The involvement of drugs in drivers of motor vehicles killed in Australian road traffic crashes

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Abstract

A multi-center case-control study was conducted on 3398 fatally-injured drivers to assess the effect of alcohol and drug use on the likelihood of them being culpable. Crashes investigated were from three Australian states (Victoria, New South Wales and Western Australia). The control group of drug- and alcohol-free drivers comprised 50.1% of the study population. A previously validated method of responsibility analysis was used to classify drivers as either culpable or non-culpable. Cases in which the driver “contributed” to the crash (n=188) were excluded. Logistic regression was used to examine the association of key attributes such as age, gender, type of crash and drug use on the likelihood of culpability. Drivers positive to psychotropic drugs were significantly more likely to be culpable than drug-free drivers. Drivers with Δ9-tetrahydrocannabinol (THC) in their blood had a significantly higher likelihood of being culpable than drug-free drivers (odds ratio (OR) 2.7, 95% CI 1.02–7.0). For drivers with blood THC concentrations of 5 ng/ml or higher the odds ratio was greater and more statistically significant (OR 6.6, 95% CI 1.5–28.0). The estimated odds ratio is greater than that for drivers with a blood alcohol concentration (BAC) of 0.10–0.15% (OR 3.7, 95% CI 1.5–9.1). A significantly stronger positive association with culpability was seen with drivers positive to THC and with BAC ≥0.05% compared with BAC ≥0.05 alone (OR 2.9, 95% CI 1.1–7.7). Strong associations were also seen for stimulants, particularly in truck drivers. There were non-significant, weakly positive associations of opiates and benzodiazepines with culpability. Drivers positive to any psychoactive drug were significantly more likely to be culpable (OR 1.8, 95% CI 1.3–2.4). Gender differences were not significant, but differences were apparent with age. Drivers showing the highest culpability rates were in the under 25 and over 65 age groups.

Introduction

Alcohol is recognised as a leading contributor to road trauma. Alcohol over-involvement in crashes has been clearly demonstrated by a number of studies which show substantial increases in crash risk when the blood alcohol concentration (BAC) exceeds 0.10 g per 100 ml (%) (Borkenstein et al., 1974, Mounce and Pendleton, 1992, Robertson and Drummer, 1994).

While many drugs detected in crash victims are liable to impair driving skills, there is still uncertainty as to whether this translates to an increased crash risk. Likely drugs include cannabis; benzodiazepines; opiate-like drugs, such as heroin, morphine and methadone; and amphetamines and other CNS stimulants.

Experimental studies using instrumented cars have provided much useful information on the role of certain drugs on performance and error rates. Many CNS active drugs, particularly cannabis, benzodiazepines, barbiturates and the sedating antihistamines reduce lane control by increasing the standard deviation of lateral position (SDLP) while the drug is having its peak activity (Brookhuis et al., 1990, Laurell and Tornros, 1986, O’Hanlon et al., 1982, O’Hanlon and Volkerts, 1986, Ramaekers et al., 2000, Robbe, 1994). Logan et al. (2000) examined the extent of driver impairment of carisoprodol, a skeletal muscle relaxant, and its major metabolite meprobamate, which has sedative properties. The authors found that at therapeutic concentrations impairment was possible with symptoms of intoxication similar to alcohol.

Unfortunately, these experimental studies may not accurately predict the effects of drugs under actual driving conditions. The association of drugs with crashes has been extensively investigated in epidemiological studies but with mixed results. Moreover, while providing evidence of an association, epidemiological methods cannot unequivocally establish that drug use causes adverse driving events.

Early studies of drugs and crash risk concluded that cannabis-users are more likely to be responsible for their crashes than drug-free drivers (Simpson, 1986, Simpson et al., 1982, Warren et al., 1981). The epidemiological design providing the strongest evidence of a causal association of drug use and crash risk is case-control responsibility analysis. Terhune et al. (1992) examined the cases of almost 2000 drivers fatally-injured in the USA to assess the contribution of drugs to crashes and found that the responsibility rate for amphetamine-positive drivers was higher than the drug-free group (Terhune et al., 1992). However, Terhune (Terhune et al., 1992) also found the responsibility rate for all THC-positive drivers was lower than that for the drug-free control group. Williams also found no evidence of an association between cannabis and the risk of injury but the numbers of drivers were small (Williams et al., 1985). In contrast, an investigation of trucker fatalities pointed to an adverse effect of Δ9-tetrahydrocannabinol (THC) over 1 ng/ml, and other psychotropic drugs, on crash risk (Crouch et al., 1993).

