The analysis of behavioral sequences involved in accidents is one of the most promising methods leading to safety improvements. In this study attention was centered on accidents attributable to lane-changing maneuvers on the Tomei Expressway. It contained two phases, an analysis of the cause-and-effect relationships of accidents and the measurement of interaction situations in real traffic.In the first phase, a chain of events (detection of other vehicles, decision, action, vehicle response, etc.) until collision was developed for each reported accident on the Tomei Expressway by examining the complete accident reports with narratives. By integrating each chain, a logical diagram tracing all events was constructed, and causes and characteristics of accidents related to lane-changing maneuvers were examined.In the second phase, in the observation of lane-changing maneuvers in traffic on the Tomei Expressway, measurements were made of motion and positional parameters involved in two-vehicle interaction.