Science’s answers to crucial questions about driving while stoned — how dangerous it is, how to test for impairment, and how the risks compare to driving drunk — have been slow to reach the general public.
“Our goal is to put out the science and have it used for evidence-based drug policy” said Marilyn A. Huestis, a senior investigator at the National Institute on Drug Abuse.
It is clear that marijuana use causes deficits that affect driving ability, Dr. Huestis said. She noted that several researchers, working independently of one another, have come up with the same estimate: a twofold increase in the risk of an accident if there is any measurable amount of THC in the bloodstream.
The estimate is based on review papers that considered the results of many individual studies. The results were often contradictory — some of the papers showed no increase in risk, or even a decrease — but the twofold estimate is widely accepted.
The estimate is low, however, compared with the dangers of drunken driving. A recent study of federal crash data found that 20-year-old drivers with a blood-alcohol content of 0.08 percent — the legal limit for driving — had an almost 20-fold increase in the risk of a fatal accident compared with sober drivers. For older adults, up to age 34, the increase was ninefold. The study’s lead author, Eduardo Romano, is a senior research scientist at the Pacific Institute for Research and Evaluation.
The difference in risk between marijuana and alcohol can probably be explained by two things, Dr. Huestis and Dr. Romano both say. First, stoned drivers drive differently from drunken ones, and they have different deficits. Drunken drivers tend to drive faster than normal and to overestimate their skills, studies have shown; the opposite is true for stoned drivers.
The deficits of being stoned really began to show up, she said, when people had to handle multiple tasks at once and were confronted with something unexpected.
Experts like Dr. Romano and Dr. Kleiman believe that facts suggest public resources are better spent combating drunken driving. Stoned driving, they say, is best dealt with by discouraging people from mixing marijuana and alcohol — a combination that is even riskier than alcohol alone — and by policies that minimize marijuana’s risk on the road. For instance, states that legalize recreational marijuana, Dr. Kleiman said, should ban establishments like pot bars that encourage people to smoke away from home.
A version of this article appears in print on February 18, 2014, on page D1 of the New York edition with the headline: Marijuana and the Sobriety Test.