Washington -- As California advocates ponder a renewed push to legalize marijuana for adults, law enforcement officials and traffic safety experts are warning of a side effect of states allowing the drug for medical or recreational use: the danger caused by people driving while high.

Research is incomplete on how much marijuana it takes to impair driving. But Gil Kerlikowske, director of the White House Office of National Drug Control Policy, said being even a little intoxicated on marijuana is unacceptable.

"Smoking marijuana has a very negative effect on your ability to operate a motor vehicle," Kerlikowske said. "It's quite dangerous to you, your passengers and others on the road."

Marijuana advocates acknowledge that driving under the influence of cannabis is ill-advised. But they argue that law enforcement's concern is overblown, and point to a 2012 study that concluded the auto accident risk posed by marijuana is on par with antihistamines and penicillin.

Legalization moves

The debate over marijuana and highway safety is set against the backdrop of last year's decision by Washington state and Colorado voters to legalize marijuana for personal use, as well as the passage of medical marijuana laws in California and 19 other states. Californians rejected full legalization in 2010, but advocates buoyed by polls showing increasing public acceptance of recreational use by adults hope to return to the ballot next year or 2016.

Law enforcement officials say that while traffic fatalities in Colorado decreased 16 percent from 2006 to 2011, deaths involving drivers testing positive for just marijuana increased 114 percent during the same period.

And in Washington, according to Chuck Hayes of the International Association of Chiefs of Police, tests confirming the presence in drivers of THC - marijuana's active ingredient - have made up 42 percent of the state's toxicology lab caseload this year, an increase from 26 percent last year.

"I'm not sure the public really understands the danger of it," said Hayes, a retired Oregon State Police captain who trains police officers to be drug-recognition experts. "A lot of education needs to be done in this area."

Threat to patients

But those favoring marijuana legalization for medical or recreational uses insist the greater danger comes from one-size-fits-all state laws that target anyone behind the wheel with traces of THC in their system, or peg violations to a particular THC blood-test threshold.

Such laws, they say, are a particular danger to medical marijuana users because THC lingers in blood and urine for days after consumption. These "zero tolerance" laws are on the books in 14 states.

"The data doesn't support the disproportionate policy response that law enforcement is asking for," said Paul Armentano, deputy director of NORML - the National Organization for the Reform of Marijuana Laws. "We're not having a public outcry saying we need a serious crackdown for antihistamine and penicillin."

Kerlikowske, a former Seattle police chief, called advocates' assertion that medical marijuana patients are unfairly affected by impaired-driving laws "a bit of a red herring."

"It's pretty obvious you're getting stopped for a reason - bad driving," he said. "In the real world, those arguments go up in smoke."

California law

California's law forces prosecutors seeking a DUI conviction for marijuana to prove that a driver was impaired because of ingesting the drug, not just that a driver took it.

The National Highway Traffic Safety Administration's 2007 National Roadside Survey of nearly 10,000 drivers nationwide found 11.3 percent tested positive for illegal drugs. The most popular: marijuana, at 8.6 percent.

A similar survey of 1,300 California drivers pulled over last year on Friday and Saturday nights found that more tested positive for drugs than for alcohol, and that the most prevalent drug detected was marijuana.

The public campaign against drunken driving has succeeded in reducing alcohol-related accidents. In turn, the reduction is "revealing the previously obscured and now growing problem of drug-impaired driving," said Chris Cochran, spokesman for the California Office of Traffic Safety, which conducted the survey.

Research into driving and drugs is continuing, but "the problem is (drugs) are causing tragedies on our roadways today," Cochran said.

Although California's legal standard is not as draconian as those of the 14 "zero tolerance" states, medical marijuana advocates see a danger in hyped-up law-enforcement concern over drugged driving.

Law enforcement is "criminalizing thousands of medical marijuana patients unnecessarily," said Kris Hermes, spokesman for Americans for Safe Access in Oakland. "For the most part, they're not behaving any differently than any other driver."

Residual traces

Hermes says his organization has no problem with police pulling over impaired drivers. But it worries that medical marijuana patients who commit minor traffic infractions, such as not using a turn signal or failing to wear a seat belt, could end up facing DUI charges if they have residual traces of THC in their systems.

"Marijuana has been in use for decades without significant risk on the roads," Hermes said. "We don't need to suddenly protect the public from a problem that doesn't exist."

Some academic studies have concluded there is clear evidence of a link between marijuana consumption and traffic accidents.

A study conducted last year at Dalhousie University Medical School in Canada found that those who drive within three hours of consuming cannabis are almost twice as likely to cause an accident as those who are drug- or alcohol-free.

However, a 2011 study by two researchers found a decrease in traffic deaths in states with medical marijuana laws. The researchers, Mark Anderson at Montana State University and Daniel Rees at the University of Colorado Denver, concluded that marijuana often is a substitute for alcohol and that users tend to consume it at home rather than driving to bars or restaurants.