Risk homeostasis: issues for future research

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Safety Science, 16 (1993) 19-33 Elsevier

Risk homeostasis:

issues for future research

T.W. Hoyes and A.I. Glendon Human Factors Research Unit, Aston Business Birmingham B4 7ET, UK

School, Aston University,

Aston Triangle,

(Received 23 March 1992; accepted 13 August 1992)

Abstract Hoyes, T.W. and Glendon, A.I., 1993. Risk homeostasis: 16: 19-33.

issues for future research. Safety Science,

Risk homeostasis theory (RHT) is considered in the context of four methodological and conceptual issues. The first question to be considered is whether the theory, in the terms in which it has been proposed, can be falsified. It is suggested that advances in understanding RHT could be made by clearly identifying what findings would constitute a falsification of it, since at present this is problematic. The second issue is whether psychologically invisible interventions might preclude a homeostatic effect. It is suggested that, conceptually at least, psychological invisibility need not preclude homeostasis. Third, is debated the issue of bi-directionality - whether a homeostatic effect can be observed when there has been a reduction in safety as well as when there has been a safety improvement. It is argued that whilst no finding would appear to be in clear violation of RHT’s being bidirectional, this is due in part to the global terms in which RHT has been postulated. Finally, it is argued that simulation exercises which mimic physical risk and utility might usefully be employed as a methodology to examine psychological factors associated with RHT.

R&urn6 La theorie sur l’homeostasie de risque (RHT) vue dans la lumiere de quatre possibilitks methodologiques et conceptuelles. La premiere question qui se pose c’est de savoir s’il est possible de falsifier la theorie telle qu’elle est proposee. On avance qu’il est possible d’approfondir les connaissances de la RHT en identifiant clairement quels sont les resultats qu’on pourrait considerer comme &ant falsifies puisque ceci constitue un probleme h l’heure actuelle. La duexieme question c’est de savoir si des interventions psychologiquement invisibles pourraient prevenir un effet homkostatique. On avance que, tout au moins sur le plan conceptuel, une invisibilitk psychologique n’emp&he pas necessairement une homeostasie. Troisiemement on discute la possibilite du “double sens” - c’est a dire s’il est possible de noter l’apparition dune homeostasie lorsqu’il y a degradation ainsi qu’amelioration de la securite. On dit que si aucun resultat ne semble aller directement a l’encontre de l’hypothese du double-sens de la RHT, c’est partiellement grace a la formulation g&r&ale de la RHT. Pour conclure on avance qu’on pourrait se servir de maniere utile en guise de methodologie des exercices de simulation reproduisant les risques physiques et utilites pour examiner les facteurs psychologiques lies a la RHT.

Elsevier Science Publishers

B.V.

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Zusammenfassung Die Risikoh(~me(~stasethe(~rie (RHT 1wird anhand van vier Fragen beziiglich der Methodik und des Konzepts betrachtet. Die erste Frage ist, nb die Theorie in ihrer vorgeschlagenen Form gefBlscht werden kiinnte. Es wird behauptet. dari. das Versttindnis der RHT vergriiRert werden kann, indem man deutlich angibt, welche Befunde eine FBlschung darstellen wiirden. da dies derzeit ein Problem ist. Die zweite Frage ist, ob psychologisrh unsichtbare Eingriffe einen homeostatischen Effekt ausschliefjen kiinnten. Es wird behauptet, da& die psychologische Unsichtbarkeit zumindest nach dem Konzept Homeost.ase nicht. unbedingt ausschiie~en mu& Die dritte Frage ist die der Bidi~ekt.ionalit~t, d.h. ob ein homeostatischer Effekt wahrgenommen werden kann, sowohl wenn eine Verringerung der Sicherheit stattgefunden hat als such wenn es eine Verbesserung der Sicherheit gegeben hat. Obwohl anscheinend keine Befunde deut.lich gegen die Bidirektionalitgt der RHT verstolSen. wird brhauptet, daR dies zum Teil auf die allgemeinen Ausdriicke, in denen die RHT postxliert w!)rden ist., zuriickzufiihren ist. Zum SchluR wird behauptet, da& Simulierungs~bungen. hei del~en das physikalische Risiko und der Nutzen nachgeahmt wird. niitzlich als eine Methodik zur PI iif~mg psschologischer. mit der RHT in Verbindung gebrachtrr Faktoren. angewandt werden k6nnt.e.

