The objective of this study was to clarify the relationship between injury severity in bicyclists involved in traffic accidents and patient outcome or type of vehicle involved in order to propose effective measures to prevent fatal bicycle injuries. Hospital records were reviewed for all patients from 2007 to 2010 who had been involved in a traffic accident while riding a bicycle and were subsequently transferred to the Shock Trauma Center of Dokkyo Medical University Koshigaya Hospital. Patient outcomes and type of vehicle that caused the injury were examined. The mechanism of injury, Abbreviated Injury Scale (AIS) score, and Injury Severity Score (ISS) of the patient were determined. A total of 115 patients’ records were reviewed. The mean patient age was 47.1 ± 27.4 years. The average ISS was 23.9, with an average maximum AIS (MAIS) score of 3.7. The ISS, MAIS score, head AIS score, and chest AIS score were well correlated with patient outcome. The head AIS score was significantly higher in patients who had died (mean of 4.4); however, the ISS, MAIS score, and head AIS score did not differ significantly according to the type of vehicle involved in the accident. The mean head AIS scores were as high as 2.4 or more for accidents involving any type of vehicle. This study provides useful information for forensic pathologists who suspect head injuries in bicyclists involved in traffic accidents. To effectively reduce bicyclist fatalities from traffic accidents, helmet use should be required for all bicyclists.