"There's no evidence marijuana us impairs driving ability..."
Just Google smoker dude. There is plenty of evidence that smoking weed impairs driving and all machine use.
Also, there is no clear standard testing method yet for weed impairment, so that needs to be worked out before weed-heads get on the highways.
And workplaces with machines will need to test for it randomly.
Lots to be worked out yet.

https://www.drugabuse.gov/publications/research-reports/marijuana/does-marijuana-use-affect-driving