ReviewDose related risk of motor vehicle crashes after cannabis use
Introduction
The effects of Δ9-tetrahydrocannabinol (THC) on the ability of drivers to operate safely have traditionally been determined in epidemiological surveys of THC users’ involvement in traffic accidents and in experimental studies to measure the drug’s influence on skills related to driving (reviews: Robbe, 1994; Berghaus et al., 1998a, Berghaus et al., 1998b; Bates and Blakely, 1999, Solowij, 1998, EMCDDA, 1999, O’Kane et al., 2002). The purpose of epidemiological studies is to determine both the severity of THC impairment and the prevalence of THC use among the driving population by measuring the frequency of cannabis use among drivers who do and do not become involved in crashes. Essentially they aim to determine if cannabis use is over represented among drivers who were involved in accidents. Experimental studies are designed to predict the effects of cannabis on driving ability by measuring their users’ performances in laboratory tests of isolated psychological functions, driving simulators and on-the-road driving tests. In the context of well-designed experiments, drugs that produce large performance impairments in many different tests can be considered potentially hazardous to drivers whereas drugs that fail to produce any impairment can be considered safe. Experimental studies often provide the earliest evidence for a drug’s hazard potential for driving.
Many excellent studies on the effects of cannabis on driving are available only as technical reports, proceedings or book chapters. That is unfortunate since reviews in general should not cover data that are not published in peer-reviewed sources. Yet, applying this rule invariably would seriously weaken any review in this field. We therefore decided to also include sources that did not appear in peer-reviewed formats, i.e. about 50% of the references, in order to fully summarize and integrate what is known about the effect of cannabis on performance and driving ability. In particular a summary of the literature relevant to the following research questions will be given:
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Does cannabis impair psychomotor, cognitive, and actual driving performance and increase the risk of becoming involved in traffic accidents?
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Is there a relation between performance impairment and cannabis dose or its concentration in plasma?
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Do combined effects of cannabis and alcohol on driving performance differ from those of either drug alone?
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Does cannabis affect all aspects of the driving task alike?
Section snippets
Prevalence of THC in crash involved drivers
Surveys conducted in widely separated localities have generally revealed the presence of THC in between 4 and 14% of drivers who sustained injury or death in traffic accidents (Cimbura et al., 1982, Terhune and Fell, 1982, Terhune et al., 1992, Chesher and Starmer, 1983, Mason and McBay, 1984, Donelson et al., 1985, Garriott et al., 1986, Daldrup et al., 1987, McLean et al., 1987, Cimbura et al., 1990, Soderstrom et al., 1995, Mercer and Jeffery, 1995, Logan and Schwilke, 1996, Drummer et al.,
Experimental studies of cannabis and performance
Determination of the effect of THC on performance has mostly been based on information provided by the field of psychopharmacology. Psychopharmacologists have devised a large number of “psychomotor” tests, characterized by contingent motor responding to an imposed discrete or continuous signal (e.g. reaction time, attention, tracking, and critical flicker/fusion frequency tests), and “cognitive” tests for measuring various mnemonic functions but also deductive reasoning. Finally, tests were
Discussion
The epidemiological literature has provided conflicting information on the role of THC in performance impairment and motor vehicle crashes. Among epidemiological studies, case-control studies are limited in number but generally provide evidence supporting an association between cannabis and increased crash risk. The majority of epidemiological studies are culpability studies and several of these show little evidence that drivers who only used cannabis are more likely to cause accidents than
Conclusions
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THC has been shown to impair cognition, psychomotor function, and actual driving performance in a dose related manner.
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The degrees of impairment observed in laboratory or actual driving tests after doses up to 300 μg/kg THC were comparable to the impairing effects of an alcohol dose producing a BAC≥0.05g/dl, the legal limit for driving under the influence in most European countries.
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There is no indication that past use of THC alone affects crash risks, but there is growing evidence that recent use
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