1. Introduction That technical safety improvements are worthy ends in themselves presupposes that relevant behaviour of those affected does not change so as to negate them. Risk homeostasis theory (RHT), a model of risk taking originally proposed by Wilde (1982a, 1982b, 1984,1985,1986a, 1986b, 1988a, 1989) suggests ways in which such negation might occur. RHT can be summarized as follows (although the model is generalizable, it is usually stated in terms of road user behaviour ) : (i) Each road user has a target (or accepted) level of risk which acts as a comparison with actual risk. Where a difference exists, one may move towards the other. Thus, when a safety improvement occurs, the target level of risk motivates behaviour to compensate - e.g. driving faster or with less attention. RHT has not been concerned with the cognitive or behavioural pathways by which homeostasis occurs, only with the consequences of adjustments in terms of accident loss. (ii) Homeostasis operates under a population-level closed loop. Thus, individuals’ experiences are only important in so far as they make up populations. For example, while a road user who made an error which resulted in his or her death can no longer adjust behaviour through a closed-loop process, the population as a whole can. RHT is silent on the role of the mere information of a safety change, but has something to say about the results of the change when fed back to the population over the long term. Therefore, demonstrating that a safety improvement has brought. about a reduction in accidents in the short term is not to refuutethe theory: accident loss may change as adjustments continue to be made. Whether RHT takes place at a population level is unclear,

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as Wilde’s original exposition (Wilde, 1982a) was in the form of an individual driver behaviour model. However, subsequent formal presentations of RHT have explicitly stated that the effect is at a population level (e.g. Wilde, 1988a). (iii) RHT does not posit risk for its own sake. For negation of safety improvements to occur, compensation must bring benefits. However, showing that a safety improvement has not been negated does not refute RHT when the utility of a change to riskier behaviour is either zero or negative. (iv) Wilde (1988a) defines accident loss as “... the sum of the cross products of the frequency of accidents and their costs.” (proposition 1). Target levels of risk might therefore vary with accident costs or accident numbers or both. While accident probability might remain unchanged as a result of some interventions (e.g. collateral strategies), a safety improvement could also be a reduction in accident costs. To summarize, RHT proposes that the amount of accident loss “accepted” by the aggregate of road users in return for benefits accruing from road use determines the behaviour of the road user population. Within RHT research, several issues have attracted particular attention. The central issue of whether the evidence supports or refutes RHT is disputed (e.g. Evans, 1985,1986a, 1986b,l991; McKenna, 1985,1987,1988,1990). This paper considers this question in the context of the debated issues.

2. Can RHT be falsified? A frequent criticism of RHT is that its stated terms preclude its falsification (e.g. Adams, 1988). In assessing this criticism, several scenarios are examined, each assuming a safety improvement. Illustrative scenarios rather than empirical studies are considered because although empirical evidence is relevant to falsification, it may be inadequate or open to different interpretations and falsification is essentially a theoretical matter. Scenario 1. Before and after an intervention, measures are taken of driving speeds, headway, number of overtakes and judged riskiness of each overtake. None of these measures provides evidence of behaviour change. Scenario 2. Numbers of road fatalities are counted before and after an intervention. The figures are re-examined after l$ years. (In 1982 Wilde tentatively suggested a time-span of 1 year for the operation of RHT; in 1988, he proposed that the maximum time span for homeostasis to operate is between 11 and 2 years; although in 1989 he maintained that RHT does not formally specify the time period of its operation.) Fewer people die in the second period - evidence for a sustained effect of the intervention. Scenario 3. Accident loss is calculated for a road user population lf years either side of an intervention. Total accident loss falls after the intervention

