Patent Publication Number: US-2022234607-A1

Title: Driving Assistance Method and Driving Assistance System

Description:
TECHNICAL FIELD 
     The present disclosure relates to a driving assistance method and a driving assistance system. 
     BACKGROUND 
     JP 2018-151765 A discloses a visual recognition assistance device that issues a warning when a driver does not visually observe a part to be monitored for a predetermined time. The device detects an object present around a vehicle by a vehicle exterior camera, specifies a type of the detected object, and detects a visual state of the driver by an in-vehicle camera. Then, based on the type of the object and the visual state of the driver, the device determines whether or not a visual observation of the driver is appropriate, and issues a warning when determining that the visual observation of the driver is inappropriate. 
     SUMMARY 
     However, it is difficult to determine whether or not a warning is necessary by detecting a part to be visually observed by the vehicle exterior camera and a visual part of the driver by the in-vehicle camera to determine whether or not the visual observation is appropriate. Furthermore, if whether or not a warning is necessary is determined only by an orientation of the face or eyes of the driver detected by the in-vehicle camera, an unnecessary warning may be issued. 
     In view of the above problems, an object of the present disclosure is to provide a driving assistance method and a driving assistance system capable of reducing an unnecessary warning by a simple method. 
     According to an aspect of this invention, there is provided a driving assistance method of, by a controller, acquiring state information of a driver during travelling of autonomous driving and issuing a warning to the driver when the controller determines that the driver is not facing forward based on the state information of the driver. In the driving assistance method, a warning is prohibited when the driver is facing a lane change direction during the travelling of autonomous driving. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a driving assistance system according to the present embodiment; 
         FIG. 2  is a flowchart illustrating vehicle speed control according to the present embodiment; 
         FIG. 3  is a flowchart illustrating lane center keeping control according to the present embodiment; 
         FIG. 4  is a diagram illustrating lane change control started with a direction indicator operation performed by a driver as a trigger; 
         FIG. 5  is a flowchart illustrating the lane change control started with the direction indicator operation performed by the driver as the trigger; 
         FIG. 6  is a diagram illustrating lane change control based on an overtaking assistance function; 
         FIG. 7  is a flowchart illustrating the lane change control based on the overtaking assistance function; 
         FIG. 8  is a diagram illustrating lane change control based on a route travel assistance function; 
         FIG. 9  is a flowchart illustrating the lane change control based on the route travel assistance function; 
         FIGS. 10A and 10B  are diagrams illustrating an example of a warning screen of a forward looking alert; 
         FIG. 11  is a flowchart illustrating control of a notification operation of the forward looking alert; 
         FIGS. 12A and 12B  are diagrams illustrating an example of a warning screen of a primary hands-on alert; 
         FIGS. 13A and 13B  are diagrams illustrating an example of a warning screen of a secondary hands-on alert; 
         FIG. 14  is a flowchart illustrating control of a notification operation of a hands-on alert that is started when a mode is switched to a hands-on mode; and 
         FIG. 15  is a flowchart illustrating control of a notification operation of a hands-on alert that is started when the forward looking alert is notified in a hands-off mode. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present disclosure are described with reference to drawings and the like. 
     First Embodiment 
       FIG. 1  is a schematic configuration diagram of a driving assistance system  100  according to a first embodiment. 
     As illustrated in  FIG. 1 , the driving assistance system  100  according to this embodiment includes an external sensor  1 , an internal sensor  2 , a navigation system  3 , an in-vehicle device  4 , a presentation device  5 , an input device  6 , an actuator  7 , and a controller  8 . The driving assistance system  100  is mounted on a vehicle (a host vehicle)  10  having an autonomous driving function. 
     The external sensor  1  is a detection device that detects an external situation that is surrounding information of the host vehicle  10 . The external sensor  1  detects, for example, other vehicles around the host vehicle  10 , a lane marker, a road type (an expressway, a general road, and the like), and the like. The external sensor  1  includes a vehicle exterior camera  11  and a radar  12 . 
     The vehicle exterior camera  11  is an imaging device that images an external situation of the host vehicle  10  and acquires imaging information on the external situation of the host vehicle  10 . The vehicle exterior camera  11  is, for example, around view monitor cameras provided on vehicle exterior sides of front, rear, and left and right doors of the host vehicle  10 , a front camera provided on a vehicle exterior or interior side of a windshield, and a rear camera provided on a rear of the host vehicle  10 . 
     The radar  12  detects an object outside the host vehicle  10  using radio waves. The radio waves are, for example, millimeter waves. More specifically, the radar  12  transmits radio waves to surroundings of the host vehicle  10  and receives radio waves reflected by an object to detect the object. The radar  12  can acquire, for example, a distance or a direction to the object as object information. 
     The external sensor  1  outputs the acquired surrounding information (imaging information and object information) of the host vehicle  10  to the controller  8 . 
     The internal sensor  2  is a detection device that detects a state of a driver. The internal sensor  2  includes a driver monitoring camera  21 , a touch sensor  22 , and the like. 
     The driver monitoring camera  21  is a monitor unit that images a state of a driver and performs image processing, and detects an orientation of a face and an open/closed state of eyes of the driver during traveling. The driver monitoring camera  21  is, for example, an infrared camera unit installed on a dashboard of the host vehicle  10 , and includes an infrared camera  211  including an LED emitting infrared light, an optical filter, a lens, and an image sensor, and an image processing chip  212 . 
     The infrared camera  211  images the face of the driver. The image processing chip  212  extracts organs (eyes, a nose, and a mouth) on the face of the driver from a driver image of the infrared camera  211  by pattern matching, and determines the orientation of the face of the driver based on sizes and a positional relation of the organs on the face. The image processing chip  212  acquires the determined orientation of the face of the driver as facial recognition information. Further, the image processing chip  212  determines the open/closed state of the eyes based on an image of peripheries of the eyes of the driver and also acquires open/closed state information as the facial recognition information. 
     The touch sensor  22  is a detection device (a detection unit) that detects that the driver keeps hands on a handle. The touch sensor  22  is provided on the handle, and detects that the driver keeps the hands on the handle by a capacitance sensor. The touch sensor  22  acquires, as handle grasping information, whether or not it is detected that the driver keeps the hands on the handle. 
     In addition, the internal sensor  2  may include, for example, a seating sensor that detects that the driver is seated with a weight sensor and a seat belt sensor that detects that a seat belt is locked. 
     The internal sensor  2  outputs the acquired facial recognition information and handle grasping information to the controller  8  as driver state information. 
