Although traffic accidents in Japan involving bicycles have been decreasing yearly, more than 120,000 per year still occur. Few data exist regarding the mechanisms underlying bicycle accidents occurring at intersections. Such dangerous situations form the backdrop of the warning and automatic braking systems being developed for motor vehicles. By clarifying cyclist behavioral characteristics at crucial times, it may be possible to introduce a similar warning system for cyclists as a countermeasure to reduce accidents. The objective of this study is to clarify the mechanism of accidents involving bicycles and to obtain useful data for the development of a warning system for cyclists. A video camera and software investigated and analyzed cyclists' speed and trajectory at an intersection where many accidents occur. Cyclists entering the intersection from one direction were recorded. The results showed that the intersection offered poor visibility to cyclists, many of whom maintained their previous speed without stopping. Given these data, an improved infrastructure and a warning system using vehicle to cycle communication is desirable. Therefore, with construction of a warning system in mind, in this study we speculated on a timely warning that will contribute to safe cycling without causing alarm to the cyclist. If the warning should take place 2 s before the cyclist crosses the intersection, it would be effective for about 80% of the cyclists in this study, so our data have potential to contribute to the development of a warning system.