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and, 1; years later, remains significantly lower - risk exposure remains constant. Scenarios 1 and 2 are typical of attempts to falsify RHT, but do they? The first does not because RHT does not predict particular behavioural pathways for its operation (Wilde, 1988a). If the behavioural pathways by which homeostasis occurs are not specified, then eliminating any number (short of all) of them, whilst contributing to our understanding of how the theory operates, cannot constitute a falsification. Thus, the findings of Lund and Zador (1984 ), who failed to find evidence of compensation in four driver behaviours, are not incompatible with RHT. Smith and Lovegrove (1983) provide limited support for RHT using specific driver behaviours (including one - speed - in common with Lund and Zador). It is possible that when looking at specific behaviours, researchers who f?tnd evidence for compensation support the theory, while those using the same criteria yet who fail to find such evidence, might have committed a methodological error. Alternatively, such findings may reflect the insensitivity of such measures as valid indicators of risk-related behaviour. Seeking a homeostatic effect through specific behavioural pathways, in the absence of a proper theory, can only provide one sort of evidence - if an effect is found, this accords with expectations, but if no effect is found then this does not necessarily refute the theory. Scenario 2 takes into account Wilde’s time-lagged criterion - there is a sustained reduction in fatalities over an extended period. This finding too can be reconciled with RHT. RHT does not specify number or rate of fatalities as outcome measures but rather total accident loss. Thus, even if the number or rate of fatalities falls, if non-fatal accidents have risen then total accident loss may have been maintained. Second, if risk exposure falls, then the reduction in fatalities may reflect increased safety per km of exposure - which is not incompatible with RHT. An examination of scenario 3 must begin with the issue of how total accident costs are measured. While algorithms and criteria have been proposed for such calculations, some costs are subjective. Assuming that reasonable estimates of total accident loss are possible, would scenario 3 constitute a falsification of RHT? At this stage (and we are now describing a level of methodological sophistication that has not been reached in RHT research to date) the findings might start to embarrass RHT proponents, and would be difficult but not impossible to square with RHT. RHT proponents could refer to Wilde’s ( 1988a) proposition 3, which explains that target level of risk is determined by four sets of utilities: benefits and losses associated with relatively risky behaviour, and benefits and losses associated with relatively safe behaviour. If the target level of risk in the population changes in favour of improved safety, then there will be fewer accidents and less accident loss. But how do we know that the target level of risk in the population has changed? The answer, that there will be fewer accidents and/or less accident loss, is unhelpfully circular.

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This may be unfair to Wilde, since he has so far only cited this proposition when he has identified a probable cause of the change in relative utilities (e.g. fuel price). But as these changes do occur, a confounding variable will usually be found. Moreover, risk homeostasis utilities are subjective. Arriving at a destination three minutes sooner could be a “positive” utility for a, utility-free for b and a cost (negative utility in models like O’Neill’s (1977) ) to c. Only population-level perceptions of the value of one outcome relative to another can be estimated. Through these scenarios, it may be seen that not only is there no evidence to date that cannot be reconciled with RHT, but more critically, that so long as RHT is postulated as it is at present, it is impossible to falsify. This feature is compounded by the proposition that compensation may occur in some domain other than that in which an intervention has been made. Two things seem to preclude falsification. The first is a definition of accident loss that cannot be satisfactorily measured. Thus, as long as something remains the same (or changes in the predicted direction) after an intervention, RHT cannot be refuted. Second, when a safety feature brings about a reduction in all measures of accident loss, a motivational change also usually occurs - then there is no case to answer, and RHT remains unfalsified. This is compounded by the usual accompaniments to safety interventions of attempts to induce motivational change (e.g. advertising). If an accident-loss reduction occurs, what caused it? Was it the intervention or gory pictures that accompanied the intervention and made everybody want to be safer? The Health Belief Model (Becker, 1974) indicates that the likelihood of taking preventive action is dependent upon three crucial factors: personal risk perception (vulnerability and severity of outcomes), perceived benefits of preventive action minus perceived barriers and cues to action such as mass media or personal experience. Until RHT advocates define accident loss and motivational changes more objectively, its falsification is hard to envisage. Risk homeostasis theory would be greatly advanced if its proponents would spell out clearly and explicitly what sort of evidence would constitute its falsification.