     The navigation system  3  is a device that guides the driver to a destination set on a map by the driver or the like. The navigation system  3  includes a host vehicle position detection device  31 , a display  32 , a speaker  33 , and the like. 
     The host vehicle position detection device  31  is a device that acquires position information of the host vehicle  10 , and includes a GPS receiver (GNSS)  311 , a gyro sensor  312 , a vehicle speed sensor  313 , and the like. The GPS receiver  311  periodically receives a signal (GPS data) transmitted from a GPS satellite. The gyro sensor  312  detects a direction of the host vehicle  10 . The vehicle speed sensor  313  detects a vehicle speed of the host vehicle  10 . 
     The host vehicle position detection device  31  outputs the received GPS data and the detected direction and vehicle speed of the host vehicle  10  to the controller  8  as the position information of the host vehicle  10 . The controller  8  constantly grasps a correct current position, vehicle speed, and traveling direction of the host vehicle  10  based on the received position information of the host vehicle  10 . Further, the controller  8  calculates a target route to a destination of the host vehicle  10  based on the current position and traveling direction of the host vehicle  10  and map information of a map database (not illustrated). The calculated target route is transmitted to the display  32  and the speaker  33  as navigation information. The map database includes speed limit information of a road, and the navigation system  3  acquires information on the speed limit of the road on which the host vehicle  10  is traveling, and outputs the information to the controller  8 . 
     The display  32  and the speaker  33  are devices that perform notification of target route information to the driver. The display  32  displays the target route information based on the navigation information from the controller  8 . The speaker  33  outputs the target route information by voice based on the navigation information from the controller  8 . 
     The navigation system  3  may use information stored in a computer of a facility outside a vehicle, such as an information processing center capable of communicating with the host vehicle  10 . For example, the navigation system  3  may acquire, as the navigation information, congestion information, indicating road congestion and speed limit information of a road, from the computer of the facility via communication. The processing executed by the navigation system  3  may be distributed and executed by a computer mounted on the host vehicle  10  and a computer outside the vehicle. 
     The in-vehicle device  4  includes various devices mounted on the vehicle and operates by an operation of the driver. The in-vehicle device  4  includes a steering wheel (a handle)  41 , an accelerator pedal  42 , a brake pedal  43 , and the like. When the in-vehicle device  4  is operated by the driver, operation information thereof is output to the controller  8 . When the controller  8  receives the operation information from the in-vehicle device  4 , the controller  8  transmits a command based on the operation information to the actuator  7  described later, and the actuator  7  executes traveling control of the host vehicle  10  based on the command from the controller  8 . 
     The in-vehicle device  4  further includes devices such as a direction indicator  44 , an audio device, an air conditioner, a hands free switch, a power window, a wiper, a light, and a horn. When these devices are operated by the driver, operation information thereof is output to the controller  8 . 
     In autonomous driving control (driving assistance control) described later, the in-vehicle device  4  such as the direction indicator  44  can be automatically operated and stopped according to the command from the controller  8  regardless of the operation of the driver. 
     The presentation device  5  is a device that notifies the driver of various notification information based on the control of the controller  8 , which will be described later. The presentation device  5  includes a meter display  51 , a head-up display  52 , a speaker  53 , and the like. 
     The meter display  51  is a display device incorporated in a meter unit and displays various notification information for the driver. The notification information includes traveling state information of the vehicle and warning information. As the traveling state information of the vehicle, for example, the vehicle speed, fuel cost, and a cruising distance of the host vehicle  10 , ON/OFF of the autonomous driving control, a type of a driving control mode, a speed limit sign, a situation of other vehicles around the host vehicle  10 , and lane information of the vicinity of the host vehicle  10  are displayed. As the warning information, for example, a forward looking alert, a hands-on alert, and an approach alert are displayed. The forward looking alert is displayed when it is determined that the driver is not watching ahead. The hands-on alert is displayed when it is detected that the driver releases the hands from the handle in a hands-on mode described later. The approach alert is displayed when it is determined that there is a risk of collision with another vehicle ahead. Details of the warning information will be described later. 
     The head-up display  52  is a display device that displays various notification information for the driver on the windshield. The notification information includes the traveling state information of the vehicle and the warning information. The head-up display  52  displays information similar to that of the meter display  51  more easily. 
     The speaker  53  is a notification device that outputs various notification information to the driver by voice. The speaker  53  notifies the driver of a lane change or various warning information during travelling of autonomous driving, for example, by voice. 
     The input device  6  is a switch operated by the driver and is provided, for example, at a position on the steering wheel where the driver can easily operate the input device  6 . The input device  6  includes a main switch  61 , a setting/coasting switch  62 , a resuming/accelerating switch  63 , a cancel switch  64 , a speed limit assistance switch  65 , an inter-vehicle distance adjustment switch  66 , a lane change assistance switch  67 , a steering control switch  68 , and the like. 
     The main switch  61  is a selector switch that switches power ON/OFF of the driving assistance system  100 . 
     The setting/coasting switch  62  is a switch that starts vehicle speed control and reduces a set vehicle speed. 
     The resuming/accelerating switch  63  is a switch that has both a resuming operation of resuming an operation at the set vehicle speed before release and an acceleration operation of increasing the set vehicle speed after the vehicle speed control is released. 
     The cancel switch  64  is a switch that releases operations of various types of control related to the autonomous driving, such as vehicle speed control and steering control. 
     The speed limit assistance switch  65  is a switch that automatically sets a target vehicle speed to a speed limit during a vehicle speed control operation. 
     The inter-vehicle distance adjustment switch  66  is a switch that changes an inter-vehicle distance (a target inter-vehicle distance) to a preceding vehicle during the vehicle speed control operation. 
     The lane change assistance switch  67  is a switch that allows the driver to perform a lane change when the driving assistance system  100  determines that the lane change of the host vehicle  10  is necessary during steering control and makes a notification for prompting the driver to change a lane by the presentation device  5 . 
     The steering control switch  68  is, for example, a touch panel on a screen of the display  32  in the navigation system  3  and is a selector switch that switches ON/OFF of a steering control function. 
     The vehicle speed control and the steering control will be described later. 
     The input device  6  may be a device such as a dial switch or a microphone capable of inputting a command by voice of the driver, in addition to a push type switch capable of inputting a command by a hand operation of the driver, a touch panel disposed on a display screen, or the like. 
     Information (input information) input to the input device  6  is output to the controller  8 . 
