This paper reviews the available material for the production of a databasethat should underlie the definition of the so-called 'design road user'.After considering the basic questions dealing with the incorporation of humanfactors knowledge into highway design guidelines, an inventory is presentedof known dose-effect relationships between road user characteristics on theone hand and behavioural as well as safety parameters on the other. Thesecharacteristics are divided into background characteristics (of either apermanent or temporary nature), driver information processingcharacteristics, and behavioural parameters as they can be observed on theroad. While useful knowledge on several characteristics is already available,the investigation of several variables that may be very relevant has hardlystarted. Among the latter are the following: 1) Life-style, i.e., compositeprofiles as interrelated patterns of rather mundane characteristics that aremore 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 - appearsto be of central relevance. This pertains, on the one hand, to the 'UsefulField of View', and on the other to the capacity to shift attention from oneobject to another; 3) Decision-making characteristics. Individuals differ inthe way in which they reach decisions. If the situation is risky, theperception of the risk and the thoroughness with which the decision is takenappear to be of prime importance; and 4) Driving behaviour parameters. Ifsafety is a desired outcome of the highway design process it should be knownwhat the accident risks associated with certain driving behaviours are. Thiscan only be the case if quantitative relationships are developed which linkbehavioural parameters to the ensuing accident probability and/or severity.