3. Psychological invisibility and RHT A second conceptual issue, raised by Slavic and Fischhoff (1982 ) and by McKenna (1985), is that of psychologically invisible changes in risk. McKenna’s position is that safety improvements can be made that are impossible for risk-takers to identify. For example, a collision-absorbent bumper looks much like a normal bumper. How can drivers compensate for safety improvements that they cannot perceive? In the short term, drivers cannot. If RHT has an open-loop component, psychological invisibility must starve it of infor-

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mation in the short term. However, RHT is said to work in a closed loop and over the long term. If information on accident loss enters that closed loop as a reference variable, what effect would this have? If the safety intervention is effective, fewer people are killed or injured. As accident loss reduces, and personal experience, media reports and other sources inform road users of this, then RHT predicts compensatory behaviour. The role of the media in informing risk perception is problematic. Reports of road accidents may increase road users’ percept,ions of road accidents as a social problem without affecting their personal risk perception. Tyler and Cook (1984) observed this phenomenon for mass media effects upon risk judgements about crime. Evans doubts the impact of national fatality data or news reports upon road-user behaviour (Evans, 1991, pp. 285 and 294), although the considers that fictional television and movie portrayals of the life-threatening use of motor vehicles as heroic or non-dangerous has an important influence upon social norms relating to driving (Evans, 1991, p. 388). However, other sources of information are available to road users. It is an open question as to whether risk information is specific to drivers and passengers of vehicles affected by the improvement or whether it generalizes to the driving population. A psychologically invisible improvement may have a generalized effect, but compensation somewhere within the driving population may still occur. The difficulty is knowing where to look for an effect. Those drivers and passengers directly affected by the intervention may enjoy a reduction in total accident loss, but the whole driving population may not. However, short-term psychological invisibility is not a means of circumventing RHT’s essential predictions. Adams (1988) suggests that psychological invisibility has little practical relevance to risk homeostasis. The introduction of safety improvements is usually accompanied by some public debate. Safety is also marketable. If one manufacturer develops cars which are safer than other cars, why keep quiet about it? Adams’ example is that of Volvo, who have sold safety. One might ask about the morality of withholding information on the relative safety of one form of transport (e.g. road, type of car). Thus, psychological invisibility need not be addressed in the abstract and empirical evidence is required in order t,o establish the possible operation of long-term effects.

4. Bi-directionality and RHT Most evidence supporting RHT relates to accident loss after safety improvements. An interesting question arises when safety is reduced (we refer to this two-way mechanism as bidirectional). RHT would predict that when risk level worsens, compensation should follow. Thus, as safety improvements are compensated, so reduced safety should be negated. Evans (1991) presents results

