I think because marijuana is illegal, people make the leap that the effects of marijuana (on driving) must be exponentially greater," he said.

Joe Rogoway, a Santa Rosa defense attorney and marijuana legalization advocate, said research shows the drug does not cause an impairment like alcohol. He cited a Rotterdam study showing people are more cautious when under the influence of marijuana.

Rogoway said legalization opponents have overstated the effects on driving for political and ideological reasons, and that alcohol remains a far more dangerous drug.

"They just don't like marijuana," he said. "They don't want it legal. That's their entire basis. It's not legitimate."

Proving marijuana impairment in court is difficult because there is no legal limit for the amount of the mind-altering chemical THC that can be in a person's bloodstream. However, some courts have agreed on a minimum level to prove the mere presence of marijuana.

Prosecutors rely almost entirely on a police officer's observations of physical symptoms, such as red eyes and reckless driving.

Diana Gomez, a chief deputy district attorney in Sonoma County, said blood testing confirms only that a person has used marijuana but not that there is impairment. She said that evidence is used to corroborate an officer's observations about a person's driving.

But since marijuana is a "sticky" substance that lodges in fatty cells of the body for many days, it is impossible to draw any conclusions from toxicology testing alone, she said.

Pilot programs are being conducted in Los Angeles, Sacramento and elsewhere that allow officers to test for drugs and marijuana at traffic stops, said Cochran of the Office of Traffic Safety.

The goal is to develop a valid way to measure marijuana impairment for use in prosecutions. "They would like to put it to court testing, eventually," Cochran said.

Some researchers believe a test will be developed within five years that is as accurate at gauging THC levels as a Breathalyzer is for alcohol, but others say blood tests will never accurately measure marijuana impairment.

Armentano advocated for the continued use of drug-recognition experts, maintaining that blood and urine tests cannot accurately show how impaired a person is. That's because impairment does not increase in direct relation to how much THC is in the blood and varies from person to person, Armentano said.

"Experts have also failed to agree on what specific THC concentrations, if any, may be consistently linked with impairment," he wrote in a paper on legal limits for cannabis.

The lack of a legal limit, combined with marijuana's growing presence, is driving an increase in demand for drug-recognition experts.

The DRE program, started in Los Angeles in the 1970s, is now a national effort administered in California by the CHP. Officers attend a course, paid for by grants from the Office of Traffic Safety, that consists of 72 hours of class time and 32 hours of field training.

They learn a 12-step process for determining impairment that begins with a breath test, includes questioning by the officer and a number of performance tests, and ends with a blood or urine test.

Capitelli, of the Petaluma force, put that training to use when he suspected the man he pulled over for speeding might be under the influence. He took the man's pulse. It was racing at about 140 beats per minute. He asked the man to stand with his feet together, eyes closed, head back, and silently count to 30. The exam, called a "Rhomberg stand," checks a person's balance and sense of time. In this case, the man counted to 30 in about 14 seconds, suggesting that he might be on a stimulant.

Capitelli shined a flashlight in his eyes and found that his pupils were "blown out," or dilated.

At the end, Capitelli suspected the man was under the influence of a stimulant and arrested him. The final step, a blood test, has not yet come back.

Capitelli is one of two drug-recognition experts in the Petaluma Police Department. All officers have some degree of DUI training, but if they are unsure about a case, they can call in a specially trained officer like Capitelli.

"We can all tell when someone is baked out of their gourd," Capitelli said. "But lower levels are hard to tell."

"District attorneys are a lot more likely to take a case to court if a cop has been through DRE school," he added.

Dave Radford, who directs an Office of Traffic Safety program to train prosecutors in how to handle DUI cases, called the DRE program "excellent," but said there is a shortage.

"The problem is we don't have enough DREs," he said.

To meet demand, the state has set a goal that 10 percent of police departments' traffic officers be DRE certified.

Not everyone is satisfied with the current state of the law. Defense attorney Rogoway said getting a conviction for driving under the influence of marijuana remains rare.

The push in the Legislature for a "zero tolerance" law would sidestep some of the imprecision of toxicology testing. A driver with any amount of marijuana in his or her blood automatically would be guilty of driving under the influence, Rogoway said.

Sonoma County prosecutors couldn't supply statistics showing the number of people cited for driving under the influence of marijuana. Until recently, those cases were lumped in with people accused of drunken driving.

But starting Jan. 1, new penal code sections distinguish between drivers suspected of being impaired by alcohol and those thought to be using drugs. However, there still isn't a marijuana-only charge under the new law.

More than a dozen other states have zero-tolerance laws. Washington has made it illegal to drive with 5 nanograms per milliliter of THC metabolites in the bloodstream — a standard Rogoway and Armentano said is essentially meaningless. California courts have agreed 30 nanograms per milliliter is the minimum to show presence of marijuana, Rogoway said.

Still, Armentano acknowledged that stoned drivers can be dangerous and advocated for a public awareness campaign.

"With the changing attitudes toward a legal classification of marijuana and with potentially greater use of marijuana by the general public, I think it's imperative that, parallel to that, there be a high-profile, public education campaign making people not only aware of the laws but of the potential risks," he said.

Gomez said there's a misconception that it is OK to drive when using marijuana recommended by doctors. In fact, it's illegal to drive while under the influence of any medication, from antihistamines to cough syrup, she said.

"The law says you can't be under the influence of anything, even NyQuil," Gomez said, referring to the Vicks cold medicine made by Procter & Gamble.

Primicerio agreed. "Once they're impaired, that's the bottom line."

(You can reach Staff Writer Jamie Hansen at jamie.hansen@pressdemocrat.com or 521-5205 and Staff Writer Paul Payne at 568-5312 or paul.payne@pressdemocrat.com.)