Target level of risk

If there is a utility model determining target risk, then it should be feasible to formulate that model in mathematical terms, or alternatively to derive the utilities and trade-offs through the standard game-type approach used in stated preference studies. However, Wilde (1988) denies that it is possible to derive quantitative utilities [T]he target level of risk is thought of as a construct that people arrive at in an intuitive manner, not as the result of explicit calculation of probabilities of particular outcomes and their positive or negative values (p. 443). Thus, we have a model whose main mechanism cannot be verified empirically because it is only an intuitive construct. Therefore, it can be concluded that what we have here is more assertion than model. (For more on risk homeostasis, the reader is directed to Chapter 5.)... 

The notions of target level of risk (see Section 5.1) and homeostasis when applied to driver behaviour can conveniently be incorporated in the flow diagram in Figure 5.1. 

FIG U RE 5.1 Homeostatic model relating the accident rate per head of population in a jurisdiction to the level of cantion in road-nser hehavionr and vice versa, with the average target level of risk as the controlling variable. (Adapted from Wilde G.J.S, 1982. Risk Analysis, 2, 209-225.)... 

What RHT does not say is that the target (i.e., accepted or tolerated) level of accident risk by road users is immutably fixed and invariable under all circumstances although this misquotation can be found in many lay and even academic and professional communications. Instead, what RHT does say is that the target level of risk depends on the balance of the expected benefits and costs of choosing risky versus cautious behaviour alternatives. This balance varies as a function of the state of the economy, and especially as a consequence of countermeasures that are intended to increase a greater desire for safety. 

The first scientific attempt to deal with the issue of compensation was the classic field-theoretical study of Gibson and Crooks (1938). One theoretical concept was the field of safe travel and a key observation was the impact of brakes on driver behaviour better brakes could make the field of safe travel—that is, the distance to the car in front—shorter. Nearly 50 years later, the launch of risk homeostasis theory (RHT) gave rise to a profound debate about risk, risk homeostasis and risk compensation (Wilde 1988, 1982 see also Chapter 5). The core issue in the debate was Wilde s determined assertion that all individuals, not only car drivers, carry an inherent target level of risk that they are seeking to maintain or restore. 

Wilde (1982, 1988) Risk homeostasis theory and target level of risk... 

Perceived costs and benefits of action alternatives - Target level of risk (8) - Desired adjustment a-b "0 2... 

SIL is defined as the relative level of risk reduction provided by a safety function, or a specified target level of risk reduction. SIL is a measurement of performance required for an SIF. Safety integrity is the probability of an SR system satisfactorily performing the required safety functions under aU the stated conditions and the stated time period. 

This theory is based on the assumption that the operator is able to perceive a risk level in the environment that is related to the actual risk level through a delayed feedback. The operator adapts his or her behaviour in such a way that there is a balance between the hazards that the operator is exposed to and what he or she can accept (target level of risk). It follows from this theory that measures to reduce hazards at the workplace will be followed by changed behaviour on the part of the operators such that their perceived level of risk remains the same. The only way to improve safety is to change the operators target risk level. The risk-homeostasis theory was primarily developed for situations where people voluntarily expose themselves to accident risks such as in traffic. We must expect, however, that employees at a workplace also to some extent control the risk that they are exposed to based on their own experiences and preferences. 

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