Medical marijuana may save lives out on the road as well as in clinics, according to a new study.

When examining 19 states that had medical marijuana laws on the books by 2014, researchers found that their average rate of traffic deaths fell 11 percent after the laws were enacted. The happy side-effect wasn’t uniform, however; only seven states saw significant reductions, while two states saw increases. Nevertheless, the authors of the new report in the American Journal of Public Health argue that the data bucks the common criticism that more pot access should increase car crashes and injuries.

Drops in traffic deaths may, in part, be explained by people swapping alcohol for pot, leading to reduced drunk driving, the study’s authors speculated. To back that up, the authors note that the lives spared tended to belong to younger people, particularly 25- to 44-year-olds—an age group frequently involved in alcohol-related traffic deaths.

But, with the laws having such varied effects among the states, there must be other factors at play. For instance, law enforcement strategies, the quality of roadways and public transportation, public programs aimed at curbing driving under the influence (of any substance), plus quality of healthcare may also influence these numbers. The researchers noted that having dispensaries—and thus the potential for easier access to marijuana—also linked to lower rates of traffic deaths.

The seven states with death rate reductions were California, Oregon, Washington, Colorado, Nevada, New Mexico, and Arizona. Rhode Island and Connecticut saw increased traffic death rates.

There were several patterns within these states. For instance, California saw an immediate 16 percent drop in traffic deaths after medical marijuana laws were enacted in 1996. However, the death rate inched back up in subsequent years. In Colorado, on the other hand, medical marijuana laws enacted in 2010 initially had no statistically significant effect, but the state later saw yearly reductions.

The findings were drawn from nationwide census data on traffic fatalities between 1985 and 2014, maintained by the National Highway Traffic Safety Administration, and took into account changes in things like laws on speeding and texting, as well as seat belt enforcement. In the study's time frame, 1.22 million people died in car crashes across the country.

The authors dug into the data amid concerns that even more could die on the road as medical marijuana laws become increasingly popular among states. Currently, 28 states and the District of Columbia have passed medical marijuana legislation. Researchers have previously found that marijuana use while driving causes slowed response time and weaving. Still, data also suggests that marijuana use can make drivers more cautious and slow.

The authors hope that the current study quells some fear while spurring more research into how public policy around marijuana laws can cut traffic deaths across the board.

American Journal of Public Health, 2016. DOI: 10.2105/AJPH.2016.303577 (About DOIs).