category: literaturenote citekey: larssonneedsystemstheoryapproach2010 title: The need for a systems theory approach to road safety authors: "Larsson, Peter; Dekker, Sidney W.A.; Tingvall, Claes" year: 2010 date: 2010-00-00 2010 doi: 10.1016/j.ssci.2009.10.006 publication: Safety Science url: "https://doi.org/10.1016/j.ssci.2009.10.006" zotero_key: W6DZEMV7 zotero_storage: DHVRF2C6 collections: transpordi_planeerimine folder: 001_artiklid/Road Safety Indicators firstAuthor: "Larsson, Peter"

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Safety Science

journal homepage: www.elsevier.com/locate/ssci

The need for a systems theory approach to road safety

Peter Larsson a,*, Sidney W.A. Dekker ^b , Claes Tingvall ^c

^a Swedish Transport Agency, Box 267, SE-781 23 Borlänge, Sweden
^b Lund University, School of Aviation, Drottningvägen 5, SE-260 70 Ljungbyhed, Sweden
^c Swedish Road Administration, Röda Vägen 1, SE-781 87 Borlänge, Sweden

article info

Article history: Received 19 January 2009 Received in revised form 19 October 2009 Accepted 21 October 2009

Keywords: Road safety Traffic safety Vision Zero Systems theory Human factors

abstract

In other hazardous complex socio-technical systems in society, e.g. nuclear power and aviation, systemstheoretical assumptions are considered a promising way to better understand and manage safety. In this paper, two fundamental approaches to road safety were assessed in the light of a systems theory approach. One approach, is based on a premise where individual road-users are solely responsible when crashes occur. In that case countermeasures are aimed at altering the behavior of the road-user in order to adapt him/her to the road transport system. The other approach, the so-called zero-tolerance position, or Vision Zero approach, to road safety is built around two axioms; the system must be adapted to the psychological and physical conditions and limitations of the human being and the responsibility for road safety must be shared between the road-users and the designers and professional operators of the system. It was found that the most important determinants of systems theory are basically not present in the road-user approach. However, even if the Vision Zero approach clearly takes step towards systems theory, it does leave room for articulating even more features of systems theory.

-2009 Elsevier Ltd. All rights reserved.

1. Introduction

According to WHO (2009) over 1.2 million people are killed in road traffic crashes annually between 20 and 50 millions suffer non-fatal injuries. There is an obvious risk that these figures will increase substantially without strengthened efforts and new initiatives, especially if the increasing traffic in the developing countries is taken into account (WHO, 2004).

One approach to road safety is mainly based on a premise where individual road-users are solely responsible when crashes occur. This view has been enabled by, and is in turn constitutive of, findings claiming that human error is the cause of approximately 95% of road crashes (WHO, 2004). Hence, remedies are primarily sought in persuading road-users to adopt error-free behavior. Such remedies consist of information, education, legislation and police surveillance (Amalberti, 2001). In other hazardous complex socio-technical systems, e.g. nuclear power safety, software safety and aviation safety, systems theory is considered as a promising way to better understand and manage safety (Leveson, 2002).

The Vision Zero, which was presented in Sweden 1996 and constitutes the road safety strategy in Sweden, intends to take a more holistic and systemic approach to road safety (Ministry of Trans- port and Communications, 1997). It may therefore be a step closer towards a safety approach based on systems theory.

The purpose of this paper is to contrast the road-user focused approach and the Vision Zero approach in road safety work with an approach based on systems theory in order to identify important features differentiating these approaches. These features may have an influence on the identification of road safety countermeasures and their implementation. Hence the research question is: could a systems theory approach improve the effectiveness of a road safety work based on a road-user approach or on a Vision Zero approach?

After an introduction in which different road safety approaches are described, the article is focused on contrasting two of them; the road-user and the Vision Zero approach, refracted through the lens of systems theory in order to highlight important features that set these approaches apart. These features may point to more constructive road safety countermeasures and how they could be implemented.

