Effects of drugs on driving
Cannabis
Cannabis (marijuana) is a very popular recreational drug. Obtained from the plant Cannabis sativa, the leaves and flowering parts contain a variety of cannabinoids that posses psychoactive effects, the predominant one being D9-tetrahy drocannabinol (D9-THC). Cannabinoids have significant behavioural and physiological effects, which contribute to changes in a person’s ability to drive safely. The drug is popular for its relaxation-promoting and euphoric properties, accompanied by sedation and, in the right dose and setting, by hallucinations. Accompanying effects include altered time and distance perception, a reduced ability to concentrate, impaired learning and recall, increased appetite and mood changes. Associated physiological effects include increased pulse and blood pressure, and blood shot eyes. Loss of the ability of the eyes to focus on objects has also been reported. The predominant effects with respect to driving are sedation and the effects on concentration, divided attention, perception, and temporal and spatial orientation. The sedative effects can be similar to those of CNS depressants, and the associated driving behaviours are similar also.
Although a number of papers have been published in the scientific literature regarding cannabis use and driving impairment, it is difficult to draw hard and fast conclusions about the exact effect of cannabis on driving: the dose needed to bring about impairment and the length of time the drug may affect impairment. This is due to differences in aspects such as time of observation of behaviour after administration, dose administered, duration of effect between individuals, lack of appropriate control groups, and cannabis use combined with alcohol or other drugs. The general picture is that there is a higher percentage of cannabinoid-positive blood or urine samples (indicating use) in drivers involved in accidents or arrested for impaired driving than in the general population.
A limited number of studies have been performed in which subjects have been administered cannabis and then tasked with driving in electronic driving simulators, or actually operating vehicles on open or closed driving courses. These studies have been reviewed by Huestis (2002), and reveal evidence of minor to moderate, but significant, reduction in driving performance at the doses given. However, some drivers actually improved in performance after the cannabis dose. This effect of improved driving performance has also been seen in some alcohol-intake studies where a slight improve ment was seen at low blood alcohol levels. In The Netherlands, on-road driving studies gave findings consistent with earlier studies (Robbe and O’Hanlon 1993, 1999), i.e. that cannabis use did impair driving ability. No correlation was found between the blood THC concentrations and the degree of effect. It was also shown that, when cannabis was taken in combination with alcohol, the effects on driving were increased. In summary, cannabis is a psychoactive drug with effects on mood, concentration and judge ment in addition to its sedating properties, all of which contribute in a dose-related fashion to impairment of vehicle operation. Compared to many other drugs, the level of impairment after mild-to-moderate single-dose recreational use is low, equivalent to a BAC in the range 0.3–0.7 g/L. Combining alcohol and cannabis produces a greater impairment than either alone, and cannabis use should be considered when deter mining what may have produced impairment in subjects with low BACs. The blood THC and metabolite concentrations are not well correlated with effect, although some authors have explored the possibility of relating these concentrations to time since last use (Huestis et al. 1992).
