Table of Contents

9.1.2 Evidence of crash risk


The relationship between raised alcohol levels and crash risk is well established, and it has been estimated that driving while intoxicated contributes to 30–50 per cent of fatal crashes, 15–35 per cent of crashes involving injury and 10 per cent of crashes not involving injury.

Increasing levels of intoxication result in disproportionate increases in the risk of a motor vehicle crash. The first case-controlled study of collision risk showed that with a blood alcohol concentration (BAC) of 0.05 per cent (g/100 mL), a driver was twice as likely to be involved in a collision as someone with no alcohol; at 0.10 per cent a driver has five times the relative risk; and at 0.20, there is a 25 times greater risk of a collision.7

Less experienced drivers have alcohol-related crashes at lower BACs than more experienced drivers. For example, a study of single vehicle fatal collisions showed that a male driver in the first five years of driving is 17 times more likely to have a fatal collision if their BAC is 0.05–0.079 and risk increases exponentially with BAC.8 This supports zero BAC for probationary drivers as mandated in our graduated licensing system. In the case of commercial vehicle drivers, ‘zero’ BAC is also mandated (refer to Appendix 4: Drivers’ legal BAC limits). Inexperienced drivers need to be educated about the real risks associated with drinking and driving.

Individuals with alcohol dependency have approximately twice the risk of crash involvement as controls, possibly because they are more likely to drive while intoxicated despite prior convictions for drink-driving.


There is limited evidence regarding crash risk and drug dependency. Approximately 13 per cent of fatal crashes are attributed to drug use. The risk is amplified with alcohol-drug and impairing drug-drug combinations.

Amphetamine-type stimulants are a particular hazard in the long-distance trucking industry. Stimulants are used to promote wakefulness but can result in rebound fatigue. An Australian study of responsibility for collision found amphetamine-type stimulants in 4.1 per cent of all fatally injured drivers and 23 per cent of fatally injured truck drivers. Low doses of stimulants improve reaction time and reduce fatigue but at a cost of poor road position, loss of attention to peripheral information, erratic driving, weaving, speeding, drifting off the road, increased risk taking and high speed collisions.913

Cannabis. Driving under the influence of cannabis is perceived by many as a low-risk activity14 and is reported to be more common than driving under the influence of alcohol.15,16 Cannabis users report frequently driving within an hour of smoking17,18 and being told about the risks appears to have little impact.19 The relationship between blood levels of tetrahydrocannabinol (THC) and crash risk is not as well understood as for other drugs because it has complex pharmacokinetics. However, just the presence of measurable levels of THC is associated with an increased crash risk.20,21 An Australian study found that a driver was 6.6 times more likely to be responsible for a fatal crash risk at levels of THC 5 mg/mL or above compared with drug-free drivers (sex and age adjusted).

Cannabis use can lead to dependence syndrome, with well-documented withdrawal symptoms including restlessness, insomnia, anxiety, aggression, anorexia, muscle tremor and autonomic effects.22 Adult lifetime prevalence rates suggest that 9 per cent of cannabis users develop cannabis dependence, with higher rates in young people.23 Cannabis is the most common substance after alcohol for which admission for detoxification is sought. Acute cannabis consumption is associated with increased road trauma.24,25 Chronic cannabis use is associated with cognitive decline,26 although less is known about the implications for safe driving. Chronic cannabis users should be carefully assessed. On-road assessment may be required to determine fitness to drive.

Sedating drugs. This is a heterogeneous group that includes all the drugs that cause mental clouding, sleepiness and poor responsiveness to the environment. It includes the benzodiazepines, sedating antihistamines, sedating antidepressants and narcotic analgesics. There is specific data on driving risk for some substances and none for others. Practitioners should be aware of the implications of their prescribing on the ability of patients to drive safely.

There is an increased risk of personal injury crashes among drivers using anti-anxiety drugs compared with the rest of the population.27 The risk is exacerbated by alcohol and other sedatives.28 There is a hangover effect, and a small dose of any sedative the following day can potentiate the effect. A meta-analysis of more than 500 studies showed that the degree of impairment of driving skill was directly related to the serum level of each substance.29 In Australian studies benzodiazepines are found in about 4 per cent of fatalities and 16 per cent of injured drivers. Ninety-eight per cent of the drivers who had diazepam at any level combined with alcohol at any level were responsible for the collision in which they were injured. In a study of drivers taken to hospital for treatment after a collision, 98 per cent of drivers who had a benzodiazepine at any level with alcohol at any level were responsible for the collision.30