“Recent smoking of marijuana and blood THC of 2-5 ng per milliliter are associated with substantial driving impairment. According to meta-analysis, the overall risk of involvement in an accident increases by a factor of about 2 when a person drives soon after using marijuana. In an accident culpability analysis, persons testing at 1 ng/ml of THC or higher were 3-7 times as likely to be responsible for a motor vehicle collision. In comparison, the risk of a vehicular collision increased by almost 5 for a driver with a blood alcohol over 0.08%. The risk of alcohol with marijuana is associated with an increase risk than either drug alone.”

Dose related risk of motor vehicle crashes after cannabis use - PubMed (nih.gov)

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