In recent years, in order to conduct an inter-vehicular distance maintenance control and the like, a device that estimates a radius of curvature of a travel route of one's own vehicle is mounted on a vehicle. As methods for estimating the radius of curvature of the travel route of the own-vehicle, there are two methods which will be described next.
A first method is a method that estimates the radius of curvature of the travel route of the own-vehicle based on such as a relative position and a relative velocity, which are measured by a distance/angle measuring instrument mounted on the own-vehicle, of a standstill object (for example, an object installed on the route such as a reflector) existing on the travel route of the own-vehicle. With the first method, a radius of curvature with fine accuracy can be estimated without generating steady error as described later. However, with the first method, time is required for recognizing a behavior of the standstill object. Therefore, with the first method, in cases such as when a change of the radius of curvature of the travel route of the own-vehicle is sharp, time is required for precisely estimating the radius of curvature, thus resulting in an inferior responsivity.
A second method is a method that estimates the radius of curvature of the travel route of the own-vehicle based on a steering angle or a yaw rate of the own-vehicle. With the second method, a responsivity that is better than the above described first method is obtained since the radius of curvature is directly estimated based on the steering angle and the yaw rate of the own-vehicle. However, with the second method, steady error in the estimate radius of curvature is generated since a zero point of a sensor for detecting the steering angle or the yaw rate changes depending on the degree of cant of a road.
Therefore, for example, in a travel route estimation device disclosed in patent literature 1 (hereinafter, referred to as a conventional technology), in order to estimate the radius of curvature with a relatively fine accuracy in an ordinary state of traveling and with a relatively fine responsivity even when traveling on a travel route with a sharp radius-of-curvature, a radius of curvature is estimated by averaging a first radius of curvature estimated by the first method and a second radius of curvature estimated by the second method.    [Patent Literature 1] Japanese Laid-Open Patent Publication No. 2001-328451