Abstract
Recent traffic accident statistics shows that the number of fatal accidents in curves is about 20% of all fatal accidents. And this figure has been about the same level for years. The cause is estimated to be excessive vehicle speeds, which results from driver's incorrect recognition or misjudgment for the curve shape and the vehicle speeds. To prevent excessive speed before entering a curve, the author has defined two systems using navigation system. One is a warning system, and the other is an advisory system. In these systems, the accuracy of curvature calculated from map data is essential for good performance. From this viewpoint, the author has studied the feasibility of these two systems.