Legal marijuana could soon be on its way to Massachusetts, if advocates have their way in the November election. Opposition to the idea is stiff, but support may be just strong enough to get it through.

Traffic Stop, Massachusetts State PoliceWith that possibility looming, experts are busy debating a question that always goes along with legalization: How high is too high to drive? And how do we measure what constitutes “too high?”

Massachusetts prosecutors were seeking answers to those questions in May, after a driver bought medical marijuana an hour before plowing into a State Police cruiser and killing the trooper inside it. The driver was stoned at the time, authorities said.

But cannabis is a decidedly different drug from alcohol, and measuring its concentration in the bloodstream is not so simple. The amount of alcohol present in the bloodstream diminishes rapidly, as do the effects. But the THC metabolites left in the wake of marijuana use can linger far longer than any degree of impairment.

Metabolites remain in body long after use

In other words, motorists who are completely sober could nonetheless face criminal charges for driving under the influence of drugs. This is a problem legalization advocates want to avoid, in Massachusetts and elsewhere.

And the two drugs affect the brain in different ways, said Dr. Kevin Hill, assistant professor of psychiatry at McLean Hospital and Harvard Medical School. They both impair motorists, but their effects on driving aren’t the same.

There is no “Breathalyzer” for marijuana intoxication. Scientists are working on a device that could serve the same purpose, but as of yet they have no way to detect THC in the body short of a blood test conducted at a police station – a costly and time-intensive way to detect impaired drivers.

The crash happened in May, prosecutors said, when David Njugana allegedly crashed into Trooper Thomas Clardy’s cruiser while in an “impaired state,” killing the trooper while he was sitting inside the cruiser. Njugana had legally bought at least three joints that day, one of which was found half-burnt in his car.

Impaired driver charged with multiple felonies

High DrivingPolice found THC in the driver’s blood and charged him with multiple felonies, including manslaughter and motor vehicle homicide while operating under the influence of drugs. He pleaded not guilty to all charges. Clardy, 44, was married and a father of seven.

Hill noted that while drunk drivers tend to make “errors of omission” (failing to check the rear view mirror), stoned drivers typically commit “errors of commission” (driving too slow).

“If you’re drunk, you run red lights,” he said. “If you are stoned, you stop at green lights.”

It’s unclear what effect these differences have on injury or fatality rates, but crashes involving marijuana are much less common than those involving alcohol. There are likely multiple reasons for this, including the far wider availability of alcoholic beverages.

There remains a good deal of debate in different states over what degree of THC concentration should qualify as “impairment.” Some states, such as Arizona, impose laws that treat any amount of THC, even inactive metabolites that no longer contribute to intoxication, as grounds for arrest. Others count any amount of active metabolites. Still others set specific limits measured in nanograms.

Massachusetts has no law defining exactly what constitutes marijuana impairment on the road.

Let us know: How do you think Massachusetts and other states should deal with high driving? Leave a comment below.


Please enter your comment!
Please enter your name here