Indeed, even among law enforcement experts, the need for a standard is debated. Many support tried-and-true policing methods that can ferret out stoned drivers.
"Everybody wants a magic number, because that makes it easy," said Sarah Kerrigan, a toxicologist at Sam Houston State University in Texas and an expert witness in numerous trials. "To have a law that says above a certain level you are impaired is not scientifically supportable. I don't think police need the tool, but my opinion may be in the minority."
But federal officials and local prosecutors argue that the lack of a standard makes convictions harder to obtain.
In October, a San Diego jury acquitted Terry Barraclough, a 60-year-old technical writer and medical marijuana user, on manslaughter charges in a fatal crash that occurred shortly after he had smoked marijuana.
A blood test showed he had high levels of active marijuana ingredients in his blood, but the jury heard conflicting expert testimony from toxicologists about the possible effects.
Martin Doyle, the deputy district attorney who prosecuted Barraclough, said the acquittal showed that the lack of a formal legal limit on marijuana intoxication makes such prosecutions tough.
"We don't have a limit in California and that made my prosecution very difficult," Doyle said. "We have a lapse in the law."
But defense attorney Michael Cindrich said the failed prosecution shows that the San Diego district attorney was targeting medical marijuana users and that any legal limit would be unfair to the people who rely on the drug to treat their problems.
Indeed, Anthony Cardoza, an attorney who represented Flores in the Chico accident, said his client was not impaired and that allegations about his green tongue were ridiculous. Flores' guilty plea was prompted by other legal issues, including a prior conviction for a drunk driving accident that caused an injury.
Marilyn Huestis, a toxicologist and one of the nation's top experts on marijuana at the National Institute on Drug Abuse who is directing several research programs, said she believed there is no amount of marijuana that a person can consume and drive safely immediately afterward.
Supporters of marijuana legalization agree that the drug can impair a driver, but argue that the effects wear off in a few hours. Huestis, however, said research was showing that the effects of marijuana can linger.
Marijuana's main ingredient — delta-9 THC — stays in the blood for an hour or more and then breaks down into metabolites that are both psychoactive and inert. But the impairing effects can linger, even after the THC is no longer in the blood, Huestis said. Because it can be absorbed into body tissue and slowly released for days, Huestis believes that heavy chronic daily users may be impaired in ways that are not yet understood.
A complicating factor is the tendency of many marijuana users to also use alcohol, which can sharply amplify impairment. Very little research has been conducted to determine whether it is possible to set limits on a combination of such substances.
Paul Armentano, deputy director for the National Organization for the Reform of Marijuana Laws, said some states had laws that can punish users even when they are not high, pointing to a tough Arizona statute that allows conviction for impaired driving when an inert metabolite is detected in the blood.
Arizona officials said they wrote the law because there was no scientific agreement on how long marijuana impairs a driver. But proponents see something more sinister: an effort to put marijuana users in constant legal jeopardy.
"We are not setting a standard based on impairment, but one similar to saying that if you have one sip of alcohol you are too drunk to drive for the next week," Armentano said.