Where the Breathalyzer won't help

  • New York Times
  • February 17, 2014 - 9:37 PM


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; walk nine steps, heel to toe, turn on one foot and go back; and stand on one leg for 30 seconds.

Score well on all three of these events, and there’s a very good chance that you are not drunk. 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 stoned driver.

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 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 stoner 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 grow 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 general public. “Our goal is to put out the science and have it used for evidence-based drug policy,” said Marilyn Huestis, a senior investigator at the National Institute on Drug Abuse.

A 2007 study found that 12 percent of the drivers randomly stopped on U.S. highways 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. “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. “And there’s always somebody who says, ‘I drive better while high.’ ”

Evidence suggests that is not the case. But it also suggests that we may not have as much to fear from stoned driving as from drunken driving.

Still, it is clear that marijuana use causes deficits that affect driving ability, Huestis said. She noted that several researchers 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.

Meanwhile, a 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 compared with sober drivers. For older adults, up to age 34, the increase was ninefold.

Eduardo Romano, a senior research scientist at the Pacific Institute for Research and Evaluation, and Huestis said the difference in risk can probably be explained by two things. 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.

Huestis also found that in laboratory studies, most people who were high could pass simple tests of memory, though they had to use more brainpower than sober people. People who were drunk were much more likely to fail.

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