Driving Miss Dopey

Interesting study from British researchers on the influence of cannabis upon driving. The methodology:

The research attempted to address these objectives using experienced cannabis users carrying out a variety of laboratory-based tasks and driving in a driving simulator under four cannabis conditions: placebo; low THC; high THC; and cannabis resin. The placebo, low and high dose THC conditions used herbal cannabis (‘grass’) cigarettes supplied by the National Institute on Drug Abuse (NIDA), while the cannabis resin condition used cannabis supplied by Customs and Excise from seized supplies.

And the findings? Drumroll please...

The results of the driving related laboratory tests conducted in general did not produce statistically significant results. Although reaction times were found to increase with dose level, there was too much variability in the data for statistical significance. This suggests that there may be an effect on the reaction time of participants responding to hazards, but it is quite a small effect which would require a much larger sample to determine whether or not it was statistically significant. This again confirms earlier observations of the effects of cannabis on the various aspects of driver performance; the effect on reaction time being somewhat difficult to predict.
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Overall, it is possible to conclude that cannabis has a measurable effect on psycho-motor performance, particularly tracking ability. Its effect on higher cognitive functions, for example divided attention tasks associated with driving, appear not to be as critical. Drivers under the influence of cannabis seem aware that they are impaired, and attempt to compensate for this impairment by reducing the difficulty of the driving task, for example by driving more slowly.
In terms of road safety, it cannot be concluded that driving under the influence of cannabis is not a hazard, as the effects on various aspects of driver performance are unpredictable. However, in comparison with alcohol, the severe effects of alcohol on the higher cognitive processes of driving are likely to make this more of a hazard, particularly at higher blood alcohol levels.