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Human error in accident scenarios: cause or consequence?
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Human error in accident The importance of the role played by human error in accidents has been stressed in a large number of safety studies. However, human error should not be regarded as the prime cause of accidents, but as a consequence of earlier malfunctions. Error is merely one link in the chain of events leading to an accident. In addition, if a human makes an error it is very rarely deliberate: it is most frequently because the external and internal conditions under which the activity in question must be performed have not allowed him or her to meet the requirements of the task. It is, however, essential to understand clearly the nature of these errors and identify the factors which explain their occurrence in order to be able to specify appropriate countermeasures. The driuer is one component of the traffic system, as is the vehicle
.-...-~~“--~~
--_-
I__L____I_--_-_
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scenarios:
_I_---__-~~~--
HUMAN
ERROR
cause or consequence?
and the environment in which it is driven. Each of these three components can be described in terms of a number of more or less permanent characteristics some of them beneficial, others not. In some cases the interactions between these components can lead to a malfunction in the system. However drivers are also, and above all, the agents which control the system, and are responsible for adjusting their actions according to the difficulties generated by the components of the system, which includes the drivers themselves. In order to make this adjustment, drivers engage in perception and cognitive and motor functions. But malfunctions in the systems can mean that one of these functions, which is usually adaptive, is no longer able to control a situation. This produces a functional failure, frequently refer to as human error.
i
As driving involves a large variety of mechanisms we need to construct a framework in order to list the failures which occur during it. The framework we have selected relates to the cognitive domain of information processing, which is just one of the types of human action involved in an activity of this type. This is why certain emotional and motivational aspects of behaviour will be considered as factors that explain failures that have been identified at the cognitive level. We have used Rasmussen’s functional architecture model as a basis for modelling the errors involved in road traffic acci dents. Nevertheless, we have added a prediction phase to this breakdown in order to take account of the anticipation and forecasting which drivers perform to cope with the dynamics of conditions. We have also made use of the distinction made by Reason between error and
RCCI-IEKCHt , KANSl’OKlS 5tLUKi Ii N” 66
JANVIIR4lAK5
2000
violation in order to take account of the specific nature of the failures identified in the decisional processes involved in initiating a given manceuvre.
Many detailed accident studies were conducted in the 1970s but since then interest among the scientific community has lessened. However, although studies of this type are not able to explain everything, they do focus research on errors that pose a genuine safety problem. It is this which reveals mechanisms that explain the production of failures with reference to the situations in which they arise and the factors which cause them. Early research in the area, however, needs to be updated in the light of advances in our understanding of driver function. This research involves the analysis of 392 accident situations. The data relating to these situations was obtained from detailed observations conducted initially at the scene of the accident which was completed in the days that followed with data collection prompted by the findings of the initial observation. Data collection was performed by multidisciplinary teams and related to the three components of the road system: drivers, vehicles and infrastructure. Each accident was the subject of a case study involving a temporal and spatial reconstruction of the accident process describing the context of the collected data. Analysis was based on this reconstruction and consisted of breaking down the accident scenario into sequences of events. The model used by the INFEE Accident Mechanisms Department identifies four interconnected phases: the driving situation, the accident, the emergency and the impact. By identifying these situations it is possible to reconstruct, in a homogeneous manner, the different sequential phases of the accidents thereby permitting analysis of each case with reference to causal
processes and cross-sectional studies of several accidents by comparing the different phases of the accidents. The analysis described- below concentrates on the accident stage of the process, which is the pivotal stage when driving ceases to be normal and deteriorates. This transitional stage is a good point at which to compare accidents in that it marks the onset of the malfunction process. We have therefore attempted to identify which failure in the sequence of failures that led to the accident impact characterises this transitional phase and explain why the driver’s situation became critical.