     The actuator  7  is a device that executes traveling control of the host vehicle  10  based on a command from the controller  8 . The actuator  7  includes a drive actuator  71 , a brake actuator  72 , a steering actuator  73 , and the like. 
     The drive actuator  71  is a device that adjusts a drive force of the host vehicle  10 . 
     Specifically, when the host vehicle  10  is an internal combustion engine vehicle equipped with an engine as a traveling drive source, the drive actuator  71  includes a throttle actuator that adjusts an amount of air supplied to the engine (a throttle opening) and a fuel injection valve that adjusts an amount of fuel supplied to the engine (a fuel injection amount). 
     Furthermore, when the host vehicle  10  is a hybrid vehicle or an electric vehicle equipped with a motor as a traveling drive source, the drive actuator  71  includes a circuit (an inverter, a converter, or the like) capable of adjusting electric power supplied to the motor. 
     The brake actuator  72  is a device that operates a brake system in response to a command from the controller  8  to adjust a braking force applied to wheels of the host vehicle  10 . The brake actuator  72  includes a hydraulic brake, or a regenerative brake. 
     The steering actuator  73  includes an assistance motor that controls a steering torque in an electric power steering system, or the like. The controller  8  controls an operation of the steering actuator  73  to control an operation of the wheels, thereby executing steering control of the host vehicle  10  described later. 
     The controller  8  includes a computer equipped with a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input/output interface (I/O interface). The controller  8  executes a specific program to execute processing for realizing the autonomous driving control (the driving assistance control) of this embodiment. For example, the controller  8  executes vehicle speed control, steering control, control of notification operations of various alerts, and the like, which will be described below. The controller  8  may be configured with one computer or may be configured with plural computers. 
     The controller  8  is configured to communicate various signals with the external sensor  1 , the internal sensor  2 , the navigation system  3 , the in-vehicle device  4 , the presentation device  5 , and the input device  6 . The controller  8  receives various types of information acquired by the external sensor  1 , the internal sensor  2 , and the navigation system  3 , the operation information from the in-vehicle device  4 , and the input information from the input device  6 . Then, the controller  8  is programmed to acquire these pieces of information as traveling information of the host vehicle  10 , and appropriately operate the actuator  7  based on the traveling information to execute the driving assistance control of this embodiment. 
     Hereinafter, the autonomous driving control (the driving assistance control) and the control of the notification operations of various alerts provided in the driving assistance system  100  according to this embodiment will be described. 
     The autonomous driving control (the driving assistance control) provided in the driving assistance system  100  includes a vehicle speed control function and a steering control function. 
     The vehicle speed control function is a function of controlling a traveling speed of a vehicle such that the traveling speed of the vehicle becomes a target value. In the vehicle speed control, when there is no other vehicles in front of the host vehicle  10 , the control is performed so as to maintain a set target vehicle speed. On the other hand, when there is a vehicle in front of the host vehicle  10 , the control is performed such that an inter-vehicle distance is kept constant with the target vehicle speed as an upper limit. 
     Furthermore, the vehicle speed control function includes a function of automatically setting a legal speed as the target vehicle speed. 
       FIG. 2  is a flowchart showing the vehicle speed control according to this embodiment. The following vehicle speed control is repeatedly executed by the controller  8 . 
     During the vehicle speed control described below, the driver can change the target vehicle speed at any time by operating the setting/coasting switch  62  or the resuming/accelerating switch  63 . Further, the driver can stop at any time automatically setting the speed limit to the target vehicle speed by turning off the speed limit assistance switch  65 . Furthermore, the driver can stop the vehicle speed control at any time by switching the main switch  61  to OFF, pressing the cancel switch  64 , or depressing the brake pedal  43 . 
     The vehicle speed control is control that is performed when the driving assistance system  100  is operating. When the main switch  61  of the input device  6  is switched to ON, a start command of the driving assistance system  100  is transmitted to the controller  8 . Upon receiving the start command, the controller  8  activates the driving assistance system  100 . 
     When the setting/coasting switch  62  is pressed by the driver during the operation of the driving assistance system  100 , a start command for the vehicle speed control is transmitted to the controller  8 . Upon receiving the start command of the vehicle speed control, the controller  8  starts the vehicle speed control. 
     When the vehicle speed control is started, in step S 110 , the controller  8  determines whether or not the navigation system  3  detects the speed limit of the road on which the host vehicle  10  is traveling and whether or not the speed limit assistance switch  65  is ON. When the speed limit is detected by the navigation system  3  and the speed limit assistance switch  65  is ON, the controller  8  proceeds to processing of step S 120 . 
     The speed limit of the road on which the host vehicle  10  is traveling may be detected by imaging the speed limit sign by the external sensor  1 . 
     In step S 120 , the controller  8  sets a detected speed limit as the target vehicle speed. 
     On the other hand, in step S 110 , when the speed limit is not detected or the speed limit assistance switch  65  is OFF, the controller  8  proceeds to processing of step S 121 . 
     In step S 121 , the controller  8  sets, as the target vehicle speed, a vehicle speed when the setting/coasting switch  62  is pressed to start the vehicle speed control. 
     When the target vehicle speed is set in step S 120  or S 121 , in step S 130 , the controller  8  controls the actuator  7  such that the vehicle speed of the host vehicle  10  becomes the set target vehicle speed. 
     Next, in step S 140 , the controller  8  determines whether there is another vehicle in front of the host vehicle  10  based on the surrounding information of the host vehicle  10  acquired by the external sensor  1 . 
     When the controller  8  determines that there is no other vehicles in front of the host vehicle  10  in step S 140 , in step S 150 , the controller  8  controls the actuator  7  so as to maintain the vehicle speed of the host vehicle  10  at the set target vehicle speed. 
     On the other hand, when the controller  8  determines that there is another vehicle in front of the host vehicle  10  in step S 140 , the controller  8  proceeds to processing of step S 151 . In step S 151 , the controller  8  controls the actuator  7  such that the inter-vehicle distance is kept constant with the set target vehicle speed as an upper limit. 
     After executing the processing in step S 150  or S 151 , the controller  8  returns to step S 110  again. In step S 110 , when the speed limit is not detected or the speed limit assistance switch  65  is OFF, the controller  8  proceeds to processing of step S 121 . In this case, since the target vehicle speed has already been set, the controller  8  maintains the set target vehicle speed. 
     The controller  8  displays information such as whether or not the vehicle speed control is executed, the target vehicle speed, and the speed limit on the meter display  51  and the head-up display  52  of the presentation device  5 . In addition, for example, when the target vehicle speed is changed or when a new speed limit is detected, the controller  8  may be set to notify the driver of the information by a character display on the meter display  51  and the head-up display  52  or by voice from the speaker  53 . 
