I didn’t see this one coming from AAA of all places, but questions are being raised about DUI laws when it comes to people smoking marijuana as opposed to drinking alcohol. How accurate is the testing used to determine when a pot smoker is “impaired” behind the wheel? In the case of wacky weed, the answer isn’t as obvious as we might believe. (AT&T Live News)

Motorists are being convicted of driving under the influence of marijuana based on arbitrary state standards that have no connection to whether the driver was actually impaired, says a study by the nation’s largest auto club.

The problem is only growing as more states contemplate legalizing the drug. At least three, and possibly as many as 11 states, will vote this fall on ballot measures to legalize marijuana for medicinal or recreational use, or both. Legislation to legalize the drug has also been introduced in a half dozen states…

The problem is that determining whether someone is impaired by marijuana, as opposed to having merely used the drug, is far more complex than the simple and reliable tests that have been developed for alcohol impairment.

This is another of those subjects where my inner libertarian immediately perks up and takes notice. Drunk driving laws based on blood alcohol content (BAC) are sketchy enough as it is in my opinion. People react to alcohol differently for a number of reasons. Small, thin people will, theoretically, get drunk faster than larger folks and the BAC measurement allegedly takes that into account. But some habitual drinkers may seem essentially unimpaired at a BAC of .10 while someone who never drinks may be slurring their words at .08. There are also significant questions about the legality of making people take a sobriety test without a warrant, and these are being looked at by the Supreme Court this year as we’ve discussed here before.

When it comes to marijuana the subject is even more cloudy if you’ll pardon the pun. The tests done for this sort of impairment rely on the levels of THC (tetrahydrocannabinol… the agent in pot which gets you high) in the blood. But it turns out there are different components of the drug which may or may not influence impairment and habitual smokers can have those remnants in their blood for weeks after last smoking it. They’re obviously no longer high, but could fail a sobriety test and lose their license anyway.

The degree to which a driver is impaired by marijuana use depends a lot on the individual, the foundation said. Drivers with relatively high levels of THC in their systems might not be impaired, especially if they are regular users, while others with relatively low levels may be unsafe behind the wheel.

Some drivers may be impaired when they are stopped by police, but by the time their blood is tested they have fallen below the legal threshold because active THC dissipates rapidly. The average time to collect blood from a suspected driver is often more than two hours because taking a blood sample typically requires a warrant and transport to a police station or hospital, the foundation said.

The study emphasizes the fact that marijuana is being legalized (or at least decriminalized significantly) in more and more places, but I’m not sure that’s really relevant to the question at hand. Alcohol is legal to consume (though not necessarily sell) everywhere in the country if you are of legal age and not on parole. And more to the point, being drunk isn’t illegal. It’s a question of whether or not you should be operating a motor vehicle at the same time. Even where possession of or using marijuana is still against the law, the DUI component is a completely separate question. And while I don’t exactly have vast experience in the area myself, it’s my understanding that marijuana use can be detected in the blood or even in a hair sample for months after the last time it was used, particular for chronic smokers.

If we can’t reliably say how impaired a person is based on a blood test for THC, then these laws seem out of bounds. I’m not arguing in favor of allowing impaired people to drive, but a legal position of just saying better safe then sorry simply isn’t a good enough reason to be arresting people and taking away their driving privileges. We may need some better science in terms of detecting impairment and the burden is on the government to make that happen, not the pot smokers.

EDIT: Blood alcohol level content values corrected by one decimal point because math is hard.