19
Human Factors in Accident Investigation
particularly to control recognised hazards in order to achieve an acceptable level
of risk.
Systems are dynamic in operation and one way of controlling ‘ what happens next’
is at the design stage by controlling and limiting what can happen next ( limiting the
number of states and the pathways from one state to another). Leveson ( 2016) sees
safety as a problem of control and emphasises the importance of safety constraints
at different levels ( Figure 2.4):
• Low level controls include, for example, fuses, relief valves and other engineering solutions
• Intermediate level controls include, for example, thermostats and devices
that monitor or limit aspects of the process. Forcing functions (Norman
2013) are excellent examples of safety constraints that limit what can happen next during a sequence of human actions in a system.
• Higher levels of control include, for example, standard operating procedures, maintenance programmes and training programmes
• The highest levels of control include, for example, safety committees, wider
policy and legal frameworks, national and international standards and
legislation.
FIGURE 2.4 Safety as matter of control – a visual summary of some key ideas from Leveson
( 2011). There is a hierarchy of control with different levels – the particular controls and the
hierarchy will depend on the organisation. ( From Bridger RS, Introduction to Human Factors
and Ergonomics, 4th Ed, CRC Press, Boca Raton, FL.)
Human Factors in Accident Investigation
particularly to control recognised hazards in order to achieve an acceptable level
of risk.
Systems are dynamic in operation and one way of controlling ‘ what happens next’
is at the design stage by controlling and limiting what can happen next ( limiting the
number of states and the pathways from one state to another). Leveson ( 2016) sees
safety as a problem of control and emphasises the importance of safety constraints
at different levels ( Figure 2.4):
• Low level controls include, for example, fuses, relief valves and other engineering solutions
• Intermediate level controls include, for example, thermostats and devices
that monitor or limit aspects of the process. Forcing functions (Norman
2013) are excellent examples of safety constraints that limit what can happen next during a sequence of human actions in a system.
• Higher levels of control include, for example, standard operating procedures, maintenance programmes and training programmes
• The highest levels of control include, for example, safety committees, wider
policy and legal frameworks, national and international standards and
legislation.
FIGURE 2.4 Safety as matter of control – a visual summary of some key ideas from Leveson
( 2011). There is a hierarchy of control with different levels – the particular controls and the
hierarchy will depend on the organisation. ( From Bridger RS, Introduction to Human Factors
and Ergonomics, 4th Ed, CRC Press, Boca Raton, FL.)