Does legalized marijuana lead to more car crashes?

By Rayla Bellis
As more states legalize recreational use of marijuana, decision makers are seeking to understand the implications for safety and public policy. A number of recent studies have examined whether states that have legalized marijuana are seeing higher incidence of car crashes and road fatalities, with differing results. While some studies have found that marijuana use could double crash risk according to the Insurance Institute for Highway Safety, others have found no link between legalization of the drug and crashes.
A recent analysis from the Highway Loss Data Institute (HLDI) found that legalization of recreational marijuana in Colorado, Oregon, and Washington has resulted in collision insurance claim frequencies that are approximately three percent higher overall than would be expected without legalization. Colorado and Washington were the first states to legalize recreational marijuana in 2012, and both began retail sales in 2014. Oregon legalized it in November 2014, and sales began in October 2015. Five other states and Washington, D.C., have since legalized recreational marijuana use, and 21 states have comprehensive medical marijuana programs.
HLDI examined collision claims filed between January 2012 and October 2016—before and after legalization—using neighboring states that have not legalized marijuana as controls. The researchers controlled for differences between the states in driver population, insured vehicle fleet, urban versus rural exposure, unemployment, weather, and seasonality. Colorado saw the largest estimated increase in claim frequency after retail marijuana sales began compared with its control states, at 14 percent higher than neighboring Nebraska, Utah, and Wyoming.
On the other hand, a study published by the American Public Health Association found no statistical difference in rates of motor vehicle deaths in states that have legalized marijuana compared with those that have not. The study used the U.S. Fatality Analysis Reporting System to determine the annual numbers of motor vehicle crash fatalities between 2009 and 2015 in Washington, Colorado, and eight control states, comparing year-over-year changes in motor vehicle crash fatality rates (per billion vehicle miles traveled) before and after recreational marijuana legalization.
While appearing to conflict, the results of these two studies do not necessarily directly contradict each other, because they analyzed slightly different things: collision claim frequencies versus vehicle fatalities. Regardless, they point to a need for further study.
Research has also indicated that current approaches for testing marijuana impairment in drivers may be flawed. A recent study from the AAA Foundation for Traffic Safety examined the lab results of drivers arrested for impaired driving. The AAA Foundation notes that unlike alcohol, where it is clear that crash risk increases significantly at higher BAC levels, there is no conclusive research indicating that drivers consistently become impaired at a specific level of marijuana in the blood. Depending on the individual, drivers with relatively high levels of THC metabolites in their system may not be impaired, while others with low levels may be unsafe to drive. Frequent users can also exhibit lingering high levels of THC metabolites long after use, while the presence of THC in the blood can decline more rapidly among occasional users.
This suggests that establishing legal limits specifying the maximum amount of active THC that drivers can have in their systems may not be a viable approach for preventing crashes. AAA is urging states to use more comprehensive enforcement measures to improve road safety, including seeking behavioral and physiological evidence of driver impairment.
Rayla Bellis is a Program Manager at SSTI.