Will traffic deaths rise as states legalize pot?

The legalization of recreational marijuana in two states — Colorado and Washington — and medical marijuana in more than 20 others has raised concern that there will be more drivers stoned behind the wheel. What’s not clear is whether that will translate into an increase in fatal crashes.

o WHAT WE KNOW

While marijuana users can perform simple tasks well while they are high, brain imaging has shown that they have to use more of their brain to do so. Their reaction times are slower, peripheral vision is decreased and multitasking impeded. As a result, when sudden or surprising things occur to complicate those tasks — such as when a pedestrian steps in front of a car — they cannot respond as well. On the other hand, marijuana users tend to be aware they are impaired and try to compensate for it.

o WHAT WE DON’T KNOW

It’s not clear how much marijuana use contributes to crash risk. Some studies have found that marijuana can double crash risk, but others have found virtually no increase.

o HOW HIGH IS TOO HIGH

Traces of THC, the psychoactive ingredient in marijuana, can be detected in the blood of some habitual marijuana users days or weeks after they last used the drug, making it hard to use blood tests to discern a current level of impairment. Most states haven’t set a THC threshold for impairment, but Colorado and Washington have settled on an intoxication blood level of five parts per billion. There’s no roadside test for THC like those for alcohol, but some states are experimenting with a saliva test.

o REAL-WORLD EXPERIENCE

In 2013, the first full year after Washington state legalized pot, nearly 25 percent more drivers tested positive for marijuana than before legalization. But there’s been no corresponding jump in accidents or arrests for intoxicated driving. A University of Colorado Medical School study found that the share of drivers involved in fatal motor vehicle crashes in Colorado who tested positive for marijuana more than doubled between 1994 and 2011. A National Safety Council study looked at the prevalence of drivers involved in fatal crashes who tested positive for marijuana in 12 states from 1992 to 2009 — before and after implementing medical marijuana laws. Only three states showed an increase — California, Hawaii and Washington — and those appeared to be a one-time increase possibly associated with differences in testing.

o WHAT NEXT?

The National Highway Traffic Safety Administration is conducting research to get a better idea of how pot affects driving. NHTSA and Washington state officials have also teamed up to assess change in marijuana use by drivers before and after the state allowed retail sale of the drug, with results due next year.

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 who have studied the issue, 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 they 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 it 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 this past 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.

A roadside survey by the National Highway Traffic Safety Administration in 2007 found 8.6 percent of drivers tested positive for THC, but it’s not possible to say how many were high at the time because drivers were tested only for the presence of drugs, not the amount.

A marijuana high generally peaks within a half hour and dissipates within three hours, but THC can linger for days in the bodies of habitual smokers.

Inexperienced pot smokers are likely to be more impaired than habitual smokers, who develop a tolerance. Some studies show virtually no driving impairment in habitual smokers.

Two recent studies that used similar data to assess crash risk came to opposite conclusions.

Columbia University researchers compared drivers who tested positive for marijuana in the roadside survey with state drug and alcohol tests of drivers killed in crashes. They found that marijuana alone increased the likelihood of being involved in a fatal crash by 80 percent.

But because the study included states where not all drivers are tested for alcohol and drugs, a majority of drivers in fatal crashes were excluded, possibly skewing the results. Also, the use of urine tests rather than blood tests in some cases may overestimate marijuana use and impairment.

A Pacific Institute for Research and Evaluation study used the roadside survey and data from nine states that test more than 80 percent of drivers killed in crashes. When adjusted for alcohol and driver demographics, the study found that otherwise sober drivers who tested positive for marijuana were slightly less likely to have been involved in a crash than drivers who tested negative for all drugs.

“We were expecting a huge impact,” said Eduardo Romano, lead author of the study, “and when we looked at the data from crashes we’re not seeing that much.” But Romano said his study may slightly underestimate the risk and that marijuana may lead to accidents caused by distraction.

Many states do not test drivers involved in a fatal crash for drugs unless there is reason to suspect impairment. Even if impairment is suspected, if the driver tests positive for alcohol, there may be no further testing because alcohol alone may be enough to bring criminal charges. Testing procedures also vary from state to state.

“If states legalize marijuana, they must set clear limits for impairment behind the wheel and require mandatory drug testing following a crash,” said Deborah Hersman, former chairman of the National Transportation Safety Board. “Right now we have a patchwork system across the nation regarding mandatory drug testing following highway crashes.”