Amrican Public Health Association (APHA)'s Public Health Journal reported that U.S. states that decriminalized marijuana show an increase of fatal traffic crashes in those cities. 

Researchers examined 2010-2017 city data from the Fatality Analysis Reporting System, a census of crash deaths that included drug or alcohol levels in drivers’ blood.

States with decriminalization marijuana laws enacted within the time period showed a 13% increase in fatal car crashes among 15- to 24-year-old males in their cities, the study said. Crashes in which a driver tested positive for marijuana were more frequent in the months immediately after decriminalization. There was no increase in fatal car crashes among women of any age and men above age 24.

The same study method was applied to states with medical marijuana laws. In those states, researchers found a decrease of fatal car crashes. The decline occurred across both genders and all age groups, the most pronounced being a 14% decrease among 15- to 24-year-old males.

Researchers said the difference may be linked to the ways marijuana policies influence behavior. Medical marijuana laws require that the drug be taken at home, so people are less likely to be driving while under the influence, researchers said. But in the states with decriminalized marijuana, drivers can use the drug whenever they want, including before or while driving.

Marijuana policies have significant effects on the number of fatal car crashes involving young men, the researchers concluded.