Safety Demonstration and Software Development

The paper reports about a study conducted for RATP, the utility organisation for public transportation in Paris and region. RATP has developed since the mid eighties a mathematically formal approach for the development of safety-critical software, based on the B method. The question raised, in the context of evolutions in software development, was: Is it possible to demonstrate the same level of safety without resorting to mathematically formal approaches? In order to respond this question, several steps were considered: 1) reminding the infeasibility of quantifying safety-critical software, and its consequences on the development process, and on the system vision, 2) situating the current RATP approach with respect to other safety-critical domains, 3) examining and comparing alternate approaches for developing safety-critical software, 4) coming back to the RATP approach, for examining underlying assumptions. The conclusion was the recommendation to pursue the mathematically formal development approach.