This paper reviews the available material for the production of a database that should underlie the definition of the so-called 'design road user'. After considering the basic questions dealing with the incorporation of human factors knowledge into highway design guidelines, an inventory is presented of known dose-effect relationships between road user characteristics on the one hand and behavioural as well as safety parameters on the other. These characteristics are divided into background characteristics (of either a permanent or temporary nature), driver information processing characteristics, and behavioural parameters as they can be observed on the road. While useful knowledge on several characteristics is already available, the investigation of several variables that may be very relevant has hardly started. Among the latter are the following: 1) Life-style, i.e., composite profiles as interrelated patterns of rather mundane characteristics that are more descriptive of an individual than simple uni-dimensional variables; 2) Attentional characteristics. Of the information-processing functions, attention - the stage preceding the actual processing by the senses - appears to be of central relevance. This pertains, on the one hand, to the 'Useful Field of View', and on the other to the capacity to shift attention from one object to another; 3) Decision-making characteristics. Individuals differ in the way in which they reach decisions. If the situation is risky, the perception of the risk and the thoroughness with which the decision is taken appear to be of prime importance; and 4) Driving behaviour parameters. If safety is a desired outcome of the highway design process it should be known what the accident risks associated with certain driving behaviours are. This can only be the case if quantitative relationships are developed which link behavioural parameters to the ensuing accident probability and/or severity.