The purpose of this research is to clarify the principal causes of broadside accidents at unsignalized intersections from various pre-crash driving patterns. Driving patterns showing a high incidence of accidents and high accident probabilities were identified by analyzing accident statistics, accident case data, and direct observational data collected at unsignalized intersections. In addition, it was found that primary parties traveling straight ahead tended to collide more often with secondary parties coming from the left side in Japan. This observation was studied from various perspectives, and the most probable causal factor was identified. These analyses have yielded information that is expected to be effective in considering measures for preventing these types of accidents.