Naturalistic driving data has been accumulated by driving data recorders to understand factors that contribute to collisions. Among the rear end conflicts at signalized intersections in the data, conflict data between the following vehicles and suddenly stopping lead vehicles were frequently observed just after their start. To investigate the following drivers' behavior in a realistic driving situation without collision danger, an instrumented vehicle equipped with a liquid-crystal display ahead of the windshield was developed, and an experiment reproducing such conflict on the display was conducted. It was found that a lead vehicle's rapid start (2.8 m/s₂ on average) before quitting its right turn caused the following vehicle's brake reaction time to be longer than a slow start (0.8 m/s₂ on average) did. This result suggests that a following driver's premature decision to start rapidly increases the risk of rear end collisions.