Alcohol Poses a “Greater Threat to Road Safety” Than Cannabis

Alcohol is among the most frequently detected controlled substances identified in drivers following a motor vehicle accident and it “remains the greatest threat to road safety,” according to data published in the Journal of the American Medical Association (JAMA) Network Open.

Canadian researchers reviewed bloodwork data for over 8,300 drivers involved in a motor vehicle accident. Over half of the study’s participants tested positive for the presence of a controlled substance. Drivers were most likely to test positive for either depressants, cannabis, or alcohol – with cannabis being more common among younger drivers (ages 19 to 24).

However, most drivers who tested positive for the presence of THC in blood did so at nominal levels – indicating that their exposure may have been several hours or even days beforehand. (THC may remain present in the blood of more habitual consumers for several days following past exposure.) Only about three percent of participants tested positive for THC at elevated levels (THC ≥ 5 ng/mL).

“These statistics suggest that although more drivers test positive for THC, alcohol remains the greater threat to road safety,” the study’s authors concluded.

A prior Canadian study reported that drivers treated for traffic-related injuries are over three times as likely to test positive for elevated levels of alcohol (BAC ≥ 0.08 percent) than elevated levels of THC (THC ≥ 5 ng/mL).

Driving simulator studies report that cannabis administration is typically associated with compensatory driving behavior, such as decreased mean speed and increased mean following distance, whereas alcohol administration is associated with more aggressive driving behavior. Nevertheless, cannabis exposure can influence certain psychomotor skills necessary for safe driving, such as reaction time and drivers’ ability to maintain lane positioning.

A study conducted by the US National Highway Traffic Safety Administration reported that drivers who test positive for any amount of THC possess, on average, a far lower risk of being involved in a traffic collision than do drivers who test positive for alcohol at or near legal limits.

By contrast, drivers who test positive for the presence of both THC and alcohol in their system tend to possess significantly higher odds of being involved in a motor vehicle accident than do those who test positive for either substance alone.