“Marijuana is a commonly found illicit substance in motor vehicle operators driving under the influence of drugs. Current evidence shows that blood levels of tetrahydrocannabinol do not correlate well with the level of impairment. In addition, although acute infrequent use of cannabis typically leads to cognitive and psychomotor impairment, this is not consistently the case for chronic heavy use.”

https://www.ncbi.nlm.nih.gov/pubmed/29933274

https://insights.ovid.com/crossref?an=00000539-900000000-96658