2. Different approaches to road safety

2.1. Introduction

In the paper two fundamental approaches to road safety will be described and contrasted with a systems theory approach. It seems that in reality none of the two approaches is totally predominant. However it is obvious that there are clear differences between

^* Corresponding author. Tel.: +46 243 783 06; fax: +46 243 783 30. E-mail address: peter.larsson@transportstyrelsen.se (P. Larsson).

the approaches and to be able to make a meaningful comparison between them and systems theory you need to ''stretch" them to their extreme applications.

2.2. The road-user approach

The road-user approach (RUA) to road safety is focused on human error as the main cause of road accidents and hence the individual road-user is solely responsible when crashes occur (WHO, 2004). This view is based on several studies, e.g. Treat et al. (1977), Sabey and Taylor (1980), claiming that road-user factors are the sole or contributory factors in approximately 95% of all road accidents. According to Wegman (2002) international research groups still support the truth of these findings.

Another important basis for the RUA is the legal view of responsibility for road accidents (Wegman, 2002; Tingvall and Lie, 2001). In all current road transport systems around the world, the roaduser has almost total legal responsibility for safety (Tingvall and Haworth, 1999). In most countries, there are general rules that the road-user, in all situations, should behave in such a way that accidents do not occur. If an accident occurs, at least one road-user has, by definition, broken the general rule and the legal system can therefore act.

An important consequence of the RUA is a main focus on accident prevention and hence the countermeasures are mainly aimed at changing the behavior to adapt the road-user to the system such as regulating and surveillance of behavior, information and education in order to make the road-user to behave correctly so that accidents will not occur (Zein and Navin, 2003; WHO, 2004; Mac kay and Tiwari, 2005). Thus the responsibility of the society is mainly to administrate this system by issuing new regulations, financing and carrying out surveillance, information campaigns and educational programs.

2.3. The Vision Zero approach

In 1997, the Road Traffic Safety Bill founded on a strategy called the Vision Zero was passed by a unanimous majority in the Swedish parliament. The decision meant that the Vision Zero strategy should form the basis for the road safety work in Sweden. As a consequence the road transport system shall be adapted to the psychological and physical conditions and limitations of the human being.

According to the Ministry of Transport and Communications (1997) the Vision Zero means that eventually no one will be killed or seriously injured within the road transport system. According to Tingvall and Haworth (1999) the Vision Zero ''addresses fatalities and those injuries where the victim does not physically recover within a certain period of time. This means that common, but not long-term disabling injuries, and non-injury accidents are more or less outside the scope of the Vision". The notion of longterm disabling injuries has recently been defined as 1% permanent medical disability or more (personal communication with Hans-Yngve Berg, Swedish Transport Agency, August 2009). This is one of the cornerstones of the Vision Zero and leads to the important starting point that the biological tolerance of the human body against external forces should be the limiting factor when designing the road transport system. Within the framework of the Vision Zero a model of safe road traffic has been developed and forms the basis for a strategy for countermeasures. The model is shown in Fig. 1.

The model describes, from a system perspective, the way a number of factors interact in order to achieve safe road traffic. Road traffic consists of a great number of individual journeys. When all individual journeys are safe they will add together to a safe road traffic. The starting point of the model and the prerequisite for a safe journey is the psychological and physical conditions and limitations of the human being. The main limiting factor is his/her ability to withstand external violence, which can be considered given and constant. Therefore the arrows have one direction. The passive safety, or injury mitigation capability of the system, is determined by the safety standard of the vehicles and the roads/streets added together. The total injury mitigation capacity of these components determines the safe speed of the system. If a higher speed is desired, the safety performance of vehicles, roads/streets and/or road-user must be increased, demonstrated

Fig. 1. A model of safe road traffic for car occupants (Swedish Road Administration, 2008) (occupants of other cars).

by the bi-directional arrows. Deficiencies in the system design must be compensated by a lower speed. According to Tingvall and Haworth (1999) the interfaces between the different components of the road transport system (humans, vehicles, roads) and their relation to speed become important. It is therefore important to define the parameters of the interfaces to make it clear what the limitation of a car is, as well as the road, in order to sort out the responsibility of the different components. Similar models for other road-user categories are under development.

