What the hell IÂ´ll start:
However, there is not sufficient evidence indicating the percentage of drivers that operate a vehicle after consuming cannabis, particularly during the time period of any intoxicating effect. As a result, there is no precise estimate of the percentage of drivers exposed to cannabis as an accident risk factor. Indeed, it is problematic to estimate the extent of exposure independent of other risk factors associated with cannabis use such as alcohol. Moreover, the demographic group most frequently using cannabis already has the greatest a priori accident risk due to driving inexperience and factors associated with youth relating to risk taking, delinquency and motivation. These demographic and psychosocial variables may relate to both drug use and accident risk, thereby presenting an artificial relationship between use of drugs and accident involvement.
Evidence of impairment from the consumption of cannabis has been reported by studies using laboratory tests, driving simulators and on-road observation. The laboratory tests generally indicate acute impairment of memory, attention and psychomotor control. Both simulation and road trials generally find that driving behaviour shortly after consumption of larger doses of cannabis results in (i) a more cautious driving style; (ii) increased variability in lane position (and headway); and (iii) longer decision times. Whereas these results indicate a ‘change’ from normal conditions, they do not necessarily reflect ‘impairment’ in terms of performance effectiveness since few studies report increased accident risk. However, the results do suggest ‘impairment’ in terms of performance efficiency given that the increased compensatory effort resulting from cannabis use limits the available resources to cope with any additional, unexpected or high demand, events.
In conclusion, cannabis impairs driving behaviour. However, this impairment is mediated in that subjects under cannabis treatment appear to perceive that they are indeed impaired. Where they can compensate, they do, for example, by not overtaking, by slowing down and by focusing their attention when they know a response will be required. However, such compensation is not possible where events are unexpected or where continuous attention is required. Effects of driving behaviour are present up to an hour after smoking but do not continue for extended periods.
"With respect to comparisons between alcohol and marijuana effects, these substances tend to differ in their effects. In contrast to the compensatory behaviour exhibited by subjects under cannabis treatment, subjects who have received alcohol tend to drive in a more risky manner. Both substances impair performance, however,[b] the more cautious behaviour of subjects who have received cannabis decreases the impact of the drug on performance, where the opposite holds true for alcohol." [/b] (Smiley, 1998, p. 19)
Thus, not only is it problematic to estimate the percentage of accident involvements associated with cannabis use alone, there is no evidence that impairment resulting from cannabis use causes accidents. Attempts to alleviate these problems by calculating risk of culpability for an accident (rather than the risk of having an accident) suggest that cannabis may actually reduce responsibility for accidents . It is evident that further epidemiological research is necessary. Such research must adopt a ‘Grand Rapids’ methodology of obtaining valid baseline data matched to positive cases, as well as including sufficient sample sizes and a valid operational definition of ‘responsibility’. Such research may benefit from differentiating between accident types and accounting for relevant covariates including driver age and sex.
To make the streets safe for itzy-bitzy, teeny-weeny children!