Impairment

Young, novice drivers' capabilities while driving can also be reduced by alcohol, illicit drugs, fatigue, as well as by distraction.

Alcohol

Use of alcohol and drugs increases the crash rate of young and novice drivers. Drink driving is particular dangerous for the young for a number of reasons. First of all, the young person's tolerance of alcohol is lower, as the body is not used to its consumption. Secondly, the driving task is more demanding for young, novice drivers than for other drivers, and thus, as they need to pay more attention to their driving task, the disrupting effect of alcohol is greater than for expert drivers. Thirdly, alcohol reduces inhibition. As young people posses less developed self-control mechanisms, they suffer a stronger euphoric and emotional impact from alcohol. Finally, studies have shown that teenagers tend to underestimate their actual level of intoxication.

The negative impact of these specific factors is even larger, when combined with other exposure factors common to young driver crashes, such as the presence of passengers.

Drugs

Different drugs have different effects on driving performance, especially when combined with alcohol or with other drugs. Drugs can be either illicit drugs or prescribed medication. The use of these drugs by the driving population is strongly related to the legal restrictions on their use in a country. As these restrictions differ between countries, it results in different patterns of use while driving.

Within Europe, illicit drug consumption gradually increases in the age group 15 to 25 years old. The pattern of use differs between boys and girls. Whereas the use of legal drugs is more common among girls, alcohol, cannabis and ecstasy are more frequently used by boys.

Compared to testing for alcohol, the drug screening of drivers creates a variety of problems. For instance, in some countries there is a political debate on the legalization of drugs, especially cannabis. Although the legalization of drugs in general is a different subject than that of the legalization of drug use in traffic, the debate will have an influence on the issues related to drug driving. Screening for the presence of drugs is not as clear cut as testing for alcohol. First of all, 'safe' intoxication levels for legal drugs still need to be determined [17]. In addition, the road-side screening methods are not sufficient for reliably assessing the momentary intoxication level in a driving. For this purpose, the European Union's ROSITA project is evaluating various toxicological measures, such as analyses of urine, saliva, sweat and hair.

Outcomes from epidemiological case-control studies indicate that use of illicit drugs in general is a source of risk for young, novice drivers. More in particular a combined use of drugs: these drivers have 25 times higher risk of a serious road crash injury. The combination of drugs and alcohol leads to a risk which even is 35 times higher [22] .

Fatigue

Fatigue is not only caused by the number of hours driving, but also by the time spent on other tasks before driving, by length and quality of sleep, or by stressful situations. Another factor that causes fatigue is the time of day the trip is undertaken. This is particularly important for young, novice drivers, as their relative crash rate at night is higher than that of adults.

Fatigue reduces the quality of many aspects of the driving task, such as adequate reactions to sudden changes, and tracking. Drivers try to compensate for the influence of fatigue, for instance by either increasing the task demands (e.g. driving faster) or lowering them (e.g. increasing the safety margins by slowing down of using larger following distances,). But crashes and observations of driving performance show that these compensations are not sufficient to remove all excess risk.

As adolescents need more sleep than adults, which they do not usually observe, fatigue may affect youngsters even more than adults.

Distraction

The driving task requires the driver's attention to be focussed on the traffic at all times. However, attention is easily and sometimes involuntary drawn to other events, objects or persons, inside or outside the vehicle, which are not related to the driving task (distraction).

Attentional requirements of some driving tasks are higher for novice drivers, so distraction may affect them more than adults.

Special attention in this field must be paid to youngsters' extensive use of mobile phones and music devices and the distraction caused by the presence of excited passengers.