Opinions about the legality of medical marijuana usually hinge on questions about whether marijuana is a gateway drug, or whether it encourages recreational drug use. Now a controversial new paper (unpublished and not peer-reviewed, but released for discussion), has added more smoke to the debate.

According to authors D. Mark Anderson and Daniel I. Rees, the passage of medical marijuana laws in 16 states have also helped to reduce traffic deaths. Can that be right? Does weed really reduce the speed?

The authors base their claim on statistics assembled using the the Fatality Analysis Reporting System, or FARS, which is a database collected by the National Highway Traffic Safety Administration. Across the 16 states where medical marijuana has been legalized, it was found that traffic fatalities dropped by as much as 9 percent.

The FARS statistics do not prove that medical marijuana laws are the cause of the decrease in traffic deaths (such statistics can only provide correlation), but until now no research has been conducted to explain the relationship. Anderson and Rees offer up a couple of theories.

First, they suggest that marijuana is a substitute for alcohol consumption. Basically, if medical marijuana laws create an uptick in marijuana use, then perhaps people are smoking grass rather than imbibing in the drink. This should, in turn, lead to less drunk driving, which explains the drop in traffic deaths.

But are marijuana and alcohol substitutes? The authors looked at data from the Behavioral Risk Factor Surveillance System and found that medical marijuana laws are associated with decreases in the number of drinks consumed, especially among 20- through 29-year-olds. Furthermore, after looking over data from the Beer Institute, they found that beer sales tend to fall after medical marijuana laws go into effect.

This first theory also relies on the assumption that driving drunk is more dangerous than driving high. For this, the authors note evidence that high drivers "reduce their velocity, avoid risky maneuvers, and increase their 'following distances,' suggesting compensatory behavior." A fair amount of other research also validates this claim. (Read this story for more on this.)

A second, purely speculative hypothesis suggests that because medical marijuana use is still not permitted in public, users stay at home to smoke rather than go out, thus getting in fewer accidents by default. If this theory panned out, it would at least provide reason for keeping marijuana use private.

While none of the evidence cited by the study is definitive (it's doubtful that proponents of medical marijuana legislation will begin citing driving safety as a key arguing point any time soon), it does provide a compelling anecdotal case. Perhaps, with more study, seat belt safety laws will end up finding a strange and unexpected bedfellow in medical marijuana laws.

Bryan Nelson ( @@brynelson ) writes about everything from environmental problems here on Earth to big questions in space.

Do medical marijuana laws decrease traffic deaths?
A controversial new paper claims that the passage of states’ medical marijuana laws have reduced traffic fatalities.