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from studies to illustrate that the complete range of behaviours in response to engineering changes may be observed - both in the direction of improved safety (e.g. reducing speed limits) and of decreased safety (e.g. increasing speed limits). McKenna (1985) points to research findings including those of Codling (1974) and Hawkett (1978) which seem to show that drivers do not compensate for wet roads. Instead, they make little adjustment in speed to suit the conditions and have more accidents. Can findings like these be reconciled with RHT, or do they represent a “refutation of risk homeostasis theory”? (McKenna, 1985, p. 494) . Several points are pertinent. (1) Wilde’s formulation of RHT concerns long- rather than short-term behaviour. Wet roads may well dry out before the sort of processes that Wilde and others describe have time to occur - i.e. before road-users can accommodate to the change in risk. However, in one sense wet roads are not a shortterm phenomenon. Although the driving population may not be able to perform risk homeostatic operations from a single day’s exposure to wet roads, they can compensate on subsequent exposure to wet roads. The time a road stays wet on any given day is now less relevant than the total time in which the driving population is exposed to wet roads. This feature of the driving environment could be brought under closed-loop control. Thus, adverse changes in safety could produce a long-term generalized response rather than a shortterm specific response. However, in the case of wet roads, feedback is given immediately in the form of reduced visibility associated with the condition and longer stopping distances so there is also short-term homeostasis. (2) Compensation in respect of environmental hazards such as wet roads, fog, and ice, is relevant to open-loop mediation in RHT. In the case of hazards like these, drivers may not immediately know through the closed-loop processes of their own experience that they are in more danger, but then they hardly need so sophisticated a device. Surely, the mere information that ice is there should lead to the same conclusion as that arrived at through long-term feedback processes - for the same level of risk, behaviour changes are essential? (3) Adams (1988) cites evidence to contradict McKenna. In Ontario, snow and ice have been shown to be associated with reductions’in both fatalities and accident severity. In Sweden the change from driving on the left to driving on the right was followed by a 40% reduction in fatalities compared with prior levels (Adams, 1988). How can evidence cited by McKenna and by Adams differ so? The answer lies in choice of denominator. Adams reports numbers of fatalities and severities. He notes that accident levels are lower in winter months than in summer months. But do not fewer people drive in winter than in summer? Yes, acknowledges Adams; but if a reduction in safety has changed the behaviour of these drivers - caused them to stay at home or to take a train rather than drive their cars - this is part of the homeostasis process and should be taken into account. RHT predicts compensation will occur at the level of time unit of exposure. Adams’ data are silent on this possibility, unless of course

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this time unit includes exposure to hazards in the cognitive sense of awareness that they exist, as well as being physically exposed to them. Since bi-directionality has been linked with falsification these issues are examined together in a new scenario: Scenario 4. Measures reflect accident loss per time unit of exposure on dry, clear days and on icy/foggy days. Results indicate increased accident loss on bad weather days. Does this falsify RHT? Wilde (1989) answers by pointing to the prevailing target risk level. Who, he asks, are these people who drive their cars in such bad conditions? His answer is that this subset of the population may differ significantly from those who stay at home in response to inclement weather. Perhaps those who venture out have a higher target level of risk. Perhaps they are poor at perceiving the actual level of risk that they face. If these possibilities are accepted, then scenario 4, like the first three, does not falsify RHT. If Wilde’s point about a subset replacing a population after a decrease in safety is accepted, then the same may hold true when there is a safety improvement. Suppose that a law restricts the speed at which motor-cycles can travel on public roads to a maximum of 50% of the speed that applies to other motorised vehicles. This law effectively reduces total accident loss. However, risktaking (or sensation-seeking) young motor-cyclists may have switched to some other form of transport - or to some other form of enjoyment. The road user population will then have changed, and with it, the aggregate target risk level. The foregoing is important for RHT researchers: there is little point in measuring any component of accident loss resulting from a reduction in safety, since it is difficult to envisage a finding that would be significant for the theory, other than to support it. Thus, findings on this issue, as with others, either support RHT or are irrelevant to it. If accident loss is reduced, it may be because of a changing population - not incompatible with RHT; if accident loss is unchanged, then RHT is supported.

