This does not parse.
What that analysis doesn't take into account is whether the next state to legalize marijuana will also have to deal with increased accident rates and insurance costs. This is IMO the more important question.
Also - in the states that went legal, there is an active population boom, but just as likely is being caused by tech startups. Population increases are more likely to cause an accident rate to increase.
We just won't know (for sure) if pot or jobs are the cause for the population increases. Maybe it's pot jobs?
Causation requires correlation. It's certainly not enough to prove causation, but it's actually a necessary starting point.
Speaking of driving and marijuana, I've noticed a large increase in pot smoke coming from cars in traffic since marijuana was legalized in CA. I expect some of that to come from passengers, but people driving while high is no joke.