Driver's prejudice is the major cause of road accidents in Japan. Prejudice implies the cognitive status of the drivers, in that the drivers perceive the accidental risk to be lower than the objective risk. In this study, the Simple and Useful Perceived Risk Estimation Method was proposed that estimates the driver's perceived risk quantitatively, using driving behaviour data. In addition, a driving simulator named the 'Toyota Educational Driver-Diagnosis System' was developed for easy assessment of the driver's prejudice. Sixty subjects participated in the experiment and the relationships between assessed prejudice and conventional aptitude tests were investigated. Risk perceptions of aged drivers were investigated by comparing the behaviour of subjects in four groups comprising non-aged, young-old, old-old and driving instructors. The results provided insights into the driver assistance required for aged drivers. This study aims to provide a basis for new types of driver assistance that will prevent prejudice in the epoch of Intelligent Transport Systems.