Recently in Japan, there is an increasing tendency of serious traffic accidents while negotiating curves. A major cause of such accidents is excessive vehicular speeds due to the driver's incorrect recognition or misjudgement for curvature of the road, the vehicular performance or road conditions. Some papers have been published on the systems which prevents such accidents by means of limiting vehicular speed before entering a curves. However, most such systems rely on informations given by local facilities along the road, so that various problems to be solved are still left for practical use. From this view-point, we are now studying a self-reliant cornering vehicular speed limitation system without using outside facilities along the road . This paper presents the base technology of such a system which forecasts the curvature of the oncoming road based on the map information integrated in car navigation system which has already been installed in production vehicles in Japan.