Safety of stereo driver assistance systems

The discrete nature of disparities observed by stereo systems results in complex behaviour of speeds measured by them and affects the efficacy of a stereo based driver assistance system. We describe a tool for a safety engineer which permits the safety of these systems to be estimated. It is based on a model which considers the true error in measured velocities of objects. Outputs from this tool show that choice of stereo system parameters so as to judiciously place the disparity change boundaries is critical to the effectiveness of such a system because the range of possible trajectories for a (possibly colliding) object reduces significantly when a feature point on that object crosses one of these boundaries. This factor also means that larger objects (e.g. trucks) are slightly better tracked by stereo than smaller ones (e.g. signs and pedestrians). Completely safe stereo based systems are also shown to issue many precautionary (and ultimately unnecessary) warnings if the stereo parameters are not chosen carefully.