Okay, 1 reply.
My actual take on it is that while the overall number of people driving under the influence of something may increase, the overall danger would stay the same or perhaps even possibly decrease.
If you factor in all the people who currently choose alcohol or a prescription drug to self medicate as it was put and currently drive under the influence, a fair number will undoubtedly choose to switch to marijuana when it becomes legal. I linked a couple of studies earlier, which showed much less loss of reflexes and response time when operating a motor vehicle while under the influence of marijuana compared to alcohol.
You should not trust the study though, look how hard it is for you guys to find articles to support the premise that weed soaked (only) drivers are the scourge of the roads.