A classification
model
By combining analysis based on models with analysis of the specific tasks from which the investigated errors originate we have attempted to avoid two pitfalls that often affect classifications, namely that they are either too closely related to the context (data driven) and therefore not widely applicable, or too generic (model driven) and therefore of little practical use. Comparing accident case studies with theoretical data has enabled us to develop a classification model that establishes a link between the major functional phases (involving a wide range of activities) which are traditionally codered by the literature, and the specific types of failures which relate to the particular processes involved in the deterioration of driving situations. For convenience, we have presented these failures using a conventional information processing modet. The linear nature of this presentation in no way implies that the driver operates in a linear manner. Qbviously, a great many feedback mechanisms exist between different modules and information processing operates to a considerable degree in loops. However, in the analysis described
here the functional loop is stopped at the moment the driver is confront-. ed by an unforeseen difficulty and a disruption occurs in the driving activity. A classification rnodel of this type is able to identify precisely during which stage of functioning the driver’s adaptive processes became unable to overcome the difficulties they were confronted with. ~ti~~a~ faihres and e~p~~nut~ry factors
A comprehensive analysis of the distribution of failures shows that perception-rel&ed aspects, involving visibiiity problems and information gathering strategies, play an important role in the functional failures that have caused the accident situation to occur. However, gathering together the problems associated with the diagnosis of situations and the forecasting of their change shows that the importance of failures associated with information processing is even greater. Compared to this set of difficulties associated with analysis of the situation, the proportion of failures associated with performing actions is very small, which shows that only a minority of accidents are caused by a lack of skill. Skill failures are much more apparent later in the accident when the driver attempts to correct the situation by means of an emergency manceuvre. Results of this type suggest that in order to prevent drivers finding themselves in accident situations countermeasures should concentrate on the cognitive aspects of driving Furthermore, even when the errcm are analyzed from the standpoint of the failure of the processes involved in the driving activity, the origin of these errors should not be sought only with regard to the knowledge which is used (experiencerelated variables). The production of cognitive failures also involves a whole set of parameters which are
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-...
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.._.
I
linked to motivations (making up for being late), emotions (impatience, fear), psychological state (level of attention) and physical state (level of vigilance), etc. Human error factors also come into play in close combination with environmental factors relating to the external conditions in which the driving task is performed (amount of traffic, complexity and legibility of infrastructure, etc.). Analysis of explanatory mechanisms for errors shows most of them to be multicausal combining both endogenous factors (which characterize the driver) and exogenous factors (characterizing the situational context). While human error plays an important role in accidents, it is therefore a long way from being explained by the human factor alone. A tywlwy of prototypical scenarios
Once the various types of failures have been identified, they are placed in prototypical malfunction scenarios, grouping together accidents that occur under very similar conditions. These scenarios describe the overall generation of error, on the basis of a number of shared parameters: the malfunction task in which the driver encountered a difficulty, the functional failure which occurred during the activity, the explanatory factors (internal or external to the driver) for this failure and the consequences in terms of subsequent actions.
The 392 studied accidents broke down into about sixty scenarios of this type which take account of differences between drivers in the ways they generate a functional failure when confronted by certain difficulties while driving. These scenarios thus provide us with a clinical presentation of the pathologies that lead to error production. On the basis of these pathologies, it is possible to prescribe remedies which are appropriate for the problems and the conditions under which they arise.
The failure classification model proposed here is both based on the large amount of detailed data collected about accidents and an examination of this data from the standpoint of contemporary theoretical models. This classification provides a clearer picture of the way malfunctions of the traffic system can lead to poor adjustment of one or other of the functions by means of which drivers usually cope with difficulties. It improves standardization of analysis of the participation of the human factor in the multicausal deterioration of road traffic situations. Of the functional failures that have been identified at the onset of the degraded situation, those generally related to analysis of the situation have considerable importance in relation to the problems associat-ed with the psychomotor perfor-
HUMIAiV
ERROR
mance of the action, which emphasizes the relevance of a cognitive analysis of the difficulties facing drivers. Identifying the mechanisms which govern these mental functions requires the connection to be made between research based on analysis of malfunctioning and research which is more concerned with showing the manner the driver operates while performing ordinary tasks. Error should not be studied in itself but as just one of the symptoms of human cognitive function, its internal protective systems and its failures, which may occasionally cause accidents. Grouping individual accident studies together to form prototypical accident scenarios nevertheless has the value of identifying generic problem situations which involve a similar mechanism. This similarity in the accident process may correspond to a similarity in the safety measures that could prevent their occurrence. These generic failure production scenarios should therefore constitute a powerful tool for implementing suitable countermeasures either directed at drivers through training, or directed at the environment of the driving task through measures relating to the infrastructure or the development of in-vehicle driving aids. It is important to base such measures on diagnosis of actual problems and the circumstances in which they arrive rather than on assumptions which will tend to apply ready-made solutions to poorly understood problems.
