Abstract
This paper proposes the simple risk assessment method about human errors, which influences the occurrence of accidents. This method is relative evolution on hierarchy structure of task. We showed the validity by presenting two sorts research examples in railway. In one research on one mission to stop trains when a meteorological disaster was carried out, we evaluated the frequency of task failure by human errors. In second research on several missions of train drivers or instruction members, we evaluated the risk of task failure. On both cases, we construct the hierarchical structure model of working conditions and mission situations.