In contrast to the RUA the Vision Zero approach (VZA) explicitly states that the responsibility for road safety is shared by the designers, administrators and professional users of the road transport system and the road-user. This shared responsibility is expressed in the following way (Ministry of Transport and Communications, 1997):

1. The designers of the system are always ultimately responsible for the design, operation and use of the road transport system and thereby responsible for the level of safety within the entire system.
2. Road-users are responsible for following the rules for using the road transport system set by the system designers.
3. If road-users fail to obey these rules due to lack of knowledge, acceptance or ability, or if injuries occur, the system designers are required to take necessary further steps to counteract people being killed or seriously injured.

3. Systems theory

3.1. General concept

The systems theory approach began to emerge in the 1930s and 1940s as a response to the limitations of the classic analysis techniques and their possibilities to cope with the more and more complex systems being built (Checkland, 1981). The traditional scientific approach to systems had up until then been reductionistic (analytic reduction) (Pariès, 2006). The reductionistic approach argues that from scientific theories which explains phenomena on one level, explanations for a higher level can be deduced (Skyttner, 2005).

Systems thinking is a response to the failure of mechanistic thinking to be able to explain social, socio-technical and biological phenomena (Skyttner, 2005). Skyttner further states that a system will lose its synergetic properties and cannot be understood if analytic reductionism is used to examine it. The system theory focuses on systems as a whole, not on the separate parts. Leveson (2002) states that:

It assumes that some properties of a system can only be treated adequately in their entirety, taken into account all facets relating the social to the technical aspects. These system properties derive from the relationships between the parts of systems: how the parts interact and fit together.

3.2. Systems theory and safety

3.2.1. Safety as an emergent property

Skyttner (2005) states that emergence is an important concept of systems theory and that it ''results from the interaction of independent parts when they stop being independent and start to influence each other". He argues that it is the relationships between the components of a system and not the nature of the components themselves that determine the properties and behavior of it. This is in line with both Leveson (2002) and Hollnagel (2004); who mean that accidents can be seen as emergent phenomena. Accidents occur when components of a system interact with each other and these interactions are not possible to foreseen because of their complexity (Hollnagel, 2004).

According to Leveson (2002) systems theory provides the theoretical foundation for systems engineering, which views each system as an integrated whole even if it is composed of diverse individual and specialized components. A basic and important assumption of systems engineering is according to Leveson (2002) ''that optimization of individual components or sub-systems will not in general lead to a system optimum; in fact improvement of a particular sub-system may actually worsen the overall system performance because of complex, non-linear actions among the components". This means e.g. that safety cannot be optimized through the optimization of the safety performance of the individual components and according to Leveson (2002) ''attempts to improve long-term safety in complex systems by analyzing and changing individual components have often proven to be unsuccessful over the long-term".

3.2.2. System and component performance is variable

Hollnagel (2004) conducts a line of arguments that socio-technical systems in the modern society tend to increase in complexity. By complex systems Hollnagel (2004) means systems which ''consist of multiple parts that depend on each other, and there is only a limited possibility of delaying processes or in carrying out actions". This notion is in line with Perrow (1984) but who also underlines that in a complex system there are unexpected interactions between the components of the system that are not obvious and anticipated. Hollnagel (2004) further reasons that performance in any complex system is necessarily variable. This is due to both the performance variability of the components of the system, and to the complexity of their interactions. Under these conditions accidents will occur if the performance variability is not under control and/or barriers are not introduced. This is in line with Skyttner (2005) who states that in a network of coupled variables each variable has a highest and lowest threshold. Within these limits the system can function normally but if the thresholds are exceeded disorder and finally collapse will occur. In case of technical components in a system the performance variability is due to imperfections of manufacturing and operation. But the variability is also partly due to the design of the system in which working conditions and combinations of input which were not, or could not, be foreseen when the system was designed (Hollnagel, 2004). In case of humans and social systems, performance is variable for many different reasons. The human tendency to adjust performance to current conditions and lack of constancy of perceptual and cognitive functions are the most important sources for this kind of variability (Hollnagel, 2004). Hollnagel (2004) underlines that these performance variabilities are not inherently bad. Instead they are necessary for system users and operators to learn and for a system to develop.

