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 fatal crashes 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 laboratory testing indicates 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 devilishly difficult, as demonstrated by a landmark study that NHTSA released on 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 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:

This analysis shows that the significant increased risk of crash involvement associated with THC and illegal drugs…is not found after adjusting for these demographic variables. This finding suggests that these demographic variables may have co-varied with drug use and accounted for most of the increased crash risk. For example, if the THC-positive drivers were predominantly young males, their apparent crash risk may have been related to age and gender rather than use of THC.

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 study provides no evidence that marijuana use increases crash risk. Furthermore, the authors note, that result is similar to what the best-designed previous studies have found: a small or nonexistent increase in crash risk.

Some news outlets, including The Hill and The Detroit News, correctly reported that NHTSA's case-control study found no association between marijuana use and crash risk. But that crucial point seems to have escaped USA Today, which summarized the study this way:

[The study] showed that marijuana users are more likely to be involved in crashes. But it also points out that marijuana is smoked mostly by young men, the group with the highest propensity for accidents anyway.

That is, at best, a highly misleading way of describing the results, since controlling for demographic variables and alcohol use eliminated the association between marijuana and crash risk. The paper compounded the misimpression left by this gloss by highlighting another study that NHTSA released on Friday: the National Roadside Survey of Alcohol and Drug Use by Drivers, which found that the share of weekend nighttime drivers who tested positive for alcohol fell from 12.4 percent in 2007 to 8.3 percent in 2013-14, while the share who tested positive for other drugs rose from 16.3 percent to 20 percent. Marijuana saw the biggest increase, from 8.6 percent to 12.6 percent. USA Today's headline: "Study Finds New Driving Threat From Dopers, Druggies."

Since the case-control study found (once confounders were taken into account) that "dopers" and "druggies" were no more likely to get into crashes than abstainers, what exactly is the nature of this threat that USA Today highlights in its headline? A Washington Post story features a similar error:

There are fewer drunk drivers on the road, but their place has been taken by people high on marijuana and prescription drugs….NHTSA conducted a second study to determine whether smoking marijuana increased the risk of crashes. They found that it did but, adding a caveat, said that pot smoking is most common among a group already at high risk for crashes: young men.

It is simply incorrect to say NHTSA's case-control study found that "smoking marijuana increased the risk of crashes," since the study found no such thing. Furthermore, it's a mistake to describe the drivers who tested positive for any amount of THC as "people high on marijuana." As NHTSA notes, "Drug tests do not necessarily indicate current impairment," since "detectable blood levels may persist beyond the impairing effects." And "whereas the impairment effects for various concentration levels of alcohol in the blood or breath are well understood, there is little evidence available to link concentrations of other drugs to driver performance."

The fact that drivers commonly identified as "drugged" may actually be sober could help explain why it is so hard to measure marijuana's impact on traffic accidents. If many or most of the drivers who test positive for THC are not in fact impaired, they may conceal the impact of those who are. "While the findings of this case control study were equivocal with regard to the crash risk associated with drug use by drivers," NHTSA warns, "these results do not indicate that drug use by drivers is risk-free." In other words, the fact that THC-positive drivers are no more accident prone, as a group, than THC-negative drivers (after adjustment for confounding) does not mean it is smart or safe to get behind the wheel when you are actually stoned.