Vehicle travel speed control method

A vehicle speed control system comprising:

• a distance acquisition section for successively acquiring a curve entrance arrival distance, which is a distance from a subject vehicle to an entrance of a curving road existing ahead of the subject vehicle;
• a speed acquiring section for acquiring an actual speed of the subject vehicle;
• a curvature radius acquisition section for acquiring a radius of curvature of the curving road;
• a target speed setting section for setting a curve-time target speed, to which the subject vehicle is controlled in traveling the curving road, based on the radius of curvature of the curving road and a target lateral acceleration, which is a target for the subject vehicle to travel the curving road;
• a speed difference calculation section for calculating a difference between the curve-time target speed and the actual speed;
• an evaluation index calculation section for successively calculating a corrected approach/separation condition evaluation index, as an index indicating an approach/separation condition of the subject vehicle relative to the entrance of the curving road in correspondence to the curve-time target speed, based on a rate of time change of a seeming area of a forward object projected on a retina of a driver, the corrected evaluation index increasing as the speed difference increases and increasing at a greater rate as the curve entrance arrival distance decreases with respect to a same speed difference;
• a threshold checking section for checking whether the corrected approach/separation condition evaluation index calculated by the evaluation index calculation section exceeds a threshold value determined by a threshold calculation equation of the corrected approach/separation condition evaluation index;
• a memory section for storing a corrected target relative speed calculation equation, which corrects a target relative speed calculation equation so that a target relative speed at the entrance of the curving road becomes the curve-time target speed and a corrected target relative speed is calculated by multiplying a target relative speed calculated by the target relative speed calculation equation and a ratio of a difference between the speed of the subject vehicle at a time of starting control and the curve-time target speed relative to the speed of the subject vehicle at the time of starting control,
• the corrected target relative speed calculation equation being set by
• an approach/separation condition evaluation index relational equation, which defines a relation among the approach/separation condition evaluation index indicating the approach/separation condition of the subject vehicle relative to the entrance of the curving road ahead of the subject vehicle, the curve entrance arrival distance and the relative speed of the subject vehicle to the entrance of the curving road, the approach/separation condition evaluation index increasing as the relative speed increases and increasing at a greater rate as the curve entrance arrival distance decreases with respect to a same relative speed of the subject vehicle relative to the entrance of the curving road,
• a tangential line equation, which indicates a tangential line of a curve represented by the approach/separation condition evaluation index relational equation, is determined by differentiating the approach/separation condition evaluation index relational equation by the curve entrance arrival distance, and indicates a relation between the approach/separation condition evaluation index and the curve entrance arrival distance, and
• a target relative speed calculation equation, which is determined based on the approach/separation evaluation index relational equation and the tangential line equation thereby to determine the target relative speed based on the curve entrance arrival distance;
• a target relative speed calculation section for calculating a corrected target relative speed based on the corrected target relative speed calculation equation stored in the memory section and the curve-entrance arrival distance actually acquired by the distance acquisition section, when the threshold check section determines that the corrected approach/separation condition evaluation index is determined to exceed the threshold value;
• a target deceleration calculation section for calculating a target deceleration based on a corrected target deceleration calculated by the target relative speed calculation section and the actual speed of the subject vehicle based on the speed acquisition section;
• and
• a control section for controlling the subject vehicle to attain the target deceleration.