Preface A large number of factors influence the number and severity of road accidents. Some of these factors are very basic in their nature, such as biological factors. Analyses pointing to the role of biological factors in contributing to accidents have often been ridiculed. However, there is no reason to doubt the importance of these factors. This study of risk factors associated with road accidents in Portugal con-firms the influence of biological factors. Although these factors are not always measured directly, the pattern of accidents and variation in risk indirectly support their importance. There are, for example, more acci-dents during those times of the day when people are more likely to be fatigued, such as when they return home after a long working day or when they drive at night. The fragility of old people is shown by the fact that their chances of sustaining fatal injuries are greater than for young-er road users. Perhaps some of the traditional scepticism to assigning biological factors any role in contributing to accidents is that these factors are regarded as impossible to change and accidents related to them therefore regarded as impossible to prevent. It is true that there is very little an individual can do about his or her age and gender and the basic biology associated with age and gender. But it is not true that accidents that have been in-fluenced by biological factors cannot be prevented. Take fatigue for example. It is natural and normal for people to get tired as the day goes along; the dip in circadian rhythm after lunch is well-known and everybody has experienced it. The potential risk associated with it can be mitigated in many ways. One may postpone trips until the dip is over. Roads may be marked by rumble strips that alert drivers when they veer off course. Cars may be equipped with fatigue detection and warning systems. Training drivers to detect signs of fatigue may help them to pull over and have a short nap before continuing their trip. In short, the mere fact that something is related to our basic nature as animals does not, by itself, place it beyond the realm of accident preven-tion. It has to be admitted, however, that some factors may be more difficult to influence than others. It seems clear that high levels of testosterone in young males make them take more risks than any other group of driv-ers. There is no doubt that this contributes to the very high risk of acci-dent involvement of young male drivers. Despite this, it is probably a bad idea to try to suppress testosterone below its normal level in young males. There is evidence that sexual hormones are related to female ac-cident involvement as well. Again, however, one should probably not try to tamper with nature. What we need to do, is to better understand the role biological factors play in our lives and accept them as part of our normal human nature. Such an enriched understanding is likely to help in counteracting some of the unwanted effects of biological factors. Oslo, September 18, 2013 Rune Elvik