Impairment due to alcohol and/or drugs is a major cause of motor vehicle crashes worldwide. Alcohol, illegal drugs and various prescribed drugs (medicines) impair driver's functional capabilities including reaction time, tracking ability, vision, divided attention and vigilance, which leads to increased crash risk.
The crash risk for drivers with a blood alcohol concentration (BAC) of 0.5 g/L is estimated to be 1.4 times higher than that of a sober driver; at 1.0 g/L nearly 5 times higher; and at 1.5 g/L around 20 times higher. For fatal crashes, the risk curve is even steeper. The crash risk associated with drugs depends on the type of drug. The greatest risk increases for illegal drugs - at least 5 times higher - were found for amphetamines, multiple drug use, and combined alcohol-drugs use. For medicines, risk increases were found for barbiturates (used for headaches/insomnia), benzodiazepines (for anxiety/insomnia), anti-depressants, and opioids/opiates (for pain/sleeping problems).
It has been estimated that 1.5 to 2% of kilometres travelled in the EU are driven with an illegal BAC. This equates to millions of drivers under the influence of alcohol. Around 25% of all road deaths in the EU are alcohol-related. On the positive side, alcohol-related fatalities have decreased somewhat faster than the number of other fatalities. The evidence on the development of drug-related traffic fatalities in EU-countries is scarce. Recent roadside surveys among randomly stopped drivers in EU-countries show alcohol prevalence rates between 0.3-2.7% and drug prevalence rates between 6.0 - 11%.
The reasons people engage in impaired driving reflect the opportunities that society provides for this, as well as personal choice or motivation. In regard to the repeat offenders group, medical (psychiatric) problems related to problematic alcohol and drugs use often underly their habitual pattern of impaired driving.
Countermeasures against impaired driving include: Policies to decrease the consumption of alcohol/drugs, to reduce drink/drug use before driving, strict legislation backed by police enforcement, provision of alternative transport means, awareness campaigns, rehabilitation courses, safety culture in companies, alcohol interlocks, and in-vehicle technology that warns/intervenes when impairment leads to critical events. Measures often work better in combination than when implemented individually, e.g. enforcement combined with campaigns, alcohol interlock combined with medical and psychological guidance.