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