This study first examined the correlation between types of acute phase head injuries to cyclists and consciousness disturbance by using data from patients at the emergency room of Dokkyo Medical University Koshigaya Hospital in Japan. The injuries were compared for cases where the cyclists experienced consciousness disturbance and otherwise. The results showed that skull fractures and brain contusions were the most frequent consequences of head injuries to cyclists in general. However, the average number of head injuries by types for cyclists suffering consciousness disturbance was higher than that for cyclists who had not experienced it, for both head fractures (0.9 versus 0.7) and brain injuries (1.9 versus 0.5). This implies that head fractures and brain injuries may increase the probability of consciousness disturbance. Next, considering a cyclist who had sustained a head injury and suffered acute consciousness disturbance in an accident while not wearing a helmet, a case study was conducted to quantify the effectiveness of a bicycle helmet in reducing the intensity of traumatic head injuries by using a whole-body model, a finite element model of the human head, and a model of the bicycle helmet on a mathematical dynamic modeling software. A comparison of the peak values computed for cases of cyclists with and without a helmet showed that in the former, strain on the skull was reduced by 95.9% in cases of skull fractures, and the von Mises stress on the brain was reduced by 23.3% for cases of brain contusions. The results suggest that helmet use reduces the risk of skull fracture and brain contusions in cyclists because it considerably reduces the impact. Moreover, the use of helmets can reduce the probability of consciousness disturbance.