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Miscellaneous Statements on Drug Policy
References on Drugs and Driving


Dr G.B. Chesher

Department of Pharmacology University of Sydney and National Drug and Alcohol Research Centre University of New South Wales.

The studies outlined above indicate that cannabis does cause dose-dependent effects on laboratory based tests of human skills. Furthermore, studies utilising driving simulators and on-road driving also indicate a degree of cannabis induced impairment of driving skills. However in these cases the extent of the impairment indicated from laboratory studies is not replicated in the simulator or in-car studies.
The effects of alcohol on the other hand can be demonstrated both in laboratory studies and in simulated or on-road driving at very much the same dose levels. Explanations for these differences between alcohol and cannabis have been suggested and rest essentially upon the difference in the awareness by the drug taker of the presence of drug impairment. This in turn may be explained by the present understanding of the quite different ways alcohol and cannabis are known to act on the brain.
Also mentioned above and in other publications our present laws on alcohol and driving have been based upon the scientific principles outlined here and in particular on the results of epidemiological studies. It is pertinent therefore to discuss briefly the nature of the epidemiological studies undertaken to date with cannabis and road crashes.
Epidemiological studies with alcohol are greatly facilitated by the pharmacokinetics of that drug. Alcohol is excreted in the breath and the ratio of the concentration on the breath and in the blood is relatively constant. Therefore the determination of the concentration of alcohol in the breath (by a 'breathalyser') provides a reasonably and acceptably accurate indication of the blood concentration. It is unfortunate therefore that cannabinoids are not excreted on the breath and the concentration of cannabinoids that can be detected on breath represent only that contained in the 'dead-space air' in the upper respiratory tract. The cannabinoids so detected do not correlate in any way with the blood concentration. In addition to this the blood concentration of cannabinoids do not show any useful relationship to the degree of impairment or the degree of subjective effects of the drug. The blood concentration of alcohol on the other hand does exhibit a reasonable correlation with the degree of impairment.
These properties of cannabis mean that the determination of the role of cannabis in road crashes by the same techniques of the case-control study as used for alcohol, is not an easy task. The pharmacokinetics of cannabis make this an exceedingly difficult task. The difficulty is not only related to the poor correlation between blood concentration and impairment, but also because it requires the collection of a blood sample-from both the crash case and the controls. The collection of the latter sample is likely to involve a high refusal rate, and this alone would almost certainly invalidate the study. One does not know the reason for the refusal!
The studies that have been undertaken to date can be described within three groups and these are:
(i) Questionnaire based surveys;
(ii) Incidence of drug detection in accident involved drivers; and
(iii) Attempts to assess whether or not the driver who has detectable drug in bloodstream was culpable in the accident.
Studies along the lines outlined above have been reviewed by Simpson.
5.1 Questionnaire based surveys
Questionnaire based surveys by definition depend upon self report data and their reliability is questionable. Furthermore, the incidence of cannabis use and the likelihood of a driver admitting to such use is likely to change across time.
5.2 Incidence of drug detection in crash involved drivers
This technique involves the analysis of blood or urine samples taken from crash involved drivers. The detection of cannabinoids in urine provides information only that the drug has been consumed within the last day or even month. It provides no indication at all of impairment. Therefore only the analysis of a blood sample is likely to be helpful. However, the detection of cannabis in a blood sample does not itself prove impairment or crash culpability. This fact has been well expressed by Compton as follows:
Knowing only the frequency with which crash-involved drivers use drugs does not allow one to know the danger posed by the drugs. It may simply reflect the general drug usage pattern in the driving public at large. For example, finding that 30% of crash-involved drivers have nicotine in their blood does not imply that nicotine was involved in the occurrence of their crashes. It may be that 30% of the general driving population smokes cigarettes and the smoking of cigarettes is unrelated to crash occurrence. Finding that a drug was overrepresented in crash-involved drivers (as compared to non-crash involved drivers) would strongly suggest it played a role in increasing crash risk. However, this approach requires knowing the drug usage rate of the general driving public, something we do not know and can not easily determine.
Furthermore, any comparisons of the incidence of cannabis detections in crash-involved drivers with those of non-crash involved drivers should be collected from a comparable population and at the same time. The patterns of cannabis use vary not only across time but also across populations.
Therefore studies reporting the incidence of drugs in the blood of crash-involved drivers is essentially meaningless without some control of the incidence of drug use in non-crash involved drivers. Nevertheless, such studies have been reported and are reviewed by Simpson who summarised that:
  • Marijuana users are certainly among drivers who are injured in road crashes (suggested by the presence of cannabinoids in urine);
  • More importantly, recent use, as indexed by the presence of THC in blood, is evident in perhaps less than 10% of injured drivers; and
  • When cannabis is detected, there is an 80% chance that alcohol will also be found.
5.3 Attempts to assess whether or not the driver who has detectable drugs in the bloodstream was culpable in the accident
Of the first attempts to assess culpability has been an ongoing series of data collected by McBay of fatal, single vehicle crashes. Culpability in single vehicle crashes is assumed to be that of the driver (assuming no mechanical fault can be found) and the choice of fatal crashes assumes that death occurred shortly after the accident; meaning that drug metabolism ceased at death and therefore the blood sample from the dead body will represent the blood picture at the time of the crash. Cannabis was detected in 7.8% of 600 such cases, but 88% of these also contained alcohol in concentrations which of themselves could have accounted for the crash.
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