In recent years, there has been known a technique for detecting a lane mark such as a white line on a road where a vehicle travels by processing an image of the road acquired with an imaging means such as a CCD camera mounted on the vehicle and for performing steering control of the vehicle or providing a driver with information on the basis of information on a lane (traffic lane) along which the vehicle travels recognized from a detection result (refer to, for example, Japanese Patent Laid-Open No. Hei 11-147473/1999) (hereinafter, referred to as Patent Document 1).
The steering force assisting device in Patent Document 1 differentiates, with respect to a plurality of horizontal lines on the image of the road, luminance for each horizontal line from the left in the lateral direction, and extracts a point where luminance changes from dark to light (positive edge point) and a point where luminance changes from light to dark (negative edge point) on the basis of respective peak of the differential values. Then, a combination of edge points, in which the positive edge point and the negative edge point appear in alternate order on each horizontal line and in which the edge points are arranged at intervals that seem to be appropriate for a white line, is extracted as a white line candidate. Then, a white line is detected among the extracted white line candidates on the basis of the positions thereof on the image.
On the road, however, there may be a structure such as, for example, a guard rail besides the lane mark such as the white line. In this case, according to the technique in Patent Document 1, a portion of the guard rail is also extracted as edge points since the portion of the guard rail changes in light and dark on the acquired image. Therefore, the portion of the guard rail is also extracted as a white line candidate and may be detected as a white line. Particularly in the case where a structure such as a guard rail is installed on the road, a lane mark such as a white line defining the lane may be missing in some parts or temporarily invisible. In this case, the portion of the guard rail is more likely to be detected as a white line.
Moreover, the structure such as a guard rail actually has a height and therefore a position where the guard rail is projected on the road surface is detected as a white line in the white line detection. Consequently, if recognizing the lane along which the vehicle travels from the position of the detected white line, the position and width of the lane would be different from that of the actual lane. Accordingly, the conventional device has a problem that the steering control of the vehicle based on the lane recognized in this manner causes inappropriate steering control of the vehicle.