As more states across the country have decriminalized marijuana, law enforcement and courts face a new challenge: how to regulate drivers who use the drug. 

Last November, Washington and Colorado residents voted to legalize recreational use of marijuana, and Massachusetts joined the growing number of states that have legalized medical marijuana. 

Massachusetts General Laws ch. 90 sec. 24 prohibits driving while under the influence of marijuana, just like driving under the influence of alcohol with a BAC of 0.08 or higher.  But, there are two issues that complicate the regulation of marijuana on roadways: how do authorities accurately measure whether a driver is too “high” to drive, and does using marijuana inhibit someone from driving safely in the first place?

Impaired Driving on Marijuana

Regardless of how one feels about legalizing recreational marijuana, most people probably initially think “Of course pot impairs your driving ability, and driving high should be illegal.”  But there appears to be some legitimate evidence to the contrary.  Marijuana does mildly impair psychomotor skills, and studies reveal that this tends to cause people to drive and react slower, but that it may not significantly increase the chances of a car wreck. 

Specifically, one study noted that drivers who took large doses of cannabis (1) drove more cautiously, (2) drove with “increased variability in lane position,” and (3) reacted more slowly.  The researchers noted that although these changes deviated from typical driving behavior, they did not indicate that marijuana impairs driving ability because few studies show increased risk of accidents.     

Nevertheless, another study suggested a very different conclusion: that using marijuana nearly doubled the chances of being involved in a car accident, and studies also suggest it takes longer for stoned drivers to hit the brakes. 

Detecting Marijuana Impairment

Now the question is raised of how authorities measure marijuana in a driver’s system. Currently, if a police officer suspected a driver had been using pot (e.g., if the car smelled of marijuana), he would likely administer a field sobriety test.  But this test mostly judges a motorist’s coordination and balance, which marijuana doesn’t significantly impair.  There is a somewhat more sophisticated procedure to determine drug intoxication, (a “Drug Recognition Expert Examination”) but many officers are not trained to conduct it, and it still doesn’t have the reliability that BAC tests do regarding alcohol.   

Further, blood tests for marijuana do not accurately reflect that a person is under its influence in the way that BAC tests can show that someone is drunk.  Cannabis can be detected in users’ blood weeks after they use it.  It seems unfair to punish someone for driving under the influence because the person had marijuana in her/ his system when she/ he had used 2 weeks before and was completely sober when driving.  Still, some people advocate a zero-tolerance policy that would apply to this exact scenario. 

Washington set a marijuana blood limit while driving at 5 nanograms per milliliter, but the same measure failed to pass in Colorado.  

Moving Forward

Without reliable testing, police and courts in Massachusetts will face a number of difficulties prosecuting OUI marijuana cases.  Police aren’t trained to detect chemicals in drivers’ blood (except for alcohol), and would have a tough time even showing you were intoxicated on marijuana with a field sobriety test.  Further, the defense could argue that marijuana does not significantly impair driving ability, unlike alcohol.  This could also make the prosecution’s case more difficult because it might have to hire experts to argue the contrary.  Without strong, incriminating evidence, these cases should prove difficult to prosecute.