The strategy for prevention is either by introducing barriers or by performance variability management. Hollnagel (2004) though states that ''barriers are valuable because they are effective against a specific type of disturbance even if the cause or origin of that disturbance is unknown". This is in line with Haddon (1980) who argues that ''.... reductions in desirable end results can often be achieved without exhaustive knowledge of their exact causes". Hollnagel (2004) further claims that multiple barriers are needed to prevent undesired events from taking place.

3.2.3. Systems as hierarchical structures

According to Leveson (2002) accidents can be viewed as a control problem. A systems theory approach treats safety as an emergent property. Such property can be controlled by a set of constraints related to the behavior of the components of the

Fig. 2. Generic model of socio-technical control (Leveson, 2002).

system. According to the systems approach accidents occur when the components interact and those interactions violate the constraints (Leveson, 2002). Such violations can be component failures, external disturbances and/or dysfunctional interactions among system components which are not adequately handled by the control system. According to Leveson (2002) such control is imposed on many levels, from the operational to the managerial. She further states that ''socio-technical systems can be modeled as hierarchy of levels of organization with control processes operating at the interfaces between levels to control processes at the lower levels". Leveson (2002) proposes a generic socio-technical control model with one hierarchical control structure for system development and one for system operation with interactions between them. The model is shown in Fig. 2.

Leveson (2004) consequently claims that ''the most important factor in the occurrence of accidents is management commitment to safety and the basic safety culture in the organization or industry".

3.3. Systems theory and road safety

There are very few references to systems theory and road safety found when a literature research is carried out. Emmerik van (2001) though concludes that the traditional, reductionalistic approach to road safety has limitations and a systems approach could overcome some of these limitations. Also Zein and Navin (2003) conclude that the ''simplistic representations of traffic safety disregard the dynamic interactions among the road environment, the vehicle, and the road-user". According to them these simplistic representations are a result of police reports that attribute more than 90% of all road traffic accidents to driver error and leads to the incorrect conclusion that improving driver behavior is the only effective road safety strategy. They further claim that a systems approach in road safety acknowledges the more complex nature of road traffic accidents where multiple factors interact resulting in an accident.

4. Analysis

4.1. Method

The method used to identify the differences between a systems theory approach on the one hand and the RUA and the VZA on the other is divided into three steps:

From the literature on systems theory, summarized in the previous section of the article, important features of systems theory with regard to safety will be identified.

2. An analysis is carried out in order to judge if these features are applicable to the road transport system.
3. The RUA and the VZA are then compared to the identified features of systems theory in order to identify differences which may influence the effectiveness of the identification of road safety countermeasures and their implementation.

4.2. Important features of systems theory with regard to safety

The following important features of systems theory with regard to safety can be identified from the literature.

Applicable to systems. Systems theory is applicable to different kinds of systems. Skyttner (2005) especially points out social and biological systems while Hollnagel (2004), Leveson (2002) apply the theory to socio-technical systems.
Safety as an emergent property. Systems theory views safety as an emergent property due to the complex relationships and interactions among the components of the system.
System and component performance is variable. Performance in any complex system is variable due to the performance variability of the system components and the complexity of their interactions. As a consequence accidents can be seen as a result of the performance variability among the components of a system and the complex relationships between them.
Systems as hierarchical structures. According to Leveson (2002) socio-technical systems can be seen or modeled as a hierarchy of levels of organization with control processes operating at the interfaces between levels to control processes at the lower levels (Fig. 3). To be able to control the system, constraints for these processes must be set, communicated, measured, controlled and enforced at each level.

4.3. Applicability to the road transport system

According to Skyttner (2005) there are many different definitions of systems depending on which scientific school one represent. Skyttner (2005) refers to an often used common sense definition: ''a system is a set of interacting units or elements that form an integrated whole intended to perform some function". From this definition it is possible to draw the conclusion that the road transport system really is a system since it consists of roadusers, vehicles and road components that interact with each other in order to ''produce" transports of people and cargo. In the literature on systems theory complexity is often mentioned as an important feature of systems which increases the unanticipated connections between components or sub-systems. According to Skyttner (2005) complex systems are characterized by for example:

Fig. 3. Schematic relationship between fatality risk and impact severity for a car occupant (Krafft et al., 1999).

A large number of elements.
Many interactions between the elements.
Attributes of the elements are not predetermined.
Interaction between elements is loosely organized.
The system is subject to behavioral influences.
The system is largely open to the environment.

