How Much Marijuana Does It Take to Impair Driving?

Posted by Lawrence Taylor on February 20th, 2017

A number of posts on this blog have addressed the problems encountered in trying to measure levels of marijuana — or, more accurately the active ingredient Tetrahydrocannabinol ("THC") — in the blood of a person when he was driving.  See, for example, Oregon Legislative Study Criticizes "Per Se" DUI Marijuana Laws and Marijuana Legalization and the California DUI.   

More importantly, these and other posts have also raised the related but unanswered question:  How much marijuana in the human body does it take to render a driver unable to safely operate a motor vehicle in the manner of a sober person (the rough definition of "driving under the influence" or "driving while intoxicated")?  See New Efforts to Push Roadside Marijuana DUI Test

The following excerpts from a recent article in The Atlantic, entitled "When Are You Too Stoned To Drive?", provide an excellent analysis of these important issues: 


…We take for granted that not being able to walk a straight line or stand on one leg means that you’re drunk, and that being drunk means it’s unacceptably dangerous to drive. But there is no clear scientific consensus when it comes to smoking pot and driving. And few of the tools police officers have long relied on to determine whether a driver is too drunk to drive, like a breathalyzer, exist for marijuana…

Most (but not all) studies find that using pot impairs one’s ability to drive. However, overall, the impairment appears to be modest—akin to driving with a blood alcohol level of between .01 and .05, which is legal in all states. (The much greater risk is in combining pot with alcohol.) The increased crash risk with pot alone “is so small you can compare it to driving in darkness compared to driving in daylight,” says University of Oslo political scientist Rune Elvik, who conducted several major meta-analyses evaluating the risk of drugged driving…

When it comes to alcohol, science and the courts have long established a direct line between number of drinks, blood alcohol level, and crash risk. As one goes up, so do the others. Not so for pot. Scientists can’t say with confidence how much pot, in what concentration, used in what period of time, will reliably make someone “high.”…

Blood levels of THC—tetrahydrocannabinol, the chemical component of pot that makes you high—spike quickly after smoking and then decline rapidly in the hours afterwards, during the window when a smoker would feel most high. What’s more, regular smokers could have THC in their blood for days or weeks after smoking, when they are clearly no longer high.

Still, laws in 18 states tie drugged driving charges to whether drivers have THC (or related compounds) in their blood. Some states prohibit driving with any amount, and some specify a threshold modeled after the .08 limit states use for blood alcohol. But the lag time between being pulled over and being transported to a hospital for a blood draw—on average, more than two hours—can lead to false negatives, while the tolerance developed by regular users (and the tendency for THC to stick around in their bloodstreams) can lead to false positives. This is why, researchers say, blood THC laws make little sense… 


Scientific facts, however, have never prevented politicians from passing expedient and politically-popular laws, or police and prosecutors from enforcing them. 
 

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