A report on drug-impaired driving finds that the relationship between a drug’s presence in the body and its effect on driving “are complex, not understood well, and vary from driver to driver.”

Though the report claims that drivers involved in fatal crashes are now more likely to have used legal or illegal drugs than alcohol, it warns that the “drug data should be interpreted with caution.” As noted by the report authors, data on the presence of drugs in crash-involved drivers are incomplete in most jurisdictions, inconsistent from state to state, and sometimes inconsistent across jurisdictions within states. In addition, the report finds that testing standards have been adapted in responses to changes in state marijuana laws. As such, data cannot be reliably compared across years or jurisdictions.

Additionally, these data are from the National Highway Traffic Safety Administration’s Fatality Analysis Reporting System, which come with a disclaimer stating that they only show the prevalence of marijuana use in tested drivers. The data tell us nothing about impairment.

It would not be surprising if more people 21 years of age and older are using marijuana in the states that have legalized, as these data might suggest. They do not, however, tell us that there are more impaired drivers on the road. That question has not yet been analyzed.

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Researchers widely accept that a drug may still be present in a driver’s system long after the impairment effects of that drug have worn off. For instance, marijuana may be detected hours, days, or even weeks after use, long after impairment has dissipated. To make an analogy, this would be like an alcohol test that showed us whether a person has consumed a glass of wine or a beer within the past week or two. It would tell us nothing about whether the person was safe to drive.

Moreover, the extent to which marijuana use impairs driving is unclear. While research shows a clear correlation between alcohol and crash risk, marijuana studies demonstrate that THC’s effects on crash risk are ambiguous. Some marijuana impairment studies demonstrate that THC is only associated with a relatively small or uncertain increase in crash risk. Others have demonstrated that heavy marijuana users experience fewer performance impairments than occasional users. Some studies show that marijuana alone does not lead to any increase in crash risk. Still others report contradictory conclusions on the relationship between THC intoxication and driving impairment.

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What we do know so far is that road safety has remained stable in states that have legalized marijuana. Data from Washington, Colorado, Oregon and Alaska (the first 4 states to legalize marijuana) show that traffic fatality rates have remained statistically unchanged post-legalization.

In fact, in Washington and Colorado, the states that have experienced legalization the longest, traffic fatality rates are lower than they were a decade prior and are below the national average for traffic fatalities (see chart from the National Highway Traffic Safety Administration).

In the interest of road safety, we should focus state and federal resources on better understanding the relationship between drug use and crash risk. When California voters approved Proposition 64: the Adult Use of Marijuana Act, legalizing marijuana in the state, they also voted to dedicate marijuana tax revenues to researching how to define and measure marijuana impairment in drivers.

More money and resources need to be invested in similar research projects so that we can establish reliable protocols for measuring marijuana and other drug-impairment in drivers.

Jolene Forman is a staff attorney with the Drug Policy Alliance.