Debate rages over how much pot smoking impairs driving

Everyone seems to agree that drivers who smoke too much marijuana before getting behind the wheel are a danger, but there's little agreement on how stoned they have to be before they should be arrested, the Los Angeles Times reports.

The problem is that unlike the blood-alcohol level for intoxication when it comes to drinking. It's harder to peg a magic number on dope smoking. With marijuana, experts are divided when it comes to the point of impairment to the point of endangering others. While the issue has been around for decades, it's taking on a new meaning as more states legalize pot for medical uses.

"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," Sarah Kerrigan, a toxicologist at Sam Houston State University in Texas and an expert witness in numerous trials, tells the Times.

The issue came up in San Diego last October when Terry Barraclough, a 60-year-old technical writer and medical marijuana user, was acquitted of manslaughter charges in a fatal crash that occurred shortly despite a blood test after the crash that showed he had high levels of marijuana ingredients in his blood.

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, experts say.

Marijuana legalization experts say the law goes too far, punishing those who are have smoked but are not high.