Planes, Trains and Automobiles — An Investigation into Safety Policy for Systems of Systems

Systems of systems comprise entities that are complex enough to be considered as systems in their own right. The interactions within a system of systems (SoS), unlike monolithic systems, are not constrained by physical design. The dynamic nature of SoS means that complex interactions are allowed to occur between the entities of a SoS. These interactions can lead to accidents. The road, rail and air transport networks are all examples of continually evolving systems of systems. Various guidelines have been developed over the years to control the behaviour of participants in these SoS. In the UK these constitute the Highway Code for all road users (including pedestrians), the Rules of the Air for civil aerospace and the Railway Rule Book for operations on the rail network. Each of these guidelines (or set of guidelines) forms a policy, more specifically a safety policy, for the interaction of entities as part of a safely operating SoS. This paper compares and contrasts the three policies in an attempt to understand how they provide a complete safety policy for their respective SoS. Using the Goal Structuring Notation, more commonly used to organise safety cases, the decomposition and expression of safety policy is illustrated. Similarities in the structure across the three example policies are identified in order to learn how better to formulate safety policy for other SoS.