"We have decades of research about the impact of cannabis on our brains."
Actually, no we don't. We have virtually none, and fewer studies on how cannabis use affects drivers.
Governments are writing cannabis laws blind. Governments have been notorious for NOT granting license to study cannabis, the US in particular, so up till the last couple of years there has been NO RESEARCH on cannabis. NONE.
In case you don't know, the active ingredients in cannabis have a half life of about 30 days, meaning if you ingest cannabis today, let's say it results in a blood concentration of X% today, in 30 days you will have half of X in blood concentration... enough for a blood test to say you're impaired...30 days after consuming the weed.
So an everyday regular user never gets below X%, so according to our laws would wake up impaired every day.