Personal Health
There is no increased crash risk after cannabis use; epidemiologic studies show no association after adjustment for confounders.
Acute cannabis impairment is on average similar to driving at a blood alcohol concentration of 0.08%, so crash risk is equivalent to legal alcohol intoxication.
Acute cannabis use modestly impairs driving, slightly increases crash risk, and combined use with alcohol further increases risk; blood THC poorly predicts impairment.
Detection of any blood THC after driving reliably indicates impairment and justifies per‑se DUI enforcement at any detectable level.