- -____- .__ ____-___
---
Human error in accident The importance of the role played by human error in accidents has been stressed in a large number of safety studies. However, human error should not be regarded as the prime cause of accidents, but as a consequence of earlier malfunctions. Error is merely one link in the chain of events leading to an accident. In addition, if a human makes an error it is very rarely deliberate: it is most frequently because the external and internal conditions under which the activity in question must be performed have not allowed him or her to meet the requirements of the task. It is, however, essential to understand clearly the nature of these errors and identify the factors which explain their occurrence in order to be able to specify appropriate countermeasures. The driuer is one component of the traffic system, as is the vehicle
.-...-~~“--~~
--_-
I__L____I_--_-_
-~--I~~-
..---~--lll_ll___
scenarios:
_I_---__-~~~--
HUMAN
ERROR
cause or consequence?
and the environment in which it is driven. Each of these three components can be described in terms of a number of more or less permanent characteristics some of them beneficial, others not. In some cases the interactions between these components can lead to a malfunction in the system. However drivers are also, and above all, the agents which control the system, and are responsible for adjusting their actions according to the difficulties generated by the components of the system, which includes the drivers themselves. In order to make this adjustment, drivers engage in perception and cognitive and motor functions. But malfunctions in the systems can mean that one of these functions, which is usually adaptive, is no longer able to control a situation. This produces a functional failure, frequently refer to as human error.
i
As driving involves a large variety of mechanisms we need to construct a framework in order to list the failures which occur during it. The framework we have selected relates to the cognitive domain of information processing, which is just one of the types of human action involved in an activity of this type. This is why certain emotional and motivational aspects of behaviour will be considered as factors that explain failures that have been identified at the cognitive level. We have used Rasmussen’s functional architecture model as a basis for modelling the errors involved in road traffic acci dents. Nevertheless, we have added a prediction phase to this breakdown in order to take account of the anticipation and forecasting which drivers perform to cope with the dynamics of conditions. We have also made use of the distinction made by Reason between error and
RCCI-IEKCHt , KANSl’OKlS 5tLUKi Ii N” 66
JANVIIR4lAK5
2000
violation in order to take account of the specific nature of the failures identified in the decisional processes involved in initiating a given manceuvre.
Many detailed accident studies were conducted in the 1970s but since then interest among the scientific community has lessened. However, although studies of this type are not able to explain everything, they do focus research on errors that pose a genuine safety problem. It is this which reveals mechanisms that explain the production of failures with reference to the situations in which they arise and the factors which cause them. Early research in the area, however, needs to be updated in the light of advances in our understanding of driver function. This research involves the analysis of 392 accident situations. The data relating to these situations was obtained from detailed observations conducted initially at the scene of the accident which was completed in the days that followed with data collection prompted by the findings of the initial observation. Data collection was performed by multidisciplinary teams and related to the three components of the road system: drivers, vehicles and infrastructure. Each accident was the subject of a case study involving a temporal and spatial reconstruction of the accident process describing the context of the collected data. Analysis was based on this reconstruction and consisted of breaking down the accident scenario into sequences of events. The model used by the INFEE Accident Mechanisms Department identifies four interconnected phases: the driving situation, the accident, the emergency and the impact. By identifying these situations it is possible to reconstruct, in a homogeneous manner, the different sequential phases of the accidents thereby permitting analysis of each case with reference to causal
processes and cross-sectional studies of several accidents by comparing the different phases of the accidents. The analysis described- below concentrates on the accident stage of the process, which is the pivotal stage when driving ceases to be normal and deteriorates. This transitional stage is a good point at which to compare accidents in that it marks the onset of the malfunction process. We have therefore attempted to identify which failure in the sequence of failures that led to the accident impact characterises this transitional phase and explain why the driver’s situation became critical.
