[Originally to be titled “Marijuana: I Was Wrong”, but looking back I was suitably careful about everything, and my reward is not having to say that.]

Five years ago, I reviewed the potential costs and benefits of marijuana legalization and concluded that there wasn’t enough evidence for a firm conclusion. I found that using some made-up math, the effects looked slightly positive, but this was very sensitive to small changes in how made-up the math was.

The only really interesting conclusion was that most of the objective costs or benefits of legalization came from road traffic accidents. Either stoned driving would increase such accidents, killing thousands. Or people using marijuana instead of alcohol would decrease those accidents, saving thousands. I concluded:

We should probably stop [emphasizing direct] health effects of marijuana and imprisonment for marijuana-related offenses, and concentrate all of our research and political energy on how marijuana affects driving.

Using the best evidence available at the time, I predicted that marijuana legalization would probably decrease road traffic accidents. Now several states have legalized marijuana, data are in, and we have some preliminary evidence on how marijuana affects driving. And I was wrong.

A study by the Highway Loss Data Institute in June of last year finds that states that legalized marijuana saw insurance claims for auto accidents increase about 3% over the general national trend for the time. An updated study by the same group finds 6% according to insurance claims, and 5.2% according to police reports.