A 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 stoned driver, the New York Times reports. In a 2012 study in the journal Psychopharmacology, only 30 percent of people under the influence of THC, the active ingredient in marijuana, failed the field test. Iits ability to identify a stoned driver seems to depend heavily on whether the driver is accustomed to being stoned. As more states legalize medical and recreational marijuana, 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 Huestis of the National Institute on Drug Abuse. “But I think it's a mishmash.” A 2007 study found that 12 percent of the drivers randomly stopped on Friday and Saturday nights had been drinking. Six percent of the drivers tested positive for marijuana — a number that is likely to go up with increased availability. Some experts and officials are concerned that the campaign against drunken driving has not gotten through to marijuana smokers.