Safe Software: Does It Cost More to Develop?

The importance of system safety has intensified in recent years given the ever-growing use of safety-critical systems in avionics, medicine, nuclear energy, and other fields. However, despite the abundance of standards which exist to provide guidance for the development of safe software for safety-critical systems, there is no consensus on how to achieve safety assurance in a cost-effective fashion. This paper reviews five software safety standards: the FAA System Safety Handbook, the US DoD MIL-STD-882D, the UK MoD DEF-STAN 00-56, NASA-STD 8719.13b and the RTCA DO-178B, and evaluates each in terms of cost effectiveness. It provides an overview of several safety-critical projects, ones that have incurred significant cost overruns as well as ones that have produced safety-critical software in a reasonably cost-effective manner. By virtue of discussing such projects we posit that it is possible to develop software, despite significant safety assurance requirements, without necessarily sacrificing cost. Specifically, projects can realize savings by using mature processes and appropriate tools to assist in development of safety-critical software.