     Next, the steering control function will be described. The steering control function of this embodiment includes a lane center keeping assistance function and a lane change assistance function. 
     The lane center keeping assistance function is a function of controlling the steering such that the host vehicle  10  travels in a vicinity of a center of a lane and assisting a handle operation of the driver. The lane center keeping control is started by switching the touch panel type steering control switch  68  of the navigation system  3  to ON while the vehicle speed control is executed. The lane center keeping control is started in a state (a hands-on mode) where the driver keeps the hands on the handle, and when a predetermined condition is satisfied, the driver can release the hands from the handle (a hands-off mode). 
       FIG. 3  is a flowchart illustrating the lane center keeping control according to this embodiment. The following lane center keeping control is repeatedly executed by the controller  8 . 
     During the lane center keeping control described below, the driver can stop the lane center keeping control at any time by switching the steering control switch  68  to OFF. Further, the driver can stop the lane center keeping control and the vehicle speed control at any time by switching the main switch  61  to OFF, pressing the cancel switch  64 , or depressing the brake pedal  43 . 
     When the steering control switch  68  is switched to ON by the driver during the operation of the vehicle speed control function, a start command of the lane center keeping control is transmitted to the controller  8 . Upon receiving the start command, the controller  8  starts the lane center keeping control. 
     When the lane center keeping control is started, the controller  8  displays that the steering control is operated on the meter display  51  and the head-up display  52  of the presentation device  5 . 
     Subsequently, in step S 210 , the controller  8  acquires the traveling information of the host vehicle  10 , and determines whether or not operating conditions of the lane center keeping assistance function are satisfied based on the acquired traveling information. Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) The lane markers on both sides are detected. 
     (2) The driver keeps the hands on the handle. 
     (3) The vehicle is traveling in the vicinity of the center of the lane. 
     (4) The direction indicator does not operate. 
     (5) The wiper does not operate at a high speed. 
     (6) When there is a highly accurate map, there are no tollgates, expressway exits, confluences, intersections, and lane reduction points within a predetermined distance (for example, about 200 m) ahead. 
     When all of these conditions are satisfied, the controller  8  operates the lane center keeping assistance function in step S 220 . On the other hand, until all the above conditions are satisfied, the controller  8  does not operate the lane center keeping assistance function. 
     The operating conditions of the lane center keeping assistance function are examples, are not limited to these conditions, and can be appropriately set so as to operate the lane center keeping assistance function if certain predetermined conditions are satisfied depending on the situation. 
     In step S 220 , the lane center keeping assistance function is operated, and the operation of the steering actuator  73  is controlled such that the host vehicle  10  travels in the center of the lane. In addition, the controller  8  operates the lane center keeping assistance function and notifies, by the meter display  51  and the head-up display  52  of the presentation device  5 , the driver to maintain a state of keeping the hands on the handle. That is, the driver is notified of a hands-on mode in which driving assistance (autonomous driving) is executed in a state where the driver keeps the hands on the handle. 
     Next, in step S 230 , the controller  8  determines whether or not hands-off conditions under which the driver may release the hands from the handle are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) The vehicle is traveling on an expressway. 
     (2) The vehicle is traveling on a road structurally separated from an opposite lane. 
     (3) The vehicle is traveling on a road with a highly accurate map. 
     (4) The vehicle is traveling at a vehicle speed equal to or lower than a speed limit. 
     (5) A reception signal of the GPS receiver (GNSS)  311  is valid. 
     (6) The driver keeps the hands on the handle. 
     (7) The driver is facing forward. 
     (8) There are no tollgates, expressway exits, confluences, intersections, and lane reduction points within a predetermined distance (for example, 800 m) ahead. 
     (9) There is no sharp curve (for example, a curvature radius R is 100 m or less) within a predetermined distance (for example, 500 m) ahead. 
     (10) The vehicle is not traveling in a tunnel (for example, a portion exceeding 500 m from a tunnel entrance). 
     (11) The accelerator pedal is not depressed. 
     When all of these conditions are satisfied, the controller  8  proceeds to processing of step S 240 . 
     On the other hand, when any one of the hands-off conditions is not satisfied, the controller  8  returns to step S 210 . When the operating conditions of the lane center keeping assistance function are satisfied in step S 210 , the controller  8  continues to operate the lane center keeping assistance function and notify the hands-on mode in step S 220 . When the operating conditions of the lane center keeping assistance function are not satisfied in step S 210 , the controller  8  stops the operation of the lane center keeping assistance function until the operating conditions of the lane center keeping assistance function are satisfied. 
     When the hands-off conditions are satisfied, in step S 240 , the controller  8  notifies the driver that the hands may be released from the handle by the meter display  51  and the head-up display  52  of the presentation device  5 . That is, the driver is notified that the mode is switched to the hands-off mode in which the driving assistance (the autonomous driving) is executed in a state where the driver releases the hands from the handle. 
     The hands-off conditions are examples, are not limited to these conditions, and can be appropriately set so as to switch to the hands-off mode if certain predetermined conditions are satisfied depending on the situation. 
     In step S 250 , based on the traveling information, the controller  8  determines whether or not hands-off release conditions for switching from the hands-off mode to the hands-on mode are satisfied. Specifically, for example, the controller  8  determines whether any one of the following conditions is satisfied. 
     (1) The vehicle is traveling on a road other than an expressway. 
     (2) The vehicle is traveling in a two-way traffic section. 
     (3) The vehicle is traveling on a road having no highly accurate map. 
     (4) The vehicle is traveling at a vehicle speed exceeding a speed limit. 
     (5) The GPS receiver (GNSS)  311  cannot receive a signal. 
     (6) After a forward looking alert is issued, the driver does not face forward within a predetermined time (for example, 5 seconds). 
     (7) The driver cannot be detected by the driver monitoring camera  21 . 
     (8) There is any one of: tollgates, expressway exits, confluences, intersections, and lane reduction points within a predetermined distance (for example, 800 m) ahead. 
     (9) When the vehicle is traveling at a vehicle speed less than a predetermined speed (for example, about 40 km/h), there is a sharp curve (for example, a curvature radius R is 100 m or less) within a predetermined distance (for example, 200 m) ahead. 
     (10) When the vehicle is traveling at a vehicle speed equal to or higher than a predetermined speed (for example, about 40 km/h), there is a sharp curve (for example, a curvature radius R is 170 m or less) within a predetermined distance (for example, 200 m) ahead. 
