Driving assistance device

1. A driving assistance device comprising one or more processors configured to control operation of a host vehicle, wherein

the one or more processors are configured to perform first following control controlling acceleration and deceleration of the host vehicle so that the host vehicle follows a first preceding vehicle running in front of the host vehicle in the same lane as the host vehicle, and perform second following control prohibiting the first following control while a lane change is being performed and controlling acceleration and deceleration of the host vehicle so that the host vehicle follows a second preceding vehicle running in front of the host vehicle in a destination lane, if the lane change of the host vehicle is started during the first following control,

the one or more processors are configured to brake the host vehicle when a target deceleration degree determined based on a relative relationship between the host vehicle and the first preceding vehicle becomes equal to or less than a target deceleration degree threshold value, and

the target deceleration degree threshold value during the lane change is smaller than the target deceleration degree threshold value before the lane change.

2. The driving assistance device according to claim 1 , wherein the one or more processors are configured to:

calculate a target acceleration degree of the host vehicle;

calculate an upper limit value of the target acceleration degree;

limit the target acceleration degree at the second following control to a value equal to or less than the upper limit value; and

control acceleration and deceleration of the host vehicle so that the acceleration degree of the host vehicle becomes the target acceleration degree.

3. The driving assistance device according to claim 2 , wherein the one or more processors are configured to calculate an allowable limit value based on a following distance between the host vehicle and the first preceding vehicle and a relative speed of the host vehicle and the first preceding vehicle at the time of start of a lane change so that the host vehicle is not braked with respect to the first preceding vehicle during the lane change, and calculate the upper limit value as a value equal to or less than the allowable limit value.

4. The driving assistance device according to claim 3 , wherein the one or more processors are configured to change a difference between the allowable limit value and the upper limit value based on a following distance between the host vehicle and the first preceding vehicle during the lane change.

5. The driving assistance device according to claim 3 , wherein the one or more processors are configured to change a difference between the allowable limit value and the upper limit value based on a lateral position of the host vehicle during the lane change.

6. The driving assistance device according to claim 1 , wherein the one or more processors are configured to:

calculate a target acceleration degree of the host vehicle;

control acceleration and deceleration of the host vehicle so that the acceleration degree of the host vehicle becomes the target acceleration degree; and

calculate the target acceleration degree so that a following distance between the host vehicle and the second preceding vehicle becomes a target following distance in the second following control, and if the following distance between the host vehicle and the second preceding vehicle at the time of start of the lane change is less than a predetermined set following distance, gradually increase the target following distance at the second following control from the following distance between the host vehicle and the second preceding vehicle at the time of start of the lane change to the predetermined set following distance.