(anecdote) I live in Colorado where cannabis has been legal for quite some time. People driving in to stationary objects while high on mj definitely hasn't become any more of a problem than it was previous to legalization (read: its not a problem.)
(even more anecdata and speculation) I live in Washington, and it is absolutely not a problem here. The main difference, to my mind, is that alcohol induces higher risk behavior, while cannabis produces risk aversion. Prohibition has been an abject failure. That said, I don't think it's very good for your brain. Worth avoiding or at least moderating your intake.
I'm in Oregon, pot is legal here too, far more likely the stationary object hit is someone on weed that thinks they are driving too fast.