In
California, Hawaii and Washington, the percentage of fatally injured drivers
with cannabis in their systems grew an average 6.6 percent after medical marijuana laws increased access to the drug. Now
with the legalization of recreational marijuana in Colorado and Washington, the
question of how cannabis impacts driving is even more immediate. Opinion and
evidence have been mixed.

For example,
in what it calls a review of the scientific evidence, the marijuana advocacy
website Norml writes that, “Marijuana
has a measurable yet relatively mild effect on psychomotor skills, yet it does
not appear to play a significant role in vehicle crashes.” And other studies show that many marijuana users think
that use of the drug actually improves driving skill. But then when you look at
the brain chemistry, the journal Clinical Chemistry writes that, “Blood THC concentrations of 2–5
ng/mL are associated with substantial driving impairment, particularly in
occasional smokers.”

Which
is it? Does marijuana help, hurt or leave unchanged a person’s driving ability?

A study in progress at the Journal of Safety Research pulled 72 young, male marijuana users
into the driving simulator to try to answer some of these questions. First, the
study asked how often subjects “drove within the hour following cannabis use in
the previous 12 months?” Then it asked participants about their risky driving
habits: how risky did these people consider their own driving and how many
traffic tickets had they gotten for things like speeding or failing to stop at
a light or sign? Finally, the study put these 72 subjects in a driving
simulator to see how “risky” they really were in simulated everyday situations.

The
more often a person had driven under the influence of cannabis, the riskier
were their driving behaviors and the more traffic tickets they had earned. The
researches write that, “Taken together, these results indicate that
self-reported driving under the influence of cannabis is associated with a risky
driving style including a broad range of reckless on-road behaviors and support
the problem driving behavior theory.”

Then another driving simulator study gave subjects THC cigarettes – how
would they perform while high compared to how they performed sober? The results
are, well, sobering. Even with low consumption, “increase in THC dosage alone
influences perception of what is a safe distance to leave between cars” (among
other risky driving behaviors). This study also happens to show that the
addition of alcohol to THC is especially dangerous – the sum of these two drugs
created about 20 percent more dangerous driving behaviors than either drug
alone.

So the
answer to whether marijuana impairs driving ability is yes, it does. Not only
are people who drive under the influence of marijuana more likely to drive in
risky ways, but it’s the marijuana and not their personalities alone that
create this risky driving.

Now in
the era of legal recreational marijuana, stay tuned for research like that in
the wake of medical marijuana exploring the percentage of fatally injured
drivers with measureable blood THC.

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Richard
Taite is founder and CEO of Cliffside Malibu, offering evidence-based,
individualized addiction treatment based on the Stages of Change model. He is
also co-author with Constance Scharff of the book Ending
Addiction for Good.