Accidents are preceded by long histories containing multitudes of events that constitute promising targets for preventive action. These antecedent events can be classified into at least four groups that occur in this order: failure types; psychological precursors; unsafe acts; and breakdown of defences. It is argued that events directly preceding an accident, such as breakdown of defences and unsafe acts, are only haphazard tokens of the more permanent weaknesses within a system, called failure types. Elimination of a type will therefore have much more impact than the elimination of one or a few tokens. It is also argued that there exist only a limited number of failure types, which are responsible for all accidents. However, in the specific area of road accidents, it is not known which types cause most of the problems. Therefore, their relative importance can only be guessed. We guessed that hardware problems and maintenance are unimportant types; that education and regulations are of moderate importance; and that incompatible goals, conditions promoting unsafe behaviour, and organizational inadequacy are the types that cause most of the accidents. The latter therefore constitute the most promising targets for accident prevention.
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