Driving assistance system

1. A driving assistance system comprising:

a target sensor configured to detect target information including a relative position of a target relative to a subject vehicle, the target information including a relative velocity of the target relative to the subject vehicle;

an operation amount sensor configured to obtain an accelerating operation amount corresponding to an operation amount of an accelerator pedal of the subject vehicle;

a speed sensor configured to detect a vehicle speed, the vehicle speed being a speed of the subject vehicle; and

an electronic control unit configured

to perform pre-collision control for avoiding a collision with the target, when a predetermined control start condition is satisfied, the predetermined control start condition including a condition that a first condition is established, the first condition being that a collision index value satisfies a predetermined index value condition, the collision index value having a correlation with a possibility of collision between the target and the subject vehicle, the predetermined index value condition being a required time to collision TTC (Time To Collision), the collision index value being calculated based on the target information, and

not to perform, when a second condition is established and even when the predetermined control start condition is satisfied, the pre-collision control when a permission condition has not been established before a point in time at which the predetermined control start condition is satisfied, the permission condition being that the accelerating operation amount is equal to or larger than a first operation amount threshold value and the vehicle speed is equal to or less than a speed threshold value, the second condition being that the accelerating operation amount is equal to or larger than a control threshold value and being included in the predetermined control start condition.

2. The driving assistance system according to claim 1 , wherein

when a permission cancellation condition is established, the permission cancellation condition being that the accelerating operation amount becomes smaller than a second operation amount threshold value, the second operation amount threshold value being equal to or smaller than the first operation amount threshold value, the electronic control unit is configured not to perform the pre-collision control, even when the predetermined control start condition is satisfied and the permission condition is established.

3. The driving assistance system according to claim 2 , wherein

the electronic control unit is configured

to repeatedly determine, in a predetermined cycle, whether the permission condition is established, and

not to perform the pre-collision control, when the permission cancellation condition is established, during a period from a point in time at which the permission condition is established for a first time, to a point in time at which the predetermined control start condition is satisfied.

4. The driving assistance system according to claim 1 , wherein

the electronic control unit is configured:

to calculate, as the collision index value, a collision possibility correlation value reduced as the possibility of collision between the target and the subject vehicle is increased, each time a predetermined time elapses;

to determine that a permission cancellation condition is established, when the collision possibility correlation value calculated at a given point in time is greater than the collision possibility correlation value calculated at a point in time earlier by the predetermined time than the given point in time, in a case where the permission condition is established; and

not to perform the pre-collision control when the permission cancellation condition is established, even when the predetermined control start condition is satisfied and the permission condition is established.

5. The driving assistance system according to claim 4 , wherein

the electronic control unit is configured

to repeatedly determine, in a predetermined cycle, whether the permission condition is established, and

not to perform the pre-collision control, when the permission cancellation condition is established, during a period from a point in time at which the permission condition is established for a first time, to a point in time at which the predetermined control start condition is satisfied.

6. The driving assistance system according to claim 1 , wherein

the electronic control unit is configured to:

calculate, as the collision index value, a collision possibility correlation value reduced as the possibility of collision between the target and the subject vehicle is increased, the predetermined index value condition being selected from a plurality of index value conditions, the predetermined control start condition being selected from a plurality of control start conditions, and the pre-collision control being selected from a plurality of pre-collision controls;

determine that the first condition is established by using a first index value condition as the predetermined index value condition, the first index value condition being one of the plurality of index value conditions, when the collision possibility correlation value becomes equal to or less than a first determination threshold value; and

perform control for generating a warning to a driver of the subject vehicle, as one of the plurality of pre-collision controls, when the permission condition has been established before a point in time at which a first control start condition is satisfied, in a case where the first control start condition is satisfied, the first control start condition being one of the plurality of control start conditions, the first control start condition including a condition, as an essential condition, that the first condition is established by using the first index value condition as the predetermined index value condition and the second condition is established.

7. The driving assistance system according to claim 6 , wherein

the electronic control unit is configured to:

determine that the first condition is established by using a second index value condition as the predetermined index value condition, the second index value condition being one of the plurality of index value conditions, when the collision possibility correlation value becomes equal to or less than a second determination threshold value, the second determination threshold value being less than the first determination threshold value; and

perform, as one of the plurality of pre-collision controls, braking control for applying braking force to the subject vehicle so as to decelerate the subject vehicle at a target deceleration calculated based on the target information, when the permission condition has been established before a point in time at which a second control start condition is satisfied, in a case where the second control start condition is satisfied, the second control start condition being one of the plurality of control start conditions, the second control start condition including a condition, as the essential condition, that the first condition is established by using the second index value condition as the predetermined index value condition and the second condition is established.