Marijuana impairment in drivers is hard to measure.

There's no established scientific standard that's equivalent to the well-known blood-alcohol content level. Pot's active ingredient, THC, can stay in your system for days, without much indication that you could be long past a state of highness.

“The presence of active THC is generally suggestive of recent marijuana use,” the Auto Club of Southern California says in a statement. But that's about as much as you can say about it, scientifically speaking.

The AAA Foundation for Traffic Safety recently analyzed lab results of drivers arrested for driving under the influence of marijuana and concluded that “legal limits for marijuana and driving are problematic,” according to the Auto Club of Southern California.

The analysis concludes with three points: Science doesn't support a universal, “specific level” of blood impairment for pot. High levels of THC in the blood can drop by the time a test is administered. One person could be high at a given THC level while another could be sober at the same level.

“There is understandably a strong desire by both lawmakers and the public to create legal limits for marijuana impairment, in the same manner as we do with alcohol,” said AAA CEO Marshall Doney. “In the case of marijuana, this approach is flawed and not supported by scientific research. It’s simply not possible today to determine whether a driver is impaired based solely on the amount of the drug in their body.”

The AAA Foundation for Traffic Safety, noting its debatable new finding that fatal crashes involving possibly stoned drivers doubled after Washington state legalized recreational pot, is recommending that lawmakers enact “comprehensive” testing laws.

Those should include positive tests for marijuana, “behavioral and psychological evidence” of weed-related impairment, and cop training that includes Advanced Roadside Impaired Driving Enforcement (ARIDE) and Drug Evaluation and Classification (DEC) standards.

“States need consistent, strong and fair enforcement measures,” Doney said.

California Assembly Bill 2740 would do exactly what the AAA is decrying — establish a blood-content limit for drivers suspected of impaired driving. The legislation seeks to set a DUI limit of “5 ng/ml or more of delta 9-tetrahydrocannabinol.”

California NORML said in a statement that the law co-authored by Assemblyman Evan Low of San Jose “would criminalize AIDS patients, cancer patients and other seriously ill medical marijuana users, since chronic users can show 5 ng/ml in the blood for days after use.”

Six states have approved similar blood-THC standards, and California lawmakers are under pressure to offer some guidelines to law enforcement, as it appears voters will be weighing a November initiative that seeks to legalize recreational pot.

But it's got to be done right. People who toked over the weekend shouldn't face an estimated $16,000 in DUI costs and possible job loss for being accused of stoned driving on Tuesday.

The AAA's analysis “is consistent with NORML’s long-held position,” NORML deputy director Paul Armentano said in an email. The position is that such a blood-THC standard would criminalize the innocent.

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