NEW YORK — If you are pulled over on suspicion of drunken driving, the police officer is likely to ask you to complete three tasks: Follow a pen with your eyes while the officer moves it back and forth; get out of the car and walk nine steps, heel to toe, turn on one foot and go back; and stand on one leg for 30 seconds.
This standard field sobriety test has been shown to catch 88 percent of drivers under the influence of alcohol. But it is nowhere near as good at spotting a driver under the influence of marijuana.
In a 2012 study published in the journal Psychopharmacology, only 30 percent of people under the influence of THC, the active ingredient in marijuana, failed the field test. And its ability to identify a stoned driver seems to depend heavily on whether the driver is accustomed to being stoned.
A 21-year-old on his first bender and a hardened alcoholic will both wobble on one foot. But the same is not necessarily true of a driver who just smoked his first joint and the person who is high five days a week. In another study, 50 percent of the less frequent smokers failed the field test.
As more states legalize medical and recreational marijuana, distinctions like these will become more important. But 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 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.
A 2007 study found that 12 percent of the drivers randomly stopped on US highways on Friday and Saturday nights had been drinking. (In return for taking part in the study, intoxicated drivers were told they would not be arrested, just taken home.) Six percent of the drivers tested positive for marijuana — a number that is likely to go up with increased availability. Some officials are concerned the campaign against drunken driving has not gotten through to marijuana smokers.
“We’ve done phone surveys, and we’re hearing that a lot of people think DUI laws don’t apply to marijuana,” said Glenn Davis, highway safety manager at the Department of Transportation in Colorado, where recreational marijuana use became legal on Jan. 1.
Some researchers say that limited resources are better applied to continuing to reduce drunken driving.
Still, it is clear that marijuana use causes deficits that affect driving ability, Huestis said. She noted that several researchers, working independently, 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 low compared with the dangers of drunken driving. One 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 twentyfold increase in the risk of a fatal accident.