A study of drivers injured in South Australia found drivers who tested positive for alcohol only, benzodiazepines only and the combinations of alcohol and THC and alcohol and benzodiazepines were significantly more likely to be culpable for the crash compared with the drug-free group (Longo et al., 2000, Longo et al., 2001). A small percentage of drivers who only tested positive for THC were culpable for the crash but this was not statistically significant. It was of interest that the blood THC concentrations were particularly low, most being <2 ng/ml. A study of injured drivers who presented to an urban emergency centre in Colorado found only alcohol and alcohol in combination with other drugs increased the likelihood they were responsible for the crash (Lowenstein and Koziol-Mclain, 2001). In this study cannabinoids including THC were only measured in urine limiting the time frame of last cannabis use.

Other case-control designs compare the drug use of drivers involved in crashes with other groups. A nested case-control design of over 200,000 drivers using driver’s license files, police reports of injurious crashes, and health insurance records showed an increased risk of motor vehicle crash involvement in the elderly population using long acting benzodiazepines (Hemmelgarn et al., 1997).

A case-control study of persons involved in injurious crashes showed that use of anti-depressants and opioid analgesics by older drivers was associated with increased risk of injurious motor vehicle collisions, but not with benzodiazepines or sedating antihistamines (Leveille et al., 1994). This contrasted with another study that found the relative risk of injurious crash involvement for current users of any psychoactive drug was significantly elevated and the increase was primarily due to benzodiazepines and cyclic antidepressants (Ray et al., 1992).

Other studies of all drivers have failed to find a positive association of drugs with road crashes. A retrospective, hospital-based case-control study failed to find a significant difference between the prevalences of drugs (opiates, cannabinoids and amphetamines) in drivers injured in road crashes and those admitted to a hospital for other reasons (Marquet et al., 1998).

A study which linked prescription records with hospital admissions from road crashes showed that people who used minor tranquilizers in the past 3 months had a five-fold higher risk of a serious road accident (Skegg et al., 1979). A similar study showed the odds ratio was elevated for those persons taking benzodiazepines, particularly within a few weeks of the first prescription (Neutel, 1995). In contrast, a similar study showed no increase in accident risk with the use of benzodiazepines and sedatives (Jick et al., 1981). An association of drug use and motor crashes has also been shown by comparisons of the prevalence of drugs in crash victims with general drug use obtained from surveys. Other studies have surveyed drivers regarding their use of drugs while driving and their perception of possible impairment (Albery et al., 2000, Walsh and Mann, 1999, Wechsler et al., 1984).

Crash investigations have provided evidence of likely mechanisms of causality. Logan (1996) showed that in a population of methamphetamine positive drivers, predominantly culpable for accident causation, the use of methamphetamine most likely contributed to risk-taking behaviour or was a result of withdrawal-related fatigue and hypersomnolence (Logan and Schwilke, 1996).

It is clear that variable conclusions have emanated from various sources of any link of crash risk to use of one or more psychotropic drugs. Some studies have failed to demonstrate a convincing association of drugs with crash risk often because they lacked a suitable control population while others lacked statistical power because of their small population sizes, limited extent of toxicology testing or their assessment of culpability.

The present study was designed to avoid these problems. A validated method of responsibility analysis (Robertson and Drummer, 1994) was used to classify culpability of drivers killed in three Australian states over 10 years. The toxicological data on the incidence of alcohol and the various drugs for the 3398 subjects have been described elsewhere (Drummer et al., 2003).

Section snippets

Study population

The study population is drivers killed in motor vehicle crashes in the three Australian states of Victoria (Vic.), New South Wales (NSW) and Western Australia (WA). Only cases that were on-road motor vehicle crashes were included. Crashes that occurred off-road, or those classified as due to natural causes or suicide were excluded.

In Victoria these data were obtained from records kept at the Victorian Institute of Forensic Medicine and the State Coroners Office at Southbank. Drivers were

Characteristics of drivers

Three thousand three hundred and ninety eight drivers, were included in the study: 1611 Victorian (47.4% of study population), 1031 NSW (30.3%) and 757 Western Australian drivers (22.3%). This represented 89% (median) of the driver fatalities identified from each jurisdiction. Cases were excluded if the deceased was not a driver, if they died of natural causes or committed suicide, if the crash did not occur on a public road and if either the toxicology data or culpability data were incomplete.

Discussion

This 10-year, multi-centre longitudinal study of driver culpability has shown a number of strong positive associations between use of some psychoactive drugs and responsibility for the crash. Of particular interest is the association of THC with driver culpability. This association showed a biological gradient, similar to that observed for alcohol. The estimated association with culpability of THC in concentrations of at least 5 ng/ml was much greater than the association of all identifiable

Acknowledgements

We thank the staff of the Victorian Institute of Forensic Medicine for their assistance in these investigations. We also thank the toxicologists in each state for their support and assistance, particularly Mr. Robert Hansson (WA) and Mr. Alan Hodda (NSW), as well as coroners, registrars and clerical assistants at the respective coroners’ courts. We acknowledge the financial assistance of VicRoads for financing many of these studies, as well as AustRoads and the NSW roads and traffic authority.

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