Dr G.B. Chesher
Department of Pharmacology University of Sydney and National Drug and
Alcohol Research Centre University of New South Wales.
6. THE USE OF 'RESPONSIBILITY ANALYSIS' OR ESTIMATION OF
'CULPABILITY' TO DETERMINE THE ROLE OF DRUGS IN) CRASHES
|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
|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
| 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.