Elsevier

Drug and Alcohol Dependence

Volume 73, Issue 2, 7 February 2004, Pages 109-119
Drug and Alcohol Dependence

Review
Dose related risk of motor vehicle crashes after cannabis use

https://doi.org/10.1016/j.drugalcdep.2003.10.008Get rights and content

Abstract

The role of Δ9-tetrahydrocannabinol (THC) in driver impairment and motor vehicle crashes has traditionally been established in experimental and epidemiological studies. Experimental studies have repeatedly shown that THC impairs cognition, psychomotor function and actual driving performance in a dose related manner. The degree of performance impairment observed in experimental studies after doses up to 300 μg/kg THC were equivalent to the impairing effect of an alcohol dose producing a blood alcohol concentration (BAC) ≥0.05 g/dl, the legal limit for driving under the influence in most European countries. Higher doses of THC, i.e. >300 μg/kg THC have not been systematically studied but can be predicted to produce even larger impairment. Detrimental effects of THC were more prominent in certain driving tasks than others. Highly automated behaviors, such as road tracking control, were more affected by THC as compared to more complex driving tasks requiring conscious control. Epidemiological findings on the role of THC in vehicle crashes have sometimes contrasted findings from experimental research. Case-control studies generally confirmed experimental data, but culpability surveys showed little evidence that crashed drivers who only used cannabis are more likely to cause accidents than drug free drivers. However, most culpability surveys have established cannabis use among crashed drivers by determining the presence of an inactive metabolite of THC in blood or urine that can be detected for days after smoking and can only be taken as evidence for past use of cannabis. Surveys that established recent use of cannabis by directly measuring THC in blood showed that THC positives, particularly at higher doses, are about three to seven times more likely to be responsible for their crash as compared to drivers that had not used drugs or alcohol. Together these epidemiological data suggests that recent use of cannabis may increase crash risk, whereas past use of cannabis does not. Experimental and epidemiological research provided similar findings concerning the combined use of THC and alcohol in traffic. Combined use of THC and alcohol produced severe impairment of cognitive, psychomotor, and actual driving performance in experimental studies and sharply increased the crash risk in epidemiological analyses.

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:

  • Does cannabis impair psychomotor, cognitive, and actual driving performance and increase the risk of becoming involved in traffic accidents?

  • Is there a relation between performance impairment and cannabis dose or its concentration in plasma?

  • Do combined effects of cannabis and alcohol on driving performance differ from those of either drug alone?

  • 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

  • THC has been shown to impair cognition, psychomotor function, and actual driving performance in a dose related manner.

  • 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.

  • 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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