Assessing Fitness to Drive

In this section

9.1.1. Effects of long-term alcohol use and other substance use on driving

9.1.2. Evidence of crash risk

9.1.3. Effects of alcohol or drugs on other diseases

See references 1, 2, 3, 4, 5, 6, 7, 8

Alcohol

In Australia, the 12-month prevalence of alcohol use disorders in 2016 was 6.1 per cent for men and 2.7 per cent for women. Alcohol dependence had a prevalence of 2.2 per cent for men and 0.8 per cent for women. Chronic heavy alcohol use carries a real risk of neurocognitive deficits relevant to driving capability including:

• short-term memory and learning impairments, which become more evident as the difficulty of the task increases
• impaired perceptual-motor speed
• impaired visual search and scanning strategies
• deficits in executive functions such as mental flexibility and problem-solving skills; planning, organising and prioritising tasks; focusing attention, sustaining focus and shifting focus from one task to another; filtering out distractions; monitoring and regulating self-action; or impulsivity.

Long-term heavy alcohol use is also associated with various end-organ pathologies that may affect the ability to drive – for example, Wernicke-Korsakoff syndrome or peripheral neuropathies experienced as numbness or paresthesia of the hands or feet. In the event of end-organ effects relevant to driving, the appropriate requirements should be applied as set out elsewhere in this publication.

Alcohol-dependent people may experience a withdrawal syndrome on cessation or significant reduction of intake, which carries some risk of generalised seizure (refer to Acute symptomatic seizures, discussion and medical standards), confusional states and hallucinations.

Other substances

Substances (prescribed, over-the-counter and illicit) are misused for their intoxicating, sedative or euphoric effects. Drivers under the influence of these drugs are more likely to behave in a manner incompatible with safe driving. This may involve, but not be limited to, risk taking, aggression, feelings of invulnerability, narrowed attention, altered arousal states and poor judgement.

Illicit substances are a heterogeneous group. Chronic effects of their use vary and are not as well understood as those of alcohol. Some evidence suggests cognitive impairment is associated with chronic stimulant, opioid and benzodiazepine use. Illicit substance users may be at risk of brain injury through hypoxic overdose, trauma or chronic illness.

End-organ damage, including cardiac, neurological and hepatic damage, may be associated with some forms of illicit substance use, particularly injection drug use. Cocaine and other stimulant misuse have been linked with cardiovascular pathology. In the event of end- organ effects relevant to driving, the appropriate requirements should be applied as set out elsewhere in this publication.

Withdrawal seizures may occur (refer to Acute symptomatic seizures, discussion and medical standards).

See references 1, 6 ,9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21

Alcohol

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.

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. 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.

People 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.

Drugs¹⁰

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. In an Australian study examining the odds of culpability associated with use of impairing drugs in injured drivers, one or more illicit drug present, but no alcohol, increased the odds of culpability 10-fold, while drivers with any impairing drug (but no alcohol) increased the odds 8.2-fold. The two most common illicit drugs detected were methylamphetamine and THC. Common impairing drugs included illicit drugs as well as benzodiazepines and sedating antihistamines. The most frequent substance combinations included alcohol with THC, alcohol with a benzodiazepine, alcohol with methylamphetamine, and THC with methylamphetamine.

Amphetamine-type stimulants^{10, 11, 16, 21}

Australian studies report amphetamine-type stimulants in 7.1 per cent of all fatal road crashes and a 14-fold increase in the odds of culpability for collisions causing injury. 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.

Cannabis^{5,10,13,14,22,23,24}

Cannabis use can lead to dependence syndrome, with well-documented withdrawal symptoms including restlessness, insomnia, anxiety, aggression, anorexia, muscle tremor and autonomic effects. Adult lifetime prevalence rates suggest that 9 per cent of cannabis users develop cannabis dependence, with higher rates in young people. Cannabis is the most common substance after alcohol for which admission for detoxification is sought.

Acute cannabis consumption is associated with increased road trauma. Australian studies have found the presence of cannabis (THC) in 11.1 per cent of all drivers injured in a road accident and 13.1 per cent of road fatalities. Chronic cannabis use is associated with cognitive decline, and the implications for safe driving should be carefully assessed.

On-road assessment may be required to determine fitness to drive. Practitioners should be aware of the potential use of illicit and prescription medicinal cannabis and the cumulative impairment to drive safely. For information on prescription medicinal cannabis, refer to Part A section 2.2.9. Drugs and driving.

Sedating drugs^{10, 11,17,25,26}

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. The risk is exacerbated by alcohol and other sedatives. 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. In Australian studies benzodiazepines were found in 8.2 per cent of fatalities and 12 per cent of injured drivers. In a culpability study of drivers taken to hospital for treatment after a collision, 100 per cent of drivers who had a benzodiazepine at any level with alcohol at any level were responsible for the collision.

People who are frequently intoxicated and who also suffer from certain other medical conditions are often unable to give their other medical problems the careful attention required, which has implications for safe driving.

Epilepsy

Many people with epilepsy are quite likely to have a seizure if they miss their prescribed medication even for a day or two, particularly when this omission is combined with inadequate rest, emotional turmoil, irregular meals and alcohol or other substances. Patients under treatment for any kind of epilepsy are not fit

to drive any class of motor vehicle if they are frequently intoxicated.

Diabetes

People with insulin-dependent diabetes have a special problem when they are frequently intoxicated. Not only may they forget to inject their insulin at the proper time and in the proper quantity but also their food intake can get out of balance with the insulin dosage. This may result in a hypoglycaemic reaction or the slow onset of a diabetic coma.