Skyttner (2005) further states that complex systems often behave in an unexpected manner where the relationships between cause and effect often are difficult to understand. The road transport system consists of a large number of components in the form of road-users, vehicles and road components that each day interact many million times. The vehicle and road-user constitute a subsystem in which humans and technology interact. The vehicle and road components are also a sub-system where technology interacts. The safety of the whole system is largely subject to the behavior of road-users and the system is not only open to the environment; the environment could be seen as a part of the system. The attributes of the components are partly predetermined and the interaction between the components is highly random even though there are rules governing road traffic. Consequently the road transport system in many aspects corresponds with the characteristics of a complex system.

In the 1950s the term socio-technical systems were introduced. The term refers to the interaction between society's complex infrastructure and human behavior. In this sense, society itself, and most of its sub-structures, are complex socio-technical systems (Trist, 1980). The road transport system is a complex infrastructure and a part of the society and is almost a prerequisite of the modern society. The humans must take part of and interact with the system in order to be able to live a normal life. From this definition the road transport system can be seen as a socio-technical system. The analysis in this section supports the conclusion that the road transport system can be seen as a complex socio-technical system and consequently systems theory is applicable to it.

4.4. The road-user approach and systems theory

4.4.1. Safety as an emergent property

The RUA to road safety is focused on human error as the main cause of road accidents and consequently the countermeasures are mainly aimed at changing the behavior of the road-user in order to adapt him/her to the road transport system. This approach emanate from the view that the system itself is flawless, were it not for the erratic behavior of unreliable people in it (Dekker, 2002). In that respect the RUA differs from systems theory where safety is viewed as an emergent property due to the complex relationships and interactions among all components of the system. The RUA more or less omit the influence of the other components of the system. This is especially valid from an injury mitigation point of view. From an accident risk reduction point of view the RUA partly have included the vehicle and the road components but only in order to assess their contribution to the behavior of the road-users.

4.4.2. System and component performance is variable

According to the RUA the countermeasures have preferably been focused on regulating and surveillance of behavior, education and information in order to decrease the performance variability of the road-user. These countermeasures have been judged successful since the variability in attitudes towards, e.g. alcohol and road traffic and the behavior in connection to that have decreased. The variability of the other components of the system has been taken into consideration but mostly from an accident risk reduction point of view. The development of countermeasures to reduce this kind of variability has mostly been introduced in isolation from the other components of the system and consequently has not been able to take the complex interactions into consideration.

4.4.3. Systems as hierarchical structures

According to the RUA society also has a responsibility for road safety. This responsibility is though focused on issuing constraints for the road-user in the form of regulations. These constraints are then communicated, controlled and enforced by the society. This procedure means that society transfers the responsibility for road safety on the individual road-user. Few constraints are put on the designers and professional users. According to Fig. 2 a systems theory approach to safety emphasize the constraints and control processes at the interfaces between the different levels in order to control processes at lower levels. These control processes are more or less non-existing in the road transport system except for the process between the regulatory agencies and the operating process.

4.5. The Vision Zero approach and systems theory

4.5.1. Safety as an emergent property

As stated earlier in this article safety is an emergent property due to the complex interactions between the components of a system. Fig. 1 shows a model of safe road traffic for passenger cars. The model schematically describes the way the three main components of the road transport system (the road-user, the vehicle and the road) interact with each other together with the speed. The interaction is more or less solely directed to control that the human tolerance to mechanical forces is not exceeded, which in VZA is the limiting factor for the road transport system. In order to minimize events where crashes occur, the model must be enlarged to also pick up a longer sequence including other interactions within the system. Fig. 1 hence implies that this interaction could be seen both from an accident risk reduction and injury mitigation point of view. Tingvall and Haworth (1999) and Krafft et al. (1999) also underlines the importance of defining the interfaces between the different components of the system in order to better understand and control the complex interactions among them. The VZA consequently see road safety as determined by the complex relationships between the components of the road transport system. But the interfaces between the components are not clearly defined and operationalized.