5. Risk homeostasis

theory and simulation

One research tool for examining RHT is simulation, although whether RHT can be investigated in this way is questionable. Because RHT is postulated as a long-term, closed-loop, population-level phenomenon, it might be argued that: (i) According to RHT, compensatory behaviour is only seen in the long term. Only where behaviour is observed over months or years can homeostasis be examined. In thirty minutes’ simulated driving, how could compensation effects be observed? (ii) RHT is postulated at a population-level - at least for serious (e.g. fatal) accidents: the behaviour of a hundred or so experimental participants is virtually irrelevant. However, where compensation occurs within a population is

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problematic. Does a homeostatic effect result from a small effect from a large proportion of the population, or could it be a large effect from a relatively small number of individuals? Taylor (1964) provides evidence that a single individual can operate homeostatically. (iii) RHT is about real situations. Whatever a simulated exercise in RHT might find, would participants behave in the same way in a real traffic environment (Wilde, 1982b)? A simulated traffic environment might be treated trivially. However, a strong case for examining RHT via simulation studies can be made. (1) Although simulations involve a very different time-span from that envisaged in RHT, a simulated environment (like driving in bad weather) offers the possibility of “collapsed experience”. Although populations are said to tend towards risk homeostasis in the long term, this is because the sort of events alleged to influence compensatory behaviour tend to take place in the long rather than the short term. If these events (e.g. accidents, near misses) could, through simulation, be available in a short time span, then why should RHT not be examined in the short term? (2) Although RHT is postulated at a population level, this does not preclude examining a relatively small number of individuals as this could be valuable in modelling risk homeostasis components. Because mechanisms for population level homeostatic effects are unspecified in the theory, seeking possible individual cognitive and behavioural pathways may be more fruitful. (3) The question of how “seriously” participants take simulations remains an issue. Experimenters can ask participants to behave as they would when driving a car, explain that otherwise there is no point in the experiment and subsequently ask them whether they felt that their behaviour was realistic. However, the extent to which simulation is treated like real driving is less of an issue where a homeostatic effect is observed. If participants are compensating for simulated safety changes, then this could indicate that concepts such as safety and risk may be generalized between real and simulated environments. Finally, using simulators in RHT is not about providing definitive answers to risk homeostasis questions. Simulations do permit experimental manipulation of variables which might mediate the effect. Simulator findings complement those from field work, secondary data sources or theoretical modelling on which the main case for RHT currently rests. That driving simulators can provide a means for testing aspects of RHT was demonstrated by Dorn, Glendon, Hoyes, Matthews, Davies and Taylor (1991) and by Hoyes, Dorn and Taylor (1992 ), who introduced an ABS feature into the Aston Driving Simulator. RHT has also been examined experimentally (e.g. Mittenecker, 1962; Cownie, 1970; Naatanen and Summala, 1975; Veling, 1984; Wilde, ClaxtonOldfield and Platenius, 1985; Wilde, 1988b). These studies provide limited

