Saturday, February 21, 2015

No link between marijuana use and car crashes found in landmark U.S. study

If Marijuana Causes Lots Of Crashes, Why Are They So Hard To Count? - Jacob Sullum, Forbes:

February 12, 2015 - "Last year, during a congressional hearing on the threat posed by stoned drivers, a representative of the National Highway Traffic Safety Administration (NHTSA) was asked how many crash fatalities are caused by marijuana each year. 'That’s difficult to say,' replied Jeff Michael, NHTSA’s associate administrator for research and program development. 'We don’t have a precise estimate.” The most he was willing to affirm was that the number is 'probably not' zero.

"Michael knows something that grandstanding politicians and anti-pot activists either do not understand or refuse to acknowledge: Although experiments show that marijuana impairs driving ability, the effects are not nearly as dramatic as those seen with alcohol, and measuring the real-world consequences has proven very difficult, as demonstrated by a landmark study that NHTSA released last Friday.

"In 'the first large-scale [crash risk] study in the United States to include drugs other than alcohol,' NHTSA found that, once the data were adjusted for confounding variables, cannabis consumption was not associated with an increased probability of getting into an accident....

"The NHTSA study included more than 3,000 drivers who were involved in crashes during a 20-month period in Virginia Beach, Virginia, plus 6,000 controls who drove in the same area during the same period but did not get into accidents.

"As usual, the study found that alcohol use was strongly correlated with crash risk. After adjustment for confounding, the crash risk for drivers with a blood-alcohol content (BAC) of 0.08 percent was twice the crash risk for sober drivers; it was six times as high for drivers with a BAC of 0.10 percent and 12 times as high at a BAC of 0.15 percent.

"But the picture for marijuana was quite different.

"Over all, drivers who tested positive for active THC were 25 percent more likely to be involved in crashes. But once the researchers took sex, age, and race/ethnicity into account, the risk ratio shrank from 1.25 to 1.05 and was no longer statistically significant...

"Further adjusting for alcohol consumption made the crash risk of cannabis consumers equal to that of drivers who tested negative for alcohol and all other drugs. In other words, the analysis, which NHTSA described as 'the most precisely controlled study of its kind yet conducted,' provides no evidence that marijuana use increases crash risk. That result, the authors note, is similar to what the best-designed previous studies have found: a small or nonexistent increase in crash risk."

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