The primary reason for the judicial rejection of target levels of risk or safety appears to hinge around the notion of uncertainty. When a risk expert says something is safe because the likelihood of its occurrence is around 1 x10-6 pa or 1 x 10-7 pa, they are really speaking of an inspired guesstimate or characterisation of uncertainty. That level of unlikeliness when describing a real world occurrence is just unpredictable. That is why our courts and parliaments have opted for demonstrating that all reasonable precautions are in place (the SFAIRP approach) as an alternative to calculating a difficult-to-defend number (the ALARP approach), after an event.
However, there is a caveat to this SFAIRP approach. Our courts require that, even if all reasonable practicable precautions are in place, that if something is prohibitively dangerous, it ought to be stopped altogether. What does this mean?
Prohibitively dangerous in risk terms means that its very bad, like a fatality, and that relatively speaking, it happens a lot, like for example, the road toll. People die on our roads regularly. So despite all the ways such fatalities can occur, because it happens a lot, it is relatively predictable. The road fatality number is generally accepted as being 1 x 10-4 pa. This is 100 to 1000 times more likely than 1 x10-6 pa or 1 x 10-7 pa. Does this mean that the road fatality death rate could be used as a benchmark for prohibitively dangerous?
Very possibly, at least on a large scale, industry wide basis. The courts discomfort with the ALARP approach is due to the hindsight driven nature of the courts. A death implies that the risk expert’s estimation of the rarity of the event prior to the occurrence was flawed. But if the whole activity was stopped because it approached 1 x 10-4 pa, then the courts would be silent on the matter. That is, the more scientific, ALARP would hold sway. It would certainly be more definite in the sense that the courts would have to accept that it has a greater predictability.
This suggests that SFAIRP = ALARP and could be used as a test to demonstrate when an activity is prohibitively dangerous.