The University of Colorado-Denver looked into fatalities in the 16 states that have legalized medical marijuana and unearthed perplexing results: The states saw an average nine percent drop in traffic deaths since their medical marijuana laws took effect.

“We went into our research expecting the opposite effect,” says study co-author Daniel Rees, a professor of economics at the University of Colorado-Denver. “We thought medical marijuana legalization would increase traffic fatalities. We were stunned by the results.” It even stunned Insurancequotes.com, which printed the story.

Several factors seem to influence this:

People who smoke dope drink less. The Beer Institute says beer purchases go down by an average of 5 percent after medical marijuana laws are passed.

Stoned people drive more carefully: A clinical trial conducted in Israel compared the simulated driving skills of people who’d consumed alcohol and those who’d smoked marijuana. The researchers found that alcohol caused these people to speed up their driving, while smoking marijuana prompted the drivers to slow down.