The right degree of configurability for safety-critical embedded software in Variable Message Signs

Traffic control and information systems are used in traffic technology for information propagation from a higher order control unit to the traffic participant. Nowadays, the user interface for the traffic participant is provided via freely programmable signs displaying e.g., traffic jam warnings or speed limits. These signs can be switched on or off and fed with arbitrary data corresponding to the present traffic situation. However, signs are manifold in size, functionality and means to communicate with them. For that reason, a certain degree of configurability of the embedded safety-critical software is needed in order to meet project-specific demands.

Configurability has its advantages in offering a certain degree of flexibility, but poses a risk on safety integrity and requires additional efforts for tools and documentation. This paper focuses on configurability of software in the field of Variable Message Signs (VMS). Possible configuration parameters are considered with regard to its importance during the life-cycle of a VMS and its safety impact. Considering pros and cons, finally an idea of an optimum degree of configurability is being given.