8.7%: The drop in traffic fatalities in states with medical marijuana laws.

Want to make the road safer? Encourage drivers to hit the bong rather than hit the bars.

That is a policy prescription that few people would recommend. But it is effectively the course that states that have enacted medical marijuana laws have followed, according to a study by economists Mark Anderson at Montana State University, Daniel Rees at the University of Colorado Denver and Benjamin Hansen at the University of Oregon.

The passage of a medical marijuana law tends to increase the number of adults reporting marijuana use, though there are exceptions. Shifting through data from the National Survey on Drug Use, the economists found that more people toked up in Rhode Island and Montana after medical marijuana was legalized, but not in Vermont — perhaps because social norms surrounding pot use in the Green Mountain state were already fairly, um, mellow.

Turning to detailed statistics for 1990 through 2009, the economists found that the 13 states that passed medical marijuana laws during the period saw traffic fatalities fall by 8.7%. That decline was driven by a 12% decrease in the fatalities in crashes where one of the drivers had been drinking.