Driving assistance system

1. A driving assistance system that performs a driver-initiative normal driving assistance mode as a driving assistance for a vehicle, the system comprising:

an electronic control unit (ECU), including a processor and a memory that stores a program executed by the processor, configured to:

recognize an operation amount performed by a driver for an operation device of the vehicle;

recognize an external environment of the vehicle;

set an appropriate operation amount range that is a range of an appropriate operation amount performed by the driver in response to the external environment;

determine whether or not the operation amount of the driver is included in the appropriate operation amount range at a first time;

change reaction force characteristics of the operation device, when the operation amount of the driver at the first time is determined to not be included in the appropriate operation amount range by:

controlling, during a normal driving assistance mode and after determining that the operation amount of the driver at the first time is not included in the appropriate operation amount range, a reaction force increase amount of the operation device with respect to an increase of the operation amount within the appropriate operation amount range to be larger than a reaction force increase amount of the operation device with respect to the increase of the operation amount within the appropriate operation amount range when the operation amount of the driver is determined to be in the appropriate operation range at the first time, and

controlling, during the normal driving assistance mode and after determining that the operation amount of the driver at the first time is not included in the appropriate operation amount range, a reaction force decrease amount of the operation device with respect to a decrease of the operation amount within the appropriate operation amount range to be larger than a reaction force decrease amount of the operation device with respect to the decrease of the operation amount within the appropriate operation amount range when the operation amount of the driver is determined to be in the appropriate operation range at the first time;

determine whether or not an explicit risk is present based on the external environment of the vehicle; and

switch a driving assistance for the vehicle from the normal driving assistance mode to a system-initiative risk avoidance assistance mode, based upon the determination that the explicit risk is present in the normal driving assistance mode.

2. The driving assistance system according to claim 1 ,

wherein the operation amount of the driver includes a steering amount of the driver for a steering wheel of the vehicle, and

the ECU is further configured to:

estimate whether or not the driver recognizes the explicit risk, if it is determined that the explicit risk is present, based on a predicted steering amount when the driver does not recognize the explicit risk or when the driver does recognizes the explicit risk, and the steering amount of the driver in the normal driving assistance mode,

switch the driving assistance for the vehicle from the normal driving assistance mode to the risk avoidance assistance mode, if it is estimated that the driver recognizes the explicit risk.

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

acquire information on a driving skill of the driver; and

set a switching timing based on the information on the driving skill of the driver, if it is determined that the driver does not recognize the explicit risk,

switch the driving assistance for the vehicle from the normal driving assistance mode to the risk avoidance assistance mode when the switching timing is set and when the vehicle is in the switching timing.

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

set a switching transition time which is a time from switching start to switching end, when switching from the normal driving assistance mode to the risk avoidance assistance mode,

gradually change an assistance torque given to the steering wheel through the time from the switching start to the switching end such that the assistance torque is changed from a reaction force giving torque in the normal driving assistance mode to a risk avoidance assistance torque in the risk avoidance assistance mode, and

set the switching transition time as a longer time if it is estimated that the driver does not recognize the explicit risk compared to a case where if it is estimated that the driver recognizes the explicit risk.

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

recognize a vehicle position on a map;

store latent risk information associated with the position on the map, in a latent risk database;

determine whether or not a latent risk is present based on the vehicle position on the map and the latent risk information; and

switch the driving assistance for the vehicle from the normal driving assistance mode to the risk avoidance assistance mode, if it is determined that the latent risk is present in the normal driving assistance mode.

6. The driving assistance system according to claim 5 , wherein the ECU is further configured to:

set a switching transition time which is a time from switching start to switching end, when switching from the normal driving assistance mode to the risk avoidance assistance mode,

estimate whether or not the driver recognizes the latent risk based on a predicted steering amount when the driver recognizes the latent risk and a steering amount of the driver, if it is estimated that the latent risk is present,

gradually change an assistance torque given to the steering wheel through the time from the switching start to the switching end such that the assistance torque is changed from a reaction force giving torque in the normal driving assistance mode to a risk avoidance assistance torque in the risk avoidance assistance mode, and

set the switching transition time as a longer time if it is determined that the latent risk is present and when it is estimated that the driver does not recognize the explicit risk, compared to a case when it is estimated that the driver recognizes the explicit risk.

7. The driving assistance system according to claim 5 ,

wherein the ECU is further configured to:

estimate whether or not the driver recognizes a driver-determined latent risk based on the target steering amount when the vehicle avoids the explicit risk in the risk avoidance assistance mode and a steering amount of the driver, if it is determined that the explicit risk is present and if it is determined that the latent risk is not present, and

continue the normal driving assistance mode if it is estimated that the driver recognizes the driver-determined latent risk.

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

set a switching transition time which is a time from switching start to switching end, when switching from the normal driving assistance mode to the risk avoidance assistance mode,

gradually change an assistance torque given to the steering wheel through the time from the switching start to the switching end such that the assistance torque is changed from a reaction force giving torque in the normal driving assistance mode to a risk avoidance assistance torque in the risk avoidance assistance mode, and

set the switching transition time as a longer time if it is estimated that the driver does not recognize the explicit risk compared to a case where if it is estimated that the driver recognizes the explicit risk.

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

recognize a vehicle position on a map;

store latent risk information associated with the position on the map in a latent risk database; and

determine whether or not a latent risk is present based on the vehicle position on the map and the latent risk information,

switch the driving assistance for the vehicle from the normal driving assistance mode to the risk avoidance assistance mode, if it is determined that the latent risk is present in the normal driving assistance mode.

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

recognize a vehicle position on a map;

store latent risk information associated with the position on the map in a latent risk database; and

determine whether or not a latent risk is present based on the vehicle position on the map and the latent risk information,

switch the driving assistance for the vehicle from the normal driving assistance mode to the risk avoidance assistance mode, if it is determined that the latent risk is present in the normal driving assistance mode.

11. The driving assistance system according to claim 6 , wherein the ECU is further configured to: estimate whether or not the driver recognizes a driver-determined latent risk based on the target steering amount when the vehicle avoids the explicit risk in the risk avoidance assistance mode and a steering amount of the driver, if it is determined that the explicit risk is present and if it is determined that the latent risk is not present, and

continue the normal driving assistance mode if it is estimated that the driver recognizes the driver-determined latent risk.