Traffic safety related to motorcycle riding constitutes a growing issue all over the world. This is notably the case in France, where the important progress recently obtained toward a safer driving by car didn't prove to be so much effective for motorcycling. The present study has the purpose to give some cues on the reasons why, by characterizing the specificities of accident scenarios involving a powered two-wheeler (PTW). It is based on an in-depth accident analysis conducted by research teams composed of a psychologist and a technician intervening on the scene of the accident and later on. A sample of 384 accident cases involving a PTW is compared to a sample of 1174 car drivers involved in an accident without PTW. The results show the important differences regarding the human errors committed by PTW riders and by car drivers. They also show the interactional aspects of these different errors in the production of accidents, in the sense the physical and behavioral characteristics of PTW lead to certain errors by car drivers (perceptive failures) and these errors lead in turn to certain errors by PTW riders (prognostic failures).