Over the years there has been an accumulation of evidence that suggests that road users in general, and drivers in particular, respond and adapt to safety treatments. This adaptive behavior may occur over time or over space. Although in same cases, red light cameras for example, the adaptation may result in a positive safety effect, in most cases the consequences can be negative due to increased risk taking behavior such as speeding, aggressiveness, or inattention.

The idea of behavioural adaptation to road safety measures is not new (See, e.g., OECD, 1990.). Wilde (1988) suggested that it is so prevalent that the level of risk in the transport system essentially remains constant despite safety interventions -- the so-called risk homeostasis theory. However, researchers such as Underwood et al. (1993), who cite several examples of adaptive behavior following safety interventions, have overwhelmingly disputed the risk homeostasis theory. The purpose of this paper is not to contribute to the philosophical debate on the presence and extent of adaptive behavior, but to make the case, based on empirical crash-based evidence, that the influence of driver adaptation on treatment effectiveness needs to be considered in applying crash modification factors for these treatments in making cost-effective infrastructure investment decisions. The empirical evidence presented focuses to some extent on results from several studies involving the authors and is not intended to be exhaustive. Treatments for which evidence is presented and discussed are curve delineation, raised pavement markers, red light cameras and pavement friction improvement.