WASHINGTON — As states liberalize their marijuana laws, public officials and safety advocates worry that more drivers high on pot will lead to a big increase in traffic deaths. Researchers, though, are divided on the question.
Studies of marijuana's effects show that the drug can slow decision-making, decrease peripheral vision and impede multitasking, all of which are critical driving skills. But unlike with alcohol, drivers high on pot tend to be aware that they are impaired and try to compensate by driving slowly, avoiding risky actions such as passing other cars, and allowing extra room between vehicles.
On the other hand, combining marijuana with alcohol appears to eliminate the pot smoker's exaggerated caution and seems to increase driving impairment beyond the effects of either substance alone.
"We see the legalization of marijuana in Colorado and Washington as a wake-up call for all of us in highway safety," said Jonathan Adkins, executive director of Governors Highway Safety Association, which represents state highway safety offices.
"We don't know enough about the scope of marijuana-impaired driving to call it a big or small problem. But anytime a driver has their ability impaired, it is a problem."
Colorado and Washington are the only states that allow retail sales of marijuana for recreational use. Efforts to legalize recreational marijuana are underway in Alaska, Massachusetts, New York, Oregon and the District of Columbia. Twenty-three states and the nation's capital permit marijuana use for medical purposes.
It is illegal in all states to drive while impaired by marijuana.
Colorado, Washington and Montana have set an intoxication threshold of 5 parts per billion of THC, the psychoactive ingredient in pot, in the blood. A few other states have set intoxication thresholds, but most have not set a specific level. In Washington, there was a jump of nearly 25 percent in drivers testing positive for marijuana in 2013 — the first full year after legalization — but no corresponding increase in car accidents or fatalities.
What worries highway safety experts are cases like that of New York teenager Joseph Beer, who in October 2012 smoked marijuana, climbed into a Subaru Impreza with four friends and drove more than 100 mph before losing control. The car crashed into trees with such force that the vehicle split in half, killing his friends.
Beer pleaded guilty to aggravated vehicular homicide and was sentenced last week to 5 years to 15 years in prison.
A prosecutor blamed the crash on "speed and weed," but a Yale University Medical School expert on drug abuse who testified at the trial said studies of marijuana and crash risk are "highly inconclusive." Some studies show a two- or three-fold increase, while others show none, said Dr. Mehmet Sofuoglu. Some studies even showed less risk if someone was marijuana positive, he testified.
Teenage boys and young men are the most likely drivers to smoke pot and the most likely drivers to have an accident regardless of whether they're high, he said.
"Being a teenager, a male teenager, and being involved in reckless behavior could explain both at the same time — not necessarily marijuana causing getting into accidents, but a general reckless behavior leading to both conditions at the same time," he told jurors.
In 2012, just over 10 percent of high school seniors said they had smoked pot before driving at least once in the prior two weeks, according to Monitoring the Future, an annual University of Michigan survey of 50,000 middle and high school students. Nearly twice as many male students as female students said they had smoked marijuana before driving.