Highway geometric design is carried out on the assumption of a design speed, which implies the notion of a "design driver". However, it is well known that driving behavior generally displays considerable variability. In this paper, two different perspectives of driving behavior variability are dealt with. The former sees variability as a consequence of the stochastic nature of physical quantities relating to driver behavior, such as speeds, acceptable friction factors, and perception-reaction times. According to the latter approach, variability is attributed to systematic differences among categories of road users; a notable example in this area is the case of aging drivers. Next, the authors present the two main philosophies for coping with driving behavior variability: one is focused on reducing variability, mainly through policy measures, whereas the other calls for engineering-based measures aimed at accommodating variability within highway design. Within the context of the variability-accommodation approach, emphasis is placed on the improvement of highway design guidelines. The concept of "forgiving design" is proposed, referring to the specification of above-minimum design requirements for locations of increased complexity, where the safety of "non-design" road users (such as older drivers) may be severely reduced. Examples of candidate cases for forgiving design are presented by the authors, and suggestions are made regarding the future incorporation of the concept in highway geometric design guidelines.
Samenvatting