4.5.2. System and component performance is variable

According to Hollnagel (2004) accidents occur due to the unpredictable aggregation of variability. The strategy for prevention is to introduce barriers and/or to control and manage performance variability among all components of a system. From the literature it is not clear how the VZA views performance variability in the road transport system. The model in Fig. 1 implies that there is a focus on road-user performance variability. This variability is due to different levels of psychological and physical conditions and biological tolerance. It can be compensated or controlled by a vehicle and/or a road that supports a correct use in order to avoid accidents or the vehicle and/or the road must be protective in order to mitigate injuries. As a consequence it seems that the road, the vehicle and/or the speed are used as barriers to manage the performance variability of the road-user. But according to the functional resonance model proposed by Hollnagel (2004) the safety of the system could also be increased if the performance variability of the vehicles and road components is decreased. It is also still unclear which level of variability of the road-user that should be addressed. The model further implies that there must be a certain level of inherent injury mitigation properties of the joint vehicle–road system to determine a speed limit in relation to it in order not to exceed the human tolerance of external violence. But these levels are not clearly defined. In reality there are different types of vehicles, e.g. passenger cars, lorries and motorcycles with varying safety levels in the system. The road environment differs a lot, from small roads with tight curves and trees, rocks, etc. in close proximity to the road to wide, straight motorways with forgiving side areas and/or different types of guardrails. In this aspect the model of safe road traffic is not clear. Should the least safe component in the road-vehicle system govern the output (speed) from the system? Consequently the VZA seems to be more focused on barriers than performance variability management.

4.5.3. Systems as hierarchical structures

As mentioned earlier in the article the VZA emphasizes the shared responsibility between the designers and professional users of the road transport system. It is however clear that this shared responsibility has not been operationalized further. The hierarchical levels according to Fig. 3 are quite well known but the control processes operating in the interface between them has not fully been mapped. For that reason constraints for these processes have not been able to be set, communicated, measured, controlled and enforced to any larger extent.

5. Conclusions

Systems theory has become an important foundation for modern safety work in different complex socio-technical systems in the society. The road transport system has been shown to be such a system and for that reason systems theory is applicable to it from a safety point of view.

It has been shown that the important features of system theory with regard to safety; safety seen as an emergent property, the variability of system performance due to component variability and the hierarchical structure of a socio-technical system, cannot be found in the RUA. The VZA is clearly a step forward towards a system theory approach since road safety is seen as an emergent property due to the complex relationships between the main components of the road transport system (the roaduser, the vehicle and the road). For that reason the VZA is preferred as a foundation for contemporary road safety work. However it is not quite clear how the VZA views and handles performance variability. This must be further scrutinized. One important feature of the VZA is the shared responsibility between road-users, system designers and professional users of the system. This indicates that the road transport system is viewed as a hierarchical system in the VZA but the control processes between the different levels and their constraints have not yet been clearly defined and operationalized. It is suggested that this should be further investigated.

6. Discussion

6.1. General

In the road safety community and in the literature it is often claimed that Haddon's matrix is a model for an integrated systems approach to road safety. Evans (1991) points out that there are complex interactions among the components of the road transport system which the matrix does not account for. Therefore it seems that Haddon´ s matrix cannot be seen as a systems theory approach. The major importance of Haddon's matrix is that it implies the significance of working with both loss reduction and crash prevention and the significance of working with all elements of the system, not only the road-user, in order to identify causes and countermeasures (O'Neill, 2002).

6.2. Implications for the road safety work

6.2.1. Safety as an emergent property

Since safety is seen as an emergent property, one of the most important conclusions that can be drawn from a road safety point of view is consequently that optimization of individual components or sub-systems will not generally lead to a system optimum and hence a safety optimum (Leveson, 2002). As a consequence an effective road safety work cannot focus on one component at a time trying to optimize the safety features of that component. An example of this is the car industry starting to understand the potential of further enhancing the injury mitigation properties of passenger cars is very much dependent on the collision speed and the design of the roadside. Fig. 3 shows the schematic relationship between the fatality risk, expressed in percent, and level of impact severity for a car occupant. The impact severity is related to the travelling speed (Nilsson, 2004) and can be expressed as Dv, deceleration, kinetic energy, crush, etc. In many car crashes including fatalities and severe injuries, the impact severity lies on the far right end of the curve (e.g. a side impact against a rigid pole). Consequently an increase of the injury mitigation properties, which means a lower impact severity given the same travelling speed, will result in a marginal decrease of the fatality risk. But if the injury mitigation properties of the road components are developed to absorb some of the energy (e.g. a deformable pole) the impact severity will begin to reach the steep part of the curve where a small increase in the properties of the car will give a greater decrease in fatality risk. This could also be achieved by developing technology in the car which can predict a collision, reduce the collision speed and optimize the passive safety systems of the car. This approach, which is often called the integrated safety concept (e.g. Kanianthra, 2007), is growing stronger within the car industry. It is hence important that the car industry and the road authorities co-operate in order to understand and identify the complex interactions between the car and the road with the goal to harmonize design of vehicles and roads.

