I think there is definable a connection between cannabis use and auto accidents. It slows your reaction time and that’s a known factor with accidents. That said, substance use data is notoriously underreported[1] in surveys. So that 20% data point is not very helpful. Also 12-15 year olds are bringing that number down in your data and also can’t drive making it even less useful for comparison.

[1]https://www.sciencedirect.com/science/article/pii/S089085672...

And substance use impairment is overreported. If the driver was impaired it's counted even if the impairment has no bearing on the accident. A drunk hits a red light runner--it's called alcohol even if he had no hope of avoiding it.

My understanding of stoned drivers is they tend to be too conservative--waiting for the stop sign to turn green etc. If that's accurate it could also mean stoned drivers are worse at avoiding the mistakes of others.