Driving assistance system

1. A driving assistance system that is configured to calculate a target steering angle of a vehicle based on a traveling environment of the vehicle and confer an assistance torque to a steering section, in accordance with a difference between the target steering angle and an actual steering angle of the vehicle so that the actual steering angle of the vehicle approaches the target steering angle, the system comprising:

an electronic control unit (ECU) programmed to:

recognize a travel state including a speed of the vehicle;

recognize a driving operation of the vehicle by a driver;

recognize a traveling environment of the vehicle;

estimate a driving readiness degree relating to a driving consciousness of the driver based on the travel state of the vehicle or the driving operation of the vehicle by the driver and the traveling environment of the vehicle;

calculate a proportional gain based on the driving readiness degree and the speed of the vehicle;

calculate the assistance torque according to a value obtained by multiplying a difference between the target steering angle and the actual steering angle by the proportional gain; and

transmit a control signal, which corresponds to the calculated assistance torque, to an actuator for performing the driving assistance,

wherein, based upon the speed being constant, calculate the proportional gain as a smaller value as the driving readiness degree becomes lower.

2. The driving assistance system according to claim 1 , wherein the ECU is further programmed to:

set a transition time based on the driving readiness degree,

wherein if a value of the proportional gain calculated previously is different from a value of the proportional gain calculated presently, change the proportional gain so as to gradually reach the value of the proportional gain calculated presently from the value of the proportional gain calculated previously through the transition time, and

set the transition time as a shorter time as the driving readiness degree becomes lower.

3. The driving assistance system according to claim 2 , wherein the ECU is further programmed to:

calculate a differential gain based on the driving readiness degree and the proportional gain,

calculate the assistance torque according to a total value obtained by adding a value obtained by multiplying a difference between the target steering angle and the actual steering angle by the proportional gain, to a value obtained by multiplying a difference between the differential value of the target steering angle and the differential value of the actual steering angle by the differential gain, and

calculate the differential gain as a smaller value as the driving readiness degree becomes lower.

4. The driving assistance system according to claim 1 , wherein the ECU is further programmed to:

calculate a differential gain based on the driving readiness degree and the proportional gain,

calculate the assistance torque according to a total value obtained by adding a value obtained by multiplying a difference between the target steering angle and the actual steering angle by the proportional gain, to a value obtained by multiplying a difference between a differential value of the target steering angle and a differential value of the actual steering angle by the differential gain, and

calculate the differential gain as a smaller value as the driving readiness degree becomes lower.