If being stoned makes you drive slower wouldn't your reflexes also be slower?
You are right that there is impairment, no matter the agent causing it. I found that, driving in residential or otherwise regular roads, there didn't seem to be an impairment, or at least the impairment was much less than if drunk. Where I found the impairment was on a Freeway, where the tendency was to drive slow, despite the effort to maintain 60 MPH.
I'm sure there are studies that would explain the difference between the impairments as between alcohol and MJ. I'm also certain there is a difference, a difference that is meaningful in terms of damage and safety is concerned.
Please do not take my comments as condoning any form of impaired driving, I do not. I just wish to explore the fact that there is a difference.
BTW, My experiences are 30+ years old so they are somewhat dated.