Another example is different Advanced Driver Assistance Systems (ADAS) in cars (e.g. night vision systems and driver alert systems) which are claimed to have ''an enormous road safety potential" (e-Safety Support, 2007). There is however a phenomenon described in the scientific literature called risk compensation or behavioral adaptation (e.g. OECD, 1990). It occurs when individuals adjust their behavior in response to perceived changes in risk. Drivers may, for example, increase the driving speed, pay less attention to the driving task, become more reckless and exhibit poorer control to such an extent that the safety margins created by the ADAS are cancelled out (Kovordányi et al., 2004). If ADAS are developed without the understanding and control of these complex interactions between the road-user and technology there is an obvious risk that the potential safety benefits of a system will be lower or even negative.

6.2.2. System and component performance is variable

According to Hollnagel (2004) performance variability of the components of a system is normal. Such variability of one component in isolation is consequently not the source for a system to collapse; it is the unforeseen aggregation of normal variability of two or more components that is. When road traffic accidents are investigated there are in many cases not possible to find the defective or erroneous component which ''caused" the accident. As a consequence many investigations conclude that the road-user was inattentive, careless, etc. Instead the conclusion in many cases could be that the components worked within their normal and accepted variability limits but there were no or too weak barriers. The countermeasures must hence be aimed at decreasing and controlling the variability of the components, not only the variability of the road-user, and/or introducing new barriers or strengthening existing ones. It is however unclear how the notion variability is defined. In the road transport system variability is of two types; violations and errors. Intentional violations like speeding and driving under the influence of alcohol, cannot be seen as normal and necessary. There are however different opinions what should be seen as violations and what should be seen as errors in the road transport system. The promoters of the RUA are more prone to see all errors as violations while the promoters of the VZA aim at a clearer distinction between the two types of variability. They hence advocate that variability through violations must be lowered substantially while errors must be allowed if they are dealt with by the designers of the system. This must be further analyzed and defined.

6.2.3. Systems as hierarchical structures

It is more or less an unspoken truth within the road safety community that there is a big difference between the road transport system and other socio-technical systems (Amalberti, 2001). As a result, it is assumed that safety strategies in those systems cannot be applied to the road transport system. One of the major arguments is that the road transport system is an ''open" system where an almost unlimited number of companies, organizations, private persons, etc. can operate, and where individual users, or operators, have excessive degrees of freedom way beyond those in other transportation systems (e.g. aviation). As a consequence it is thought not to be possible to apply the model of socio-technical control (Fig. 2) to the road transport system. However the road transport system could be seen as a large organization which produces transportation. The management level of the organization is the parliament and/or the government which are responsible for the strategies, policies, legislation, etc. Then there are many different divisions and/or subdivisions responsible for the conditions and tools of production e.g. the road infrastructure designers and maintainers (e.g. a road administration and the municipalities) and the vehicle industry but also for the production itself, e.g. hauliers, taxi companies, organizations organizing their own transports. Taking that perspective it seems possible to apply the model of socio-technical control on the road transport system. It is probably even more important to apply such a model on the road transport system to be able to control the often larger permitted variabilities of the system components, particularly the performance variability of road-user, compared to other complex sociotechnical systems.

6.2.4. Summary

This paper has tried to show that the road safety work can benefit by learning from other hazardous complex socio-technical systems in society and that taken-for-granted assumptions around the road transport system can actually hamper a more productive approach to road safety improvements.

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