The concepts of risk compensation and risk homeostasis are often used to describe, or to explain drivers' tendencies to react to traffic system changes whether in roads, vehicles, weather conditions or in their own skills. However, it is important to distinguish between the general phenomenon and mechanisms underlying it. This paper first points out that to understand the basic mechanisms it is necessary to split accidents and exposure into smaller entities to arrive at basic units of exposure which also represent fundamental driver tasks. Risk-related behaviour should be considered at several hierarchical levels with different mechanisms to produce `risk compensation'. At a high level, trip decisions modify populations at risk in different circumstances, sometimes attenuating, sometimes amplifying population risk differences. At a low level of vehicle control and guidance in real dynamic traffic situations, simpler control mechanisms which result in behavioural adaptation can be identified. All these effects influence the end result of accident risk as separate mechanisms. (A)
Abstract