Pubdate: Tue, 24 May 2016
Source: Boston Globe (MA)
Copyright: 2016 Globe Newspaper Company
Contact: http://services.bostonglobe.com/news/opeds/letter.aspx?id=6340
Website: http://bostonglobe.com/
Details: http://www.mapinc.org/media/52
Author: Kay Lazar

HOW MUCH MARIJUANA IS TOO MUCH FOR DRIVERS?

Just how much marijuana is too much when getting behind the wheel? Is 
any amount safe?

Those questions emerged anew after prosecutors alleged last week that 
a driver had visited a medical marijuana dispensary an hour before 
his car struck a State Police cruiser, killing the trooper inside.

That motorist, who was authorized to use marijuana for medical 
purposes and had traces of the drug detected in his blood, was 
impaired, authorities said.

But gauging marijuana impairment is not clear-cut. The risks from 
drinking and driving have been studied for years, but the data about 
marijuana's effect on motorists are considerably less robust.

Scientists know that both marijuana and alcohol can impair driving 
skills, said Dr. Kevin Hill, an assistant professor of psychiatry at 
McLean Hospital and Harvard Medical School. But the drugs affect the 
brain differently.

With too much alcohol, drivers are likely to make "errors of 
omission," such as not checking their rearview mirror, while 
marijuana users tend toward "errors of commission," such as driving 
too slowly, Hill said.

"If you're drunk, you run red lights," he said. "If you are stoned, 
you stop at green lights."

There is no reliable breath test that detects THC, the ingredient 
that causes marijuana's high - no routine standard akin to the limit 
of 0.08 blood alcohol concentration used to define drunken driving.

And there is little agreement among scientists about the level of THC 
in someone's blood that signals impairment.

Washington and Colorado are among a handful of states that have 
adopted specific measures - 5 nanograms of THC in blood levels - to 
define impaired driving from marijuana use. Massachusetts has no such law.

Massachusetts also does not separately track arrests involving 
drivers impaired specifically by THC, according to the state 
Department of Transportation. (The state does record driving arrests 
that involve any drug use, from marijuana to opioids, and that number 
is increasing.)

Washington state, which began recreational-marijuana sales nearly two 
years ago, tracks THC-involved accidents and found that fatal 
accidents involving "marijuana-positive" drivers climbed from 58 in 
2013, the year before recreational sales were allowed, to 92 last 
year, according to preliminary data.

Authorities in Washington caution that the state's blood testing for 
THC does not distinguish between active levels and what's known as 
"carboxy THC," the chemical that lingers long after the marijuana 
high has dissipated.

"As such," a report from the state concluded, "the actual impairment, 
or contributing crash effect by marijuana, if any, is unknown."

The driver implicated in the Massachusetts trooper's death in March 
had legally purchased at least three marijuana cigarettes that day, 
and one was found half-burnt in his car after the crash.

Prosecutors last week said 30-year-old David Njuguna had THC in his 
blood and was in an "impaired state" when his Nissan Maxima hit the 
parked cruiser on the Massachusetts Turnpike, killing Trooper Thomas 
L. Clardy, 44, a married father of seven.

Njuguna has pleaded not guilty to several charges, including 
manslaughter and motor vehicle homicide while operating under the 
influence of drugs.

Authorities did not reveal how much THC was in Njuguna's blood, or 
disclose the medical condition that led to him being authorized to 
use medical marijuana. Officials also did not say how often Njuguna 
used the drug.

Determining impairment in marijuana users is difficult because THC 
lingers in a person's bloodstream for days, so regular users may have 
measurable amounts of the chemical in their blood but may not be 
impaired, said Hill, the McLean psychiatrist.

By comparison, alcohol declines in the bloodstream much more quickly, 
said Hill, an addiction specialist who researches marijuana.

The 5 nanogram blood THC limit used in Colorado, one of the few 
states where marijuana is legal for medicinal and recreational use, 
may not be a reliable indicator of who is too stoned to drive, Hill said.

"That blood level of cannabis is not a good proxy of determining 
impairment," Hill said. "If I used a lot of marijuana Tuesday night, 
I may still have a blood level above that threshold on Thursday, but 
I may not be impaired to drive."

A study released earlier this month by the AAA Foundation for Traffic 
Safety attempted to establish a benchmark for THC impairment, but 
concluded that such a threshold could not be "scientifically supported."

As researchers work to establish a reliable measure, more states are 
legalizing marijuana.

The drug is allowed for medicinal use in at least 23 states and the 
District of Columbia, and a few states, including Colorado and 
Washington, have made recreational use legal.

The first medical marijuana dispensary in Massachusetts opened in 
June, and voters may be asked in November whether marijuana should be 
allowed for recreational use for adults older than 21.

Use of marijuana among teen drivers is a particular concern. 
Campaigns to reduce drunken driving have decreased the number of 
teenagers nationwide who drink and drive, data show. But some public 
health officials now worry the increasing number of states legalizing 
marijuana for medical use may translate to more teens smoking and driving.

These younger drivers, whose brains are still developing, are more 
likely to be distracted behind the wheel, and marijuana use 
accentuates distractions, said Marisa Silveri, an associate professor 
of psychiatry at Harvard Medical School.

"It's divided attention, for kids who already have less ability to 
inhibit the things that get their attention, [marijuana] makes it 
more dangerous for them," said Silveri, a McLean Hospital researcher 
who studies the effects of marijuana and drug use on young brains.
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