Challenges of improving safety in very safe transport systems

•Railways in Norway are a very safe transport system.•A low number of accidents make estimates of risk difficult.•Incident reporting has been introduced.•Changes in the number of incidents reported can be interpreted in many ways.•There is evidence of behavioural adaptation to safety barriers.

This paper discusses some challenges that may arise when trying to improve safety in systems that are already very safe. Railways in Norway are used as a case of a very safe transport system. The following challenges in improving safety are discussed: (1) A low number of accidents per unit of time makes it difficult to estimate both the current level of accident risk and changes over time in the level of accident risk. (2) Partly as a result of the low number of accidents, incident reporting has been introduced; however it is not always clear how to interpret changes in the number of incidents reported. One reason for this is that some incidents have a low potential for developing into accidents, because multiple safety barriers (defences-in-depth) stop incidents from escalating. (3) Knowledge of the effectiveness of safety barriers combined with a good safety record may lead to excessive reliance on the safety barriers and behavioural adaptation to them. The existence of these challenges is illustrated by means of data from Norwegian railways. It is discussed whether attaining a very high level of safety may lead to loss of information and loss of motivation that may slow down further progress in improving safety.