Iowa City, IA: Subjects who operate a vehicle under the influence of cannabis tend to drive more slowly while those under the influence of alcohol tend to increase their speed, according to driving simulator data published in the journal Traffic Injury Prevention.
Investigators with the University of Iowa assessed simulated freeway driving performance in subjects following their use of cannabis, alcohol, and placebo.
Both cannabis use and alcohol use were associated with increased incidents of weaving, a finding that is consistent with prior studies. Compared to the placebo group, those under the influence of cannabis “spent more time at speeds significantly below the speed limit.” By contrast, those under the influence of alcohol “spent close to 40 percent of their time more than 10 percent above the speed limit.”
Several other studies have similarly reported that drivers under the influence of cannabis tend to exhibit more compensatory driving behaviors, while those under the influence of alcohol tend to drive in a more reckless manner.
The study’s authors concluded: “This comparative analysis sought to assess the effects of cannabis on driving and put them into context by looking at the magnitude of the effects compared to those of alcohol on driving. … “Ultimately, three primary conclusions can be drawn with respect to the effects of acute cannabis use on freeway driving relative to those of alcohol: 1) driving after acute use of cannabis in this population produced impaired lateral control that was similar in magnitude and effect to driving under the influence of alcohol at approximately .05 BAC; 2) the effects on lateral control are not dissimilar to those observed in other driving environments, and 3) driving after acute use of cannabis and alcohol produced opposite effects on speed control, with slower driving following cannabis use and faster driving following alcohol use.”
Full text of the study, “Assessing the impact of cannabis use on freeway driving performance and practices: A comparative analysis with placebo and alcohol-influenced driving,” appears in Traffic Injury Prevention. Additional information on cannabis and driving is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’