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support for RHT. However, are these experimental methodologies appropriate to RHT investigation? Can experimental “game” conditions be generalized to risk taking on the roads? Wilde et al. (1985) cite Jackson, Hourany and Vidmar (1972) as evidence for generalization of risky behaviour. However, Jackson et al. refer to an operationalized concept of risk which is independent of homeostatic behaviour. Whether RHT findings can be generalized from situations involving the possibility of harm to those which do not is problematic. In the absence of the possibility of harm, RHT studies may become investigations of optimization strategies. Three years before his own simulation work, Wilde wrote: “. . simulation of risk, like a sham duplicating the real thing, is a contradiction in terms” (Wilde, 1982a). Veling’s (1984) “simulator” investigation illustrates this point. Here, participants were required to maximize their “points” in a game-type approach. In such a design - Veling’s is fairly typical - there is a right and a wrong response. One type of behaviour maximizes points and minimizes penalties, another minimizes points and maximizes penalties. It could be argued that Veling’s design, and others like it, tested nothing more sophisticated than the hypothesis that participants were bright enough to understand this. How this parallels RHT behaviour is uncertain. In RI-IT, risk is traded for perceived benefits. There cannot be a right course of action, only a chosen course of action, and unless this important distinction is acknowledged in RHT simulations, something other than risk homeostasis is being assessed. A more subtle reason for suggesting that RHT cannot be examined by gametype approaches is that any effect involving a general optimization process is likely to be cognitively mediated - i.e. compensation may operate at quite a high level of consciousness. Compensation processes described by Wilde and others appear not to be analogous to this. As Wilde (1988a, p. 443) wrote: “They [the factors that determine individual target levels of risk] are probably so thoroughly internalized that most individuals are not consciously aware of most of them most of the time”. He goes on to say (Wilde, 1988a, p. 444 ): “The comparison [between target level of risk and experienced level of risk 1 would normally be expected to occur at an intuitive and moderately conscious, i.e. ‘pre-attent,ive’ level (Ben-Davidet al., 1972 )“. We have found no evidence that, processing optimization strategies within a game-reward experimental design is at this same “pre-attentive” level. This accords with demand characteristics which emphasise links between personal task performance and financial reward. Thus, this sort of design, along with attempts to mimic real physical risk, appear to measure processes at a different level of cognitive complexity to that applying to risk compensation. A further issue is that of the extent to which risk-taking is a general trait which manifests itself across different types of risk. While Slavic (1962 ), Kogan and Wallach ( 1964 ) and Weinstein ( 1969) found little evidence t.o suggest that risk taking can be generalized, Jackson et al. (1972) identified distinct monetary, physical, social and ethical risk taking factors which were substan-

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tially correlated. At the second order factor loading, there was a single dimension on which all four facets of risk taking were salient, which they interpreted as generalized risk taking. Wilde’s own findings do not offer much support for the generality of risk taking, and overall, there is little empirical evidence for generalizing non-physical risk-taking to physical risk-taking. Finally, if the mechanisms postulated by RHT are cognitively mediated (on which there is little evidence) then decisions to change behaviour so as to negate safety improvements should be subject to ethical deliberation. In game type experimental designs, moral questions are inapplicable: maintaining a constant level of risk is morally legitimate when no harm can occur to oneself or others. But why examine RHT in a simulator at all? If its conceptual roots lie in the road traffic data it seeks to explain, is not laboratory investigation of dubious ecological validity? The reply is that simulation is something we might prefer to avoid, but that it is necessary if manipulative experiments are to be carried out - required to examine causality and other issues. For example, do homeostatic operations occur at the pre-attentive level that Wilde (1988a) suggests, or do they demand attentional resources? Can the cognitive mediation of risk information received be given an algorithmic description? If so, do individual differences exist? Do attentional reductions follow safety improvements? Such questions as these cannot be answered by reference solely to accident data. If they are to be tackled, controlled laboratory-based experiments involving simulation of physical risk could be fruitful.

6. RHT research in the future Comparisons of RHT with flat earth theory (Evans, 1991, p. 299) are as unhelpful as they are inaccurate. The RHT debate to date mostly hinges on the extent to which various studies are or are not compatible with its tenets. However, because few of these studies were carried out with the explicit intention of testing RHT, it is unsurprising that they are open to various interpretations. The problems of using such studies are that first, most published road accident data are at a high level of aggregation, second there is a large number of variables involved, and finally the large amount of unexplained variance in field studies of accidents and safety measures. Given these parameters for the debate, it is unsurprising that the arguments have been rehearsed over more than a decade so far. A starting point for a new direction for research into RHT is required. That the target level of risk may not be accessible to individuals’ conscious thought processes (or measurable by researchers’ scales or by verbal protocol analysis), but is ascribed to populations from behavioural outcomes such as accident loss, is a post hoc attribution based upon selected observations. It is

