Marijuana and Driving - More Data Needed

"The decreased speed during the simulated drive could be interpreted as an attempt to compensate for perceived cognitive impairment, Alternatively, marijuana may not have affected decision making and judgment and the reduction in speed would improve safety margins, While the clinical significance of a 3% to 5% decrease in speed may be questioned, previous research suggests such a decrease will result in approximately a 7% decrease in all injuries and a 15% decrease in fatalities (Nilsson 1981), Use of an alternate task design in which subjects are requested to drive as quickly and as safely as possible rather than following a posted speed limit may provide more insight into compensatory strategies employed while driving under the influence of marijuana, Use of a more challenging road paradigm (e.g., icy or gravel roads) which capitalizes on the use of practice effects may aid in identifying differences in driving performance under the influence of marijuana, There was significant between-subject variability in driving measures and future studies would be further strengthened by using a within-subjects design."

Source: 

Anderson, Beth M.; Rizzo, Matthew; Block, Robert I.; Pearlson, Godfrey D.; O'Leary, Daniel S., "Sex differences in the effects of marijuana on simulated driving performance," Journal of Psychoactive Drugs (San Francisco, CA: Haight Ashbury Publications, March 1, 2010), Vol. 42, No. 1.
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