     (11) The vehicle is traveling in a tunnel (for example, a portion exceeding 500 m from a tunnel entrance). 
     (12) The driver holds the handle and depresses the accelerator pedal. 
     (13) The approach alert is issued. 
     When any one of these conditions is satisfied, the controller  8  releases the hands-on mode. 
     In (6) of the hands-off release conditions here, the hands-off mode is not released until a predetermined time elapses after the forward looking alert is issued. That is, under the hands-off conditions, a condition that the driver is facing forward (7) is set as a switching condition to the hands-off mode, whereas under the hands-off release conditions, the hands-off mode is not immediately released even when the driver is not facing forward. As a result, for example, it is possible to prevent the hands-off mode from being released, for example, in a case where the driver simply checks the right and the left, instead of distracted driving. 
     When any one of the hands-off release conditions is satisfied, the controller  8  releases the hands-off mode and returns to step S 210 . When the operating conditions of the lane center keeping assistance function are satisfied in step S 210 , in step S 220 , the controller  8  continues to operate the lane center keeping assistance function and notifies that the mode is switched to the hands-on mode. When the operating conditions of the lane center keeping assistance function are not satisfied in step S 210 , the controller  8  stops the operation of the lane center keeping assistance function until the operating conditions of the lane center keeping assistance function are satisfied. 
     On the other hand, when the hands-off release conditions are not satisfied in step S 250 , the controller  8  continues the hands-off mode (step S 260 ) and returns to the processing of step S 250 . That is, the controller  8  continues the hands-off mode until the hands-off release conditions are satisfied. 
     The hands-off release conditions are examples, are not limited to these conditions, and can be appropriately set such that the hands-off mode is released if certain predetermined conditions are satisfied depending on the situation. 
     The processing from step S 210  to step S 260  is repeatedly executed until the driver stops the lane center keeping control. 
     When the lane center keeping control is started or when the hands-on mode and the hands-off mode are switched, the driver may be notified of the start or switching by character display on the meter display  51  and the head-up display  52  or by voice from the speaker  53 . 
     Next, the lane change assistance function will be described. 
     The lane change assistance function is a function of controlling the steering and assisting the lane change when the driver operates the direction indicator  44  or when the driving assistance system  100  proposes a lane change and the driver presses the lane change assistance switch  67 . The lane change control is one type of steering control and is the driving assistance (the autonomous driving) control that is executed in the state where the driver keeps the hands on the handle, that is, in the hands-on mode. 
       FIG. 4  is a diagram illustrating lane change control started with a direction indicator operation performed by a driver as a trigger. 
     As illustrated in  FIG. 4 , when the driver turns on the direction indicator  44  at a time t 1  during a steering control operation, if a predetermined condition is satisfied, a lane change procedure (LCP) is started, and the steering control is executed. 
     After the start of the LCP, when a predetermined condition is satisfied, a lane change manoeuvre (LCM) is started at a time t 2  after several seconds from the start of LCP, the lane change is performed, and the LCM is completed at a time t 3  within several seconds from the start of the LCM. 
     The direction indicator  44  is turned off automatically or by a driver operation at a time t 4  within several seconds after the completion of the LCM, and LCP is completed. 
     A time from t 1  (turning on the direction indicator) to t 2  (LCM start) and t 3  (LCM completion), and a time from t 3  (LCM completion) to t 4  (turning off the direction indicator) are all preferably about several seconds, but are not limited thereto, and can be freely set depending on the situation. 
       FIG. 5  is a flowchart illustrating the lane change control started with the direction indicator operation performed by the driver as the trigger. The following lane change control is executed by the controller  8 . 
     During the steering control operation (the steering control switch  68  is ON), when the driver operates the direction indicator  44 , a start command of the lane change procedure (LCP) is transmitted to the controller  8 . 
     When the controller  8  receives an LCP start command, in step S 310 , the controller  8  determines whether or not LCP start conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) After the start of LCP, the driver keeps the hands on the handle within a predetermined time (for example, about 2 seconds). 
     (2) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (3) There is no vehicle detected in a destination to which the vehicle is to be changed (there is a space in a lane change direction). 
     (4) There is a lane in a lane change direction. 
     (5) A lane marker in the lane change direction is a broken line. 
     (6) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     (7) The time is within a predetermined time (for example, within about 1 second) after the driver performs a direction indicator operation. 
     When all of these conditions are satisfied, in step S 320 , the controller  8  starts the lane change procedure (LCP). 
     On the other hand, when even any one of the LCP start conditions is not satisfied, the controller  8  stops the lane change procedure (LCP). 
     In step S 320 , the controller  8  starts the lane change procedure (LCP), and controls the operation of the actuator  7  (in particular, the steering actuator  73 ) so as to assist the handle operation necessary for the lane change. In addition, the controller  8  displays information such as the start of the lane change procedure (LCP) and the lane change direction on the meter display  51  and the head-up display  52  of the presentation device  5 . In addition to the display on the meter display  51  and the head-up display  52 , the driver may be notified of the start of the lane change procedure (LCP) by voice from the speaker  53 . 
     The LCP start conditions are examples, are not limited to these conditions, and can be appropriately set such that LCP is started if certain predetermined conditions are satisfied depending on the situation. 
     Next, in step S 330 , the controller  8  determines whether or not lane change stop conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether any one of the following conditions is satisfied. 
     (1) After the start of the LCP, the lane change manoeuvre (LCM) cannot be started within a predetermined time (for example, 5 seconds). 
     (2) After the start of the LCP and before the start of the LCM, the vehicle speed falls below a predetermined speed (for example, about 50 km/h). 
     (3) After the start of the LCP and before the start of the LCM, there is no space in an adjacent lane necessary for the lane change. 
     (4) Before the start of the LCM, the lane marker is not detected. 
     (5) Before the start of the LCM, it is determined that there is no adjacent lane in the lane change direction, or that there is no adjacent lane within a certain distance ahead. 
     (6) Before the start of the LCM, it is determined that there is a curve having a predetermined curvature radius (for example, a curvature radius R is 250 m or less) within a certain distance ahead. 
     (7) Before the start of the LCM, it is determined that there is a section within a certain distance ahead where a type of the lane marker indicates that a lane change to an adjacent lane in the lane change direction is prohibited. 
     (8) Before the start of the LCM, the radar  12  detects shielding, radio interference, or an axial deviation. 
     (9) The hands-on alert is issued. 
     (10) The driver turns off the direction indicator. 
