The magnitude of cannabis’ influence on driving performance is far less than that of alcohol, according to a systematic review and meta-analysis published in the journal Addiction.

Canadian researchers analyzed data from 57 studies assessing the influence of cannabis and alcohol on driving behavior and crash risk. 

They acknowledged that cannabis exposure was typically associated with deviation in drivers’ lateral positioning (lane weaving) and a decrease in their average speed. Cannabis use was “not associated with an increase in crashes in experimental studies.” Authors also found “no compelling evidence” that cannabis influenced hazard response time, headway variability, time out of lane, speed variability, speed exceedance, or time speeding. 

They concluded, “Specifically, for the measures reported here, there are no instances where the average effect of cannabis is equal to or greater than the driving performance decrements associated with BAC concentration ranging from 0.04 to 0.06 percent.”

Consistent with other studies, authors acknowledged that the combined use of cannabis and alcohol “is generally more detrimental to driving performance relative to non-intoxication or to either drug in isolation.”

NORML has long acknowledged that acute cannabis intoxication can influence driving performance, particularly if consumed with alcohol, and has opined that anyone inhaling cannabis “should refrain from driving for a period of several hours.” 

Full text of the study, “Effects of cannabis and alcohol on driving performance and driver behavior: A systematic review and meta-analysis,” appears in Addiction. Additional information is available in the NORML fact sheet, ‘Marijuana and Psychomotor Performance.’

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