A classification
model
By combining analysis based on models with analysis of the specific tasks from which the investigated errors originate we have attempted to avoid two pitfalls that often affect classifications, namely that they are either too closely related to the context (data driven) and therefore not widely applicable, or too generic (model driven) and therefore of little practical use. Comparing accident case studies with theoretical data has enabled us to develop a classification model that establishes a link between the major functional phases (involving a wide range of activities) which are traditionally codered by the literature, and the specific types of failures which relate to the particular processes involved in the deterioration of driving situations. For convenience, we have presented these failures using a conventional information processing modet. The linear nature of this presentation in no way implies that the driver operates in a linear manner. Qbviously, a great many feedback mechanisms exist between different modules and information processing operates to a considerable degree in loops. However, in the analysis described
here the functional loop is stopped at the moment the driver is confront-. ed by an unforeseen difficulty and a disruption occurs in the driving activity. A classification rnodel of this type is able to identify precisely during which stage of functioning the driver’s adaptive processes became unable to overcome the difficulties they were confronted with. ~ti~~a~ faihres and e~p~~nut~ry factors
A comprehensive analysis of the distribution of failures shows that perception-rel&ed aspects, involving visibiiity problems and information gathering strategies, play an important role in the functional failures that have caused the accident situation to occur. However, gathering together the problems associated with the diagnosis of situations and the forecasting of their change shows that the importance of failures associated with information processing is even greater. Compared to this set of difficulties associated with analysis of the situation, the proportion of failures associated with performing actions is very small, which shows that only a minority of accidents are caused by a lack of skill. Skill failures are much more apparent later in the accident when the driver attempts to correct the situation by means of an emergency manceuvre. Results of this type suggest that in order to prevent drivers finding themselves in accident situations countermeasures should concentrate on the cognitive aspects of driving Furthermore, even when the errcm are analyzed from the standpoint of the failure of the processes involved in the driving activity, the origin of these errors should not be sought only with regard to the knowledge which is used (experiencerelated variables). The production of cognitive failures also involves a whole set of parameters which are
_____-_I
-__---
-...
-_.------____
-.
.._.
I
linked to motivations (making up for being late), emotions (impatience, fear), psychological state (level of attention) and physical state (level of vigilance), etc. Human error factors also come into play in close combination with environmental factors relating to the external conditions in which the driving task is performed (amount of traffic, complexity and legibility of infrastructure, etc.). Analysis of explanatory mechanisms for errors shows most of them to be multicausal combining both endogenous factors (which characterize the driver) and exogenous factors (characterizing the situational context). While human error plays an important role in accidents, it is therefore a long way from being explained by the human factor alone. A tywlwy of prototypical scenarios
Once the various types of failures have been identified, they are placed in prototypical malfunction scenarios, grouping together accidents that occur under very similar conditions. These scenarios describe the overall generation of error, on the basis of a number of shared parameters: the malfunction task in which the driver encountered a difficulty, the functional failure which occurred during the activity, the explanatory factors (internal or external to the driver) for this failure and the consequences in terms of subsequent actions.
The 392 studied accidents broke down into about sixty scenarios of this type which take account of differences between drivers in the ways they generate a functional failure when confronted by certain difficulties while driving. These scenarios thus provide us with a clinical presentation of the pathologies that lead to error production. On the basis of these pathologies, it is possible to prescribe remedies which are appropriate for the problems and the conditions under which they arise.
The failure classification model proposed here is both based on the large amount of detailed data collected about accidents and an examination of this data from the standpoint of contemporary theoretical models. This classification provides a clearer picture of the way malfunctions of the traffic system can lead to poor adjustment of one or other of the functions by means of which drivers usually cope with difficulties. It improves standardization of analysis of the participation of the human factor in the multicausal deterioration of road traffic situations. Of the functional failures that have been identified at the onset of the degraded situation, those generally related to analysis of the situation have considerable importance in relation to the problems associat-ed with the psychomotor perfor-
HUMIAiV
ERROR
mance of the action, which emphasizes the relevance of a cognitive analysis of the difficulties facing drivers. Identifying the mechanisms which govern these mental functions requires the connection to be made between research based on analysis of malfunctioning and research which is more concerned with showing the manner the driver operates while performing ordinary tasks. Error should not be studied in itself but as just one of the symptoms of human cognitive function, its internal protective systems and its failures, which may occasionally cause accidents. Grouping individual accident studies together to form prototypical accident scenarios nevertheless has the value of identifying generic problem situations which involve a similar mechanism. This similarity in the accident process may correspond to a similarity in the safety measures that could prevent their occurrence. These generic failure production scenarios should therefore constitute a powerful tool for implementing suitable countermeasures either directed at drivers through training, or directed at the environment of the driving task through measures relating to the infrastructure or the development of in-vehicle driving aids. It is important to base such measures on diagnosis of actual problems and the circumstances in which they arrive rather than on assumptions which will tend to apply ready-made solutions to poorly understood problems.