One would think car crash rates would be higher, especially among younger drivers, in states where people are smoking more pot. But researchers at Columbia University’s Mailman School of Public Health found an 11 percent reduction in traffic fatalities on average in medical-marijuana states.

The study also found the presence of medical-marijuana dispensaries correlated with fewer traffic fatalities.

Dr. Silvia Martins, faculty member in the Epidemiology Department at Columbia’s Mailman School of Public Health and the study’s senior author, theorized that lower traffic fatality rates in legal pot states might be related to reduced levels of drunk driving, as young people substitute weed for booze.

“We found evidence that states with the marijuana laws in place compared with those which did not, reported, on average, lower rates of drivers endorsing driving after having too many drinks,” Martins said in a written statement, reported the Washington Post.

There was little evidence of a reduction in traffic fatality rates for people 45 or older, who are disproportionately registered in larger numbers in state medical-marijuana programs.

The largest drop in traffic fatality rates in states with medical-marijuana laws occurred among drivers between 15 and 44, according to the study.

Researchers took into account factors such as whether states had graduated driver licensing laws, household income, unemployment rates, state speed limits, seat belt laws and bans on cellphones and texting while driving.

The study was carried out as more states passed medical-marijuana laws—nine between 2010 and 2014—and concerns rose about the possible effect on public safety.

Previous research has shown that driving while stoned has a measurable but relatively mild effect on psychomotor skills, yet it does not appear to play a significant role in vehicle crashes, particularly when compared to alcohol.

Past studies have found that drivers who were high tended to reduce their speed and increase headways—suggesting that stoned drivers are aware of their impairment and make an effort to compensate.

The recent Colombia University study, published online in the American Journal of Public Health, analyzed National Highway Traffic Safety Administration data on traffic fatalities from 1985 to 2014.

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