Appendix 2: The effects of alcohol on driving ability

New Zealand Police: Enforcing drink-driving laws.

Alcohol directly affects the central nervous system, which affects the cognitive process and impairs decision-making. Alcohol is easily absorbed into the bloodstream and, unlike food, does not have to be digested. It directly affects all vital organs, including the brain, and has varying effects on different people.

Drinking alcohol undermines people's ability to make choices – especially where rational decision-making processes call for complex and subtle judgements. It also slows a person's reactions, dulling their judgement and vision.

The risk of a crash increases as the driver's alcohol level increases. Drivers with a high blood-alcohol concentration are more likely to crash than those who are sober.

Figure 13 describes the typical effects on a person and predictable effects on driving at different blood-alcohol concentrations.

Figure 13
Effects of different blood-alcohol concentrations on drivers

Blood-alcohol concentration (mg/ml) Typical effects Predictable effects on driving
0.02 Some loss of judgement


Slight body warmth

Altered mood
Decline in visual functions, such as rapid tracking of a moving target

Decline in ability to perform two tasks at the same time (divided attention)
0.05 Exaggerated behaviour

May have loss of muscle control (such as focusing eyes)

Impaired judgement

Lower alertness

Released inhibition
Reduced co-ordination

Reduced ability to track moving objects

Difficulty steering

Reduced response to emergencies
0.08 Muscle co-ordination becomes poor (such as balance, speech, vision, reaction time, hearing)

More difficult to detect danger

Impaired judgement, self-control, reasoning, and memory
Reduced concentration

Short-term memory loss

Difficulty controlling speed

Reduced information-processing capability (such as signal detection, visual search)

Impaired perception
0.10 Clear deterioration of reaction time and control

Slurred speech, poor co-ordination, and slowed thinking
Reduced ability to stay in lane and brake appropriately
0.15 Much less muscle control than normal

Vomiting possible

Major loss of balance
Substantial impairment in controlling vehicle, attention to driving, and in necessary processing of visual and auditory information

Source: The National Highway Traffic Safety Administration, United States Department of Transport, Washington.

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