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an example of inductive reasoning. There is now a need to complete the scientific cycle - accepted as critical by both proponents and opponents of RHT - by carrying out field or laboratory studies using experimental designs which can build up a body of relevant evidence for or against aspects of RHT using hypothetico-deductive processes. So far, proponents of RHT have been reluctant or unable to specify possible cognitive or behavioural pathways along which RHT might operate. It is population-level effects, particularly accidents, which have so far been of interest. If, however, the utility criterion is to be satisfied then the number of ways in which homeostasis can occur is limited to a few possibilities such as increased speed, increased risk of overtaking and attentional reductions. Treating possible pathways to homeostasis as imponderables is neither helpful nor necessary. If RHT opponents can point to possible behavioural pathways that satisfy the utility criterion, but for which no evidence of compensatory behaviour exists, then RHT proponents should explain why those pathways could escape homeostasis. Not to provide this explanation would leave RHT as an atheoretical statistical description of behaviour. To RHT opponents a challenge is also apposite. Rather than seeing homeostasis as a dynamic which reflects safety, consideration should also be given to behaviour outside that dynamic. Vehicle speed and driver’s attention level at any given moment are determined by relevant utilities and by prevailing road, passenger and traffic conditions. Thus, we do not usually drive through urban areas at 70 mph even when it is possible because any positive time utility is unlikely to be matched by the probability of accident loss or legal sanction. Neither would we be likely to drive at 10 mph in similar circumstances, as any positive utility gained from reduced accident loss probability is unmatched by the negative time utility. Balance is constantly maintained between benefits of risky driving and a desire to arrive safely at our destination. If RHT opponents accept that outside a safety dynamic, a cost/benefit appraisal partly determines risk-taking behaviour, why should this influence not apply when some change to the level of safety is introduced? If, on the other hand, they reject this utility-driven mechanism, what is to be its replacement? All contributors to the risk homeostasis debate, would we imagine, agree that “behavioural compensation in respect of safety changes can occur”. The two words that should bring about consensus here are “compensation” and “can”. By “compensation” we mean some change in behaviour in the direction of negation. By “can” is meant that the operation may not necessarily operate always and everywhere. We commend the statement - compensation can occur _ as a st,arting point for RHT research. An important task for RHT research is then to determine relevant psychological dimensions of RHT rather than to cite single cases as evidence for or against, the theory in the all-or-nothing way that has so far characterized the debate. More simulated risk-taking research is needed, for example simulated air

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traffic control, car-in-traffic or shipping exercises. This is one way in which RHT can break free of the statistical level of explanation at which it has so far existed, and start to enjoy a psychological description, for example in terms of personality theory or cognitive modelling. Risk homeostasis theory has in the past had to contend with charges that it is a pessimistic view of safety. Whilst Wilde (1989) is correct to point out that the question of whether RHT is pessimistic or optimistic is irrelevant (it could be both pessimistic and true) there are reasons why it is not necessarily a negative theory. As Wilde has pointed out, there is no reason why a motivational intervention should not result in a reduction in accident loss. Where the target level of risk can be reduced, RHT would predict such a reduction. Moreover, RHT does not posit risk for its own sake: some benefit has to accrue from maintaining risky behaviour. RHT would predict that where safety interventions include controls which limit specific compensatory behaviours, then safety interventions would be more likely to succeed - a point made by other researchers. For example, if compulsory seat-belt wearing were accompanied by assiduous traffic policing, compensatory speed increases may be prevented. Opponents of RHT believe that when an environment is made safer, a reduction in accident loss will follow. Proponents of RHT believe that environmental improvements allow people to gain real benefits (such as arriving at their destinations sooner) without increasing their risk of having an accident. RHT does not state, either explicitly or implicitly, that accidents are inevitable, nor that nothing can be done to improve safety. It does suggest that safety interventions can lead to changes in the behaviour of those affected by them, and that consequently, some interventions are more likely than others to be successful. If this point is accepted, then further investigation of RHT provides opportunities to improve safety and to reduce accident loss.

Acknowledgements We would like to express our thanks to Frank P. McKenna and to this paper’s two referees for aiding our discussion of RHT.

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