We have distinguished between three types of safety defences: Barriers are formally planned to interrupt (break, divert) a specified unwanted sequence of events. Safety mechanisms are aimed at increasing the robustness of the employees' work processes to prevent that these may cause the initiation of unwanted event sequences and/or to interrupt such sequences. Capacity for improvisation enables an organization to handle situations that were not foreseen in the design and formal planning processes. The requirements for real-time situation assessment increases from human barriers to improvisation, and the specific context in which task performance will take place is gradually more difficult to predict. We have pointed to various issues which should be taken into consideration when designing or arranging for human and organizational defences. In order to improve our understanding of the human and organisational contributions to safety defences, we need to study incidents in which individuals or groups have intervened to recover dangerous situations, with a focus on contrasts between successful and unsuccessful interventions.
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