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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.

In the absence of a separate control group (as used in the assessment of crash probability with alcohol as described above) an alternative of a 'culpability index' is currently being employed in drug studies. The basic construct is first to formulate a means of determining the responsibility or culpability of a driver involved in a crash. There have been several means of constructing this 'culpability index' and this must be done with each of the accident cases by observers who have no information as to the drug status of each driver. The responsibility (or culpability) ratio is then determined as the proportion of drug-bearing drivers who were determined to be culpable, to the non-drug bearing drivers who were deemed to be culpable. The null hypothesis predicts a culpability ratio of 1.00 (ie, the drug has had no causal relationship with crashes).
To date there have been six studies employing this technique (two of which have involved the re-analysis of earlier generated data). These are briefly outlined below:
1. Warren and others re-analysed the data of Cimbura and found a culpability index for cannabis of 1.7, the same as that found for alcohol. However, the original data comprised a total of 484 drivers and pedestrians, 3.7% of whom were positive for cannabis. However, 88% of these people were also positive for alcohol. This left a very small number from which to assess a culpability ratio for cannabis alone.
2. Terhune also has previously collected data independently re-analysed to estimate a culpability ratio. All BACs over 0.10% were judged significantly more culpable than the drug-free group. The cannabis group also had a higher culpability ratio than the drug-free group, but this was only marginally significant (58.8% vs 34.4%). This estimation was also compromised by the small sample size for cannabis only (n=17). The cannabis plus alcohol group was analysed separately.
3. Donelson began a very ambitious project but was unfortunately thwarted by funding problems which precluded the complete analysis of the collected data. However, a random sample of 415 cases was analysed. The results cautiously suggested a finding consistent with those of Warren et al. and Terhune above.
4. Williams et al. in a study involving 440 cases, demonstrated as in the above studies that alcohol had a higher culpability ratio compared with culpable drug-free drivers (92% vs 71%). However, those drivers in whom only cannabis was detected were less likely to be responsible for the crashes (53% vs 71%).
5. Terhune et al. reported a very comprehensive study involving 1 882 cases. They found that alcohol was the dominant drug in fatal crashes, although the basic focus of their research was to describe the effect of drugs other than alcohol. They reported that fully 40% of the drivers had only alcohol in their systems and another 11% had alcohol combined with drugs. Among the drivers with BACs at or above 0.10% (n=625) their responsibility rate:
... was an extraordinary 94%, well above that found for any other single substance.
 Of cannabis, the authors stated that while cannabinoids were detected in 7% of the drivers, the psychoactive agent THC was found in only 4%. Of the drivers with only one substance in their system, only 1.1% had cannabis alone, either as the THC the psychoactive compound or had the inactive metabolite carboxy THC. The presence of the inactive metabolite and the absence of detectable THC infers less recent ingestion of cannabis—assuming an efficient analysis.
 The THC only drivers had a responsibility rate below that of the drug-free drivers—ie. as with the study by Williams et al. (1985) they were considered to be less likely to have been a cause of the crash than the drug-free drivers.
 The report also indicated the range of THC concentrations found in the blood. There were 109 cases of THC alone; of these, 22.9% contained what the authors called a 'trace' ie. 1 to 2 nanograms THC per millilitre of blood (ng/ml); 69.7% contained 'low' concentrations between 3 to 19 ng/ml; and 7.3% contained a 'high' concentration of equal to or greater than 20 ng/ml.
6. Drummer reported a study of 1 045 fatalities in New South Wales, Victoria and Western Australia and used the technique of responsibility analysis (culpability index).
 As with other studies, the dominant drug was alcohol, being found overall in 36% of all driver fatalities, 33% of which were over the legal limit of 0.05g%. Cannabis was found in 11% of cases of which 56% (n= 63) also contained alcohol (mean BAC 0.16 g% Ī 0.08g%). There was no significant difference in the BAC of the alcohol only drivers and those with alcohol plus cannabis.
 Assessment of the culpability ratio by Drummer provided the same result as those of Williams et al. and Terhune et al; there was a trend to a decrease in relative risk when either THC or the metabolite carboxy THC was measured in blood or urine. The relative risk was 0.6 relative to drug-free drivers, although this was not significant statistically.


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