     When any one of these conditions is satisfied, the controller  8  stops the lane change assistance control. The hands-on alert is issued when the driver does not keep the hands on the handle within a predetermined time (for example, about 2 seconds) after the start of the LCP. 
     On the other hand, when the lane change stop conditions are not satisfied, in step S 340 , the controller  8  controls the operation of the actuator  7  (in particular, the steering actuator  73 ) to execute the lane change manoeuvre (LCM) of the host vehicle  10 . 
     Upon completion of the LCM, the controller  8  turns off the direction indicator  44  in step S 350 . When the direction indicator  44  is turned off, the LCP ends. In addition, the direction indicator  44  may be turned off by a driver operation. 
     The lane change stop conditions are examples, are not limited to these conditions, and can be appropriately set such that the lane change is stopped if certain predetermined conditions are satisfied depending on the situation. 
     Next, the lane change control proposed by the driving assistance system  100  will be described. The lane change assistance function proposed by the driving assistance system  100  includes the overtaking assistance function for overtaking a vehicle having a slow speed, and the route travel assistance function for traveling according to a route to a destination. The overtaking assistance function is a function of controlling the steering and assisting an overtaking operation when a vehicle having a speed lower than the set speed is detected ahead. The route travel assistance function is a function of controlling the steering when the driver sets a destination and assisting the lane change necessary for traveling according to the route. 
       FIG. 6  is a diagram illustrating the lane change control based on the overtaking assistance function. The lane change control based on the overtaking assistance function is started with the proposal of the driving assistance system  100  as a trigger when there is a vehicle having a speed lower than the set speed in front of the host vehicle  10 . 
     During the travelling of autonomous driving (the steering control operation), when a vehicle having a speed lower than the set speed is detected in front of the host vehicle  10 , if a predetermined condition is satisfied, a notification of prompting the driver to change the lane to an overtaking lane is made by the presentation device  5  at a time t 1-1 . After the notification of the lane change, when the lane change assistance switch  67  is pressed by the driver at a time t 1-2  within a predetermined time, the direction indicator  44  is turned on at a time t 1-3  within several seconds and the lane change procedure (LCP) is started. 
     When the LCP is started, the steering control is executed, and if a predetermined condition is satisfied, the lane change manoeuvre (LCM) is started at a time t 1-4  within several seconds from the start of the LCP, and the lane change to the overtaking lane is performed. At a time t 1-5  within several seconds after the start of the LCM, the lane change manoeuvre (LCM) for changing the lane to the overtaking lane is completed. At a time t 1-6  within several seconds after the completion of the LCM, the direction indicator  44  is turned off automatically or by a driver operation and LCP is completed. 
     A time from t 1-2 . (when the lane change assistance switch  67  is pressed) to t 1-3  (turning on the direction indicator), a time from t 1-3  (turning on the direction indicator) to t 1-4  (LCM start) and t 1-5  (LCM completion), and a time from t 1-5  (LCM completion) to t 1-6  (turning off the direction indicator) are all preferably about several seconds, but are not limited thereto, and can be freely set depending on the situation. 
     Next, when the host vehicle  10  overtakes a preceding vehicle, if a predetermined condition is satisfied, a notification of prompting the driver to change the lane to a traveling lane is made by the presentation device  5  at a time t 2-1 . After the notification of the lane change, when the lane change assistance switch  67  is pressed by the driver at a time t 2-2 , the direction indicator  44  is turned on at a time t 2-3  within several seconds and the lane change procedure (LCP) is started. 
     When the LCP is started, the steering control is executed, and if a predetermined condition is satisfied, the lane change manoeuvre (LCM) is started at a time t 2-4  within several seconds from the start of the LCP, and the lane change to the traveling lane is performed. At a time t 2-5  within several seconds after the start of the LCM, the lane change manoeuvre (LCM) to the traveling lane is completed. At a time t 2-6  within several seconds after the completion of the LCM, the direction indicator  44  is turned off automatically or by a driver operation and the LCP is completed. 
     A time from t 2-2 . (when the lane change assistance switch  67  is pressed) to t 2-3  (turning on the direction indicator), a time from t 2-3  (turning on the direction indicator) to t 2-4  (LCM start) and t 2-5  (LCM completion), and a time from t 2-5  (LCM completion) to t 2-6  (turning off the direction indicator) are all preferably about several seconds, but are not limited thereto, and can be freely set depending on the situation. 
       FIG. 7  is a flowchart illustrating the lane change control based on the overtaking assistance function. The following lane change control is executed by the controller  8 . 
     In step S 401 , during the steering control operation (in a state where the steering control switch  68  is ON), the controller  8  determines, based on the traveling information of the host vehicle  10 , whether or not a vehicle having a speed lower than the set speed is detected in front of the host vehicle  10 . 
     When a vehicle having a speed lower than the set speed is detected in front of the host vehicle  10 , in step S 402 , the controller  8  determines whether or not proposal conditions of lane change to the overtaking lane are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) The vehicle is in the hands-off mode. 
     (2) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (3) There is space for lane change after 5 seconds. 
     (4) There is a lane in a lane change direction. 
     (5) A lane marker in the lane change direction is a broken line. 
     (6) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     (7) The vehicle speed of the host vehicle  10  is lower than the set target vehicle speed by a predetermined speed (for example, 5 km/h) or more. 
     (8) The vehicle speed of the preceding vehicle is lower than the set target vehicle speed by a predetermined speed (for example, 10 km/h) or more. 
     (9) The inter-vehicle distance between the host vehicle  10  and the preceding vehicle is less than the inter-vehicle distance set based on a speed difference (for example, in a case where the inter-vehicle distance is set to 50 m when the host vehicle is traveling at a speed of 100 km/h and the speed difference is 10 km/h, or in a case where the inter-vehicle distance is less than 50 m when the host vehicle is traveling at 100 km/h and the speed difference is 10 km/h.) 
     (10) A speed of a preceding vehicle after the lane change is higher than that of the preceding vehicle in the travelling lane by a predetermined speed (for example, about 5 km/h) or more. 
     When all of these conditions are satisfied, it is estimated that a lane change is necessary. 
     The proposal conditions of the lane change to the overtaking lane are examples, are not limited to these conditions, and can be appropriately set such that it is estimated that the lane change is necessary if certain predetermined conditions are satisfied depending on the situation. 
     When all of the proposal conditions of the lane change to the overtaking lane are satisfied, in step S 403 , the controller  8  prompts the driver to change the lane by notification of the presentation device  5 . 
