So, which does reduce traffic fatalities more: seatbelts or marijuana?
Ok, that's not putting it exactly right. The question is really about laws, and the relative effects of laws that require passengers in cars to wear seatbelts compared to laws that allow people with glaucoma to ease their suffering with toke or two off a (medially prescribed) fatty.
And that's not really putting it right either. My real interest is in how moral intuitions, especially moral intuitions that are formalized in law, lead to consequences at larger scales. This sort of idea connects directly back to work, especially in anthropology, that looks at how different moral norms, widely shared in a group, influence their own spread, often through influencing how well the group does economically. More generally, moral norms have lots of interesting and often unintended effects. So, you know, if people think that's it's morally wrong to kill cows, then one effect of this is that you wind up with cows wandering around everywhere, eventually starving to death because there's so many (sacred) cows.
One set of moral intuitions I find really interesting is those surrounding recreational drugs, and a recent paper looks at what I thought was a really interesting example of some downstream consequences of changing the rules about their use (hat tip: Geoffrey Miller). The piece, by Mark Anderson and Daniel Rees (the former at Montana State, the latter at University of Colorado Denver), still a working paper, investigates a non-obvious relationship between legalization of medical marijuana and traffic fatalities. (I mention their affiliation because the authors are from two very relevant states in this issue. It turns out that Montana has an impressive fraction of users ("Whoa, dude... the sky here really is big,") and, as they put it: "Denver has more marijuana dispensaries than liquor stores or Starbucks coffee shops.")
Here's how the story goes. Suppose that it's true - and there is debate about this - that when pot is more readily available, alcohol consumption goes down. That is, the two are substitutes, as economists would say; when people use more pot, they use less alcohol. (The alternative possibility is that they are complements, that increased use of one increases the use of the other.)
Now suppose that it's also true that people who consume alcohol are more likely to get into fatal car crashes than people who consume pot. Andersen and Rees talk a little bit about this, saying that "drivers under the influence of marijuana reduce their velocity," a point which resonates with my memory of a friend of mine who was pulled over for doing fifteen in a sixty-five. Anyway, if that's true, then to the extent that pot is used more, and there is substitution for alcohol, the result is more slow-moving safe (?) drivers and fewer (dangerous) drunk drivers.
To look at this question, they looked at data from states that legalized the medical use of marijuana. In such states, use of marijuana does increase according to poll data, and there is evidence of a concurrent reduction in alcohol use. (Insert usual caveats regarding causality here, and please see the paper for details. Also make sure you check out how many people in Montana seem to enjoy the wacky weed. Also, as an aside, I've heard the objection that a big difference between driving drunk and driving stoned is that there's a cheap test, the breathalyzer, for alcohol, but no equivalent for pot. To this I would reply that there's an even cheaper test to see if someone is high, which is to have a little conversation, take a brief pause, and then hit them with: "What were we just talking about?" If they can't answer...)
Anyway, to the point, they conclude, to put it in their words in the abstract, "legalization is associated with a nearly 9 percent decrease in traffic fatalities, most likely to due to its impact on alcohol consumption." And, to return to the point at the beginning of the post here, they cite a study (Carpenter & Stehr, 2008) that "found that mandatory seatbelt laws decrease traffic fatalities among 14- through 18-yearolds by approximately 8 percent" (p. 19).
Of course there are many other questions raised by all these issues. For instance, what other side effects of the change in legal regime are there? After medical marijuana is legalized, does the consumption of Doritos go up? More seriously, what would the effect of decriminalization be? How many people are dying on America's highways because marijuana use is seen as bad bad bad but alcohol use is, shmeh, fine?
Ok, gotta go... glaucoma acting up again...
Anderson, D. M., & Rees,D.I. (2011). Medical Marijuana Laws, Traffic Fatalities, and Alcohol Consumption. IZA Discussion Paper No. 6112