At least 20 states could pass laws this year legalizing medical or recreational marijuana use. As many as 11 states will vote on marijuana on ballot measure in this fall's election.
While legalizing marijuana will almost certainly decrease the number of nonviolent offenders held in custody by law enforcement, legal pot presents a new challenge for authorities in the form of impaired drivers under the influence of marijuana.
But current legal limits for marijuana imposed on motorists are arbitrary and unscientific, according to research by the AAA Foundation for Traffic Safety. Because law enforcement agencies are operating under a flawed framework, innocent individuals could potentially be wrongly convicted of impaired driving while genuinely unsafe motorists go free.
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The study also found that fatal car accidents involving marijuana more than doubled once cannabis became legal within a state, suggesting an urgency to create a standard to measure drug-impaired driving.
While many states where marijuana is illegal have zero tolerance policies, six states, Colorado, Montana, Nevada, Ohio, Pennsylvania and Washington, set limits for THC, the psychoactive chemical found in cannabis, for impaired driving. The regulations mirror legal limits established for blood alcohol content (BAC) levels.
Though both are held to a similar standard, marijuana and alcohol are two entirely different substances. No research has reliably pinned down how high THC levels need to be to cause impairment, the AAA notes. Crash risk with higher BAC level is scientifically supported.
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Marijuana also isn't metabolized like alcohol. THC can linger in the bloodstream for days or weeks after a driver has used, so a suspected offender could be beyond the legal limit long after he or she has used marijuana. A 2013 study published in the journal Clinical Chemistry found that THC levels are still detectable in the blood of heavy smokers even after a month-long abstinence.
The rise in fatal car accidents in Washington after marijuana only reinforces the need for drug-driving enforcement. Between 2013 and 2014, the percentage of drivers who recently used marijuana in such accidents jumped 8 to 17 percent, though the study authors caution that these drivers were not necessarily impaired or at fault for the crash.
"Marijuana can affect driver safety by impairing vehicle control and judgment," Marshall Doney, President and CEO of AAA, said in a statement. "States need consistent, strong and fair enforcement measures to ensure that the increased use of marijuana does not impact road safety."
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Previous research has shown that drivers who test positive for marijuana are more likely to be involved in accidents. A 2011 Columbia University study found that motorists who either test positive or report driving within three hours of using were more than twice as likely as other drivers to be involved in car crashes.
As with any major public policy shift, getting used to legal marijuana will require both law enforcement and ordinary citizens to adapt. Even if the science isn't in on just who exactly is "too high to drive," using a drug and then getting behind the wheel is a bad idea whether marijuana is legal or not.