     Subsequently, in step S 404 , when the controller  8  detects that the lane change assistance switch  67  is pressed by the driver, the controller  8  proceeds to processing of step S 405 . On the other hand, if the lane change assistance switch  67  is not pressed within a predetermined time, the controller  8  stops the lane change control. 
     In step S 405 , the controller  8  determines whether or not lane change procedure (LCP) start conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) The driver keeps the hands on the handle within a predetermined time after the lane change assistance switch  67  is pressed. 
     (2) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (3) There is no vehicle detected in a destination to which the lane is to be changed (there is a space in a lane change direction). 
     (4) There is a lane in a lane change direction. 
     (5) A lane marker in the lane change direction is a broken line. 
     (6) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     (7) The vehicle speed of the host vehicle  10  is lower than the set target vehicle speed by a predetermined speed (for example, 5 km/h) or more. 
     (8) The vehicle speed of the preceding vehicle is lower than the set target vehicle speed by a predetermined speed (for example, 10 km/h) or more. 
     (9) The speed of the preceding vehicle after the lane change is higher than that of the preceding vehicle in the travelling lane by a predetermined speed (for example, about 5 km/h) or more. 
     (10) The time is within a predetermined time after the lane change assistance switch  67  is pressed. 
     When any one of the LCP start conditions is not satisfied, the controller  8  stops the lane change control. 
     On the other hand, when all of the LCP start conditions are satisfied, in step S 406 , the controller  8  turns on the direction indicator  44  within a predetermined time of several seconds after the driver presses the lane change assistance switch  67 , and starts the lane change procedure (LCP). When the lane change procedure (LCP) is started, the controller  8  controls the operation of the actuator  7  (in particular, the steering actuator  73 ) so as to assist the handle operation necessary for the lane change. In addition, the controller  8  displays information such as the start of the lane change procedure (LCP) and the lane change direction on the meter display  51  and the head-up display  52  of the presentation device  5 . In addition to the display on the meter display  51  and the head-up display  52 , the driver may be notified of the start of the lane change procedure (LCP) by voice from the speaker  53 . 
     The LCP start conditions are examples, are not limited to these conditions, and can be appropriately set such that LCP is started if certain predetermined conditions are satisfied depending on the situation. 
     Next, in step S 407 , the controller  8  determines whether or not the lane change stop conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether any one of the following conditions is satisfied. 
     (1) Before the start of the LCP, the vehicle speed exceeds the speed limit. 
     (2) Before the start of the LCP, the driver keeps the hands on the handle and depresses the accelerator pedal. 
     (3) After the start of the LCP, a predetermined time is exceeded (after the start of the LCP, the LCM cannot be started within a predetermined time). 
     (4) Before the start of the LCM, the vehicle speed falls below a predetermined speed (for example, about 50 km/h). 
     (5) After the start of the LCP and before the start of the LCM, there is no space in an adjacent lane necessary for the lane change. 
     (6) Before the start of the LCM, the lane marker is not detected. 
     (7) Before the start of the LCM, it is determined that there is no adjacent lane in the lane change direction, or that there is no adjacent lane within a certain distance ahead. 
     (8) Before the start of the LCM, it is determined that there is a curve having a predetermined curvature radius (for example, a curvature radius R is 250 m or less) within a certain distance ahead. 
     (9) Before the start of the LCM, it is determined that there is a section within a certain distance ahead where a type of the lane marker indicates that a lane change to an adjacent lane in the lane change direction is prohibited. 
     (10) Before the start of the LCM, the radar  12  detects shielding, radio interference, or an axial deviation. 
     (11) The hands-on alert is issued. 
     (12) The driver turns off the direction indicator. 
     When any one of these conditions is satisfied, the controller  8  stops the lane change control. The hands-on alert is issued when the driver does not keep the hands on the handle within a predetermined time after the lane change assistance switch  67  is pressed. 
     On the other hand, when the lane change stop conditions are not satisfied, in step S 408 , the controller  8  controls the operation of the actuator  7  (in particular, the steering actuator  73 ) and to laterally move the host vehicle  10 , and then executes the lane change manoeuvre (LCM) for changing the lane to the overtaking lane. 
     After the completion of the lane change manoeuvre (LCM) for changing the lane to the overtaking lane, in step S 409 , the controller  8  turns off the direction indicator  44  and ends the LCP. The direction indicator  44  may be turned off by a driver operation. 
     The lane change stop conditions are examples, are not limited to these conditions, and can be appropriately set such that the lane change is stopped if certain predetermined conditions are satisfied depending on the situation. 
     Next, in step S 410 , it is determined whether or not the proposal conditions of the lane change to the traveling lane are satisfied. Specifically, for example, it is determined whether all of the following conditions are satisfied. 
     (1) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (2) There is space for the lane change after a predetermined time. 
     (3) There is a lane in a lane change direction. 
     (4) A lane marker in the lane change direction is a broken line. 
     (5) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     (6) The vehicle speed of the host vehicle  10  is lower than the set target vehicle speed by a predetermined speed (for example, 5 km/h) or more. 
     (7) The vehicle speed of the preceding vehicle is lower than the set target vehicle speed by a predetermined speed (for example, 10 km/h) or more. 
     (8) The speed difference between the vehicle speed of the preceding vehicle after the lane change and the vehicle speed of the host vehicle is within a predetermined speed (for example, about 5 km/h). 
     If all of these conditions are satisfied, it is estimated that a lane change to return to the traveling lane is necessary. 
     The proposal conditions of the lane change to the traveling lane are examples, are not limited to these conditions, and can be appropriately set such that it is estimated that the lane change to return to the traveling lane is necessary if certain predetermined conditions are satisfied depending on the situation. 
     When all of the proposal conditions of the lane change to the traveling lane are satisfied, in step S 411 , the controller  8  prompts the driver to change the lane to the traveling lane by notification of the presentation device  5 . 
     Subsequently, in step S 412 , when the controller  8  detects that the lane change assistance switch  67  is pressed by the driver, the controller  8  proceeds to processing of step S 413 . On the other hand, if the lane change assistance switch  67  is not pressed within a predetermined time after the lane change is prompted, the controller  8  stops the lane change control. 
     In step S 413 , the controller  8  determines whether or not lane change procedure (LCP) start conditions are satisfied based on the traveling information of the host vehicle  10 . The LCP start conditions here are the same as those of step S 405 . When any one of the LCP start conditions is not satisfied, the controller  8  stops the lane change control for changing the lane to the traveling lane. 
     On the other hand, when all of the LCP start conditions are satisfied, in step  414 , the controller  8  turns on the direction indicator  44  within a predetermined time after the driver presses the lane change assistance switch  67 , and starts the lane change procedure (LCP). When the LCP is started, the controller  8  controls the operation of the actuator  7  so as to assist the handle operation necessary for the lane change, and notifies the driver of information such as the start of the LCP and a lane change direction by the presentation device  5 . 
     Next, in step S 415 , the controller  8  determines whether or not the lane change stop conditions are satisfied based on the traveling information of the host vehicle  10 . The lane change stop conditions are the same as those of step S 407 . When any one of the lane change stop conditions is satisfied, the controller  8  stops the lane change control. 
     On the other hand, when the lane change stop conditions are not satisfied, in step S 416 , the controller  8  executes the lane change manoeuvre (LCM) for changing the lane to the overtaking lane after laterally moving the host vehicle  10 . 
     Upon completion of the LCM, the controller  8  turns off the direction indicator  44  and ends the LCP in step S 417 . The direction indicator  44  may be turned off by a driver operation. 
     Next, the route travel assistance function for traveling according to a route to a destination will be described. The route travel assistance function is a function of controlling the steering to assist the lane change necessary for traveling according to the route when an occupant such as a driver sets the destination. 
       FIG. 8  is a diagram illustrating the lane change control based on the route travel assistance function. The lane change control based on the route travel assistance function is started with a proposal of the driving assistance system  100  as a trigger when the vehicle arrives at a lane change point necessary for traveling according to a route in a case where the driver sets a destination in the navigation system  3 . The case where the vehicle arrives at the lane change point necessary for traveling according to the route is, for example, a case where the vehicle approaches an exit road, a branch road, or a lane reduction point, or the like. 
     During the travelling of autonomous driving (the steering control operation), when the vehicle arrives at the lane change point necessary for traveling according to a route toward a destination set by the driver or the like at a time t 1-1 , if a predetermined condition is satisfied, a notification of prompting the driver to change the lane is made by the presentation device  5 . After the notification of the lane change, when the lane change assistance switch  67  is pressed by the driver at a time t 1-2 , the direction indicator  44  is turned on at a time t 1-3  within several seconds and the lane change procedure (LCP) is started. 
     Similarly to the lane change control based on the overtaking assistance function, when the LCP is started at a time t 1-3 , the steering control is executed. After the start of the LCP, when a predetermined condition is satisfied, the lane change manoeuvre (LCM) is started and the lane change is performed at a time t 1-4  within several seconds, and the LCM is completed at a time t 1-5  within several seconds after the start of the LCM. After the completion of the LCM, at a time t 1-6 , the direction indicator  44  is turned off automatically or by the driver operation and LCP is completed. 
     When it is necessary to change lanes multiple times, the same control as shown in  FIG. 8  is performed multiple times. 
     A time from t 1-2  (when the lane change assistance switch  67  is pressed) to t 1-3  (turning on the direction indicator), a time from t 1-3  (turning on the direction indicator) to t 1-4  (LCM start) and t 1-5  (LCM completion), and a time from t 1-5  (LCM completion) to t 1-6  (turning off the direction indicator) are all preferably about several seconds, but are not limited thereto, and can be freely set depending on the situation. 
       FIG. 9  is a flowchart illustrating the lane change control based on the route travel assistance function. The following lane change control is executed by the controller  8 . 
     In step S 501 , during the steering control operation (the steering control switch  68  is ON), the controller  8  determines, based on the traveling information, whether or not a lane change is necessary for the host vehicle  10  to travel according to a route to a set destination. When the lane change is not necessary, the lane change control is stopped. 
     When the controller  8  determines that a lane change is necessary for the host vehicle  10  to travel according to the route, in step S 502 , the controller  8  determines whether or not proposal conditions of a lane change along the route are satisfied. Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) A destination is set in the navigation system  3 . 
     (2) The vehicle is in the hands-off mode. 
     (3) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (4) The lane change to a target lane is possible (the lane change is not prohibited). 
     (5) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     When all of the proposal conditions of the lane change along the route are satisfied, it is estimated that the lane change along the route is necessary, and in step S 503 , the controller  8  prompts the driver to change the lane by the notification of the presentation device  5 . 
     The proposal conditions of the lane change along the route are examples, are not limited to these conditions, and can be appropriately set such that it is estimated that the lane change along the route is necessary if certain predetermined conditions are satisfied depending on the situation. 
     Subsequently, in step S 504 , when the controller  8  detects that the lane change assistance switch  67  is pressed by the driver, the controller  8  proceeds to processing of step S 505 . On the other hand, if the lane change assistance switch  67  is not pressed within a predetermined time, the controller  8  stops the lane change control. 
     In step S 505 , the controller  8  determines whether or not the lane change procedure (LCP) start conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether all of the following conditions are satisfied. 
     (1) The driver keeps the hands on the handle within a predetermined time (for example, within about 2 seconds) after the lane change assistance switch  67  is pressed. 
     (2) The vehicle speed is a predetermined speed (for example, about 60 km/h) or higher. 
     (3) There is no vehicle detected in a destination to which the lane is to be changed (there is a space in a lane change direction). 
     (4) There is a lane in a lane change direction. 
     (5) A lane marker in the lane change direction is a broken line. 
     (6) The vehicle is travelling on a straight line or a gentle curve (for example, a curvature radius R is 250 m or more). 
     When any one of these conditions is not satisfied, the controller  8  stops the lane change control. 
     On the other hand, when all of the LCP start conditions are satisfied, in step S 506 , the controller  8  turns on the direction indicator  44  within a predetermined time of several seconds after the driver presses the lane change assistance switch  67 , and starts the lane change procedure (LCP). When the lane change procedure (LCP) is started, the controller  8  controls the operation of the actuator  7  (in particular, the steering actuator  73 ) so as to assist the handle operation necessary for the lane change. In addition, the controller  8  displays information such as the start of the lane change procedure (LCP) and the lane change direction on the meter display  51  and the head-up display  52  of the presentation device  5 . In addition to the display on the meter display  51  and the head-up display  52 , the driver may be notified of the start of the lane change procedure (LCP) by voice from the speaker  53 . 
     The LCP start conditions are examples, are not limited to these conditions, and can be appropriately set such that the LCP is started if certain predetermined conditions are satisfied depending on the situation. 
     Next, in step S 507 , the controller  8  determines whether or not the lane change stop conditions are satisfied based on the traveling information of the host vehicle  10 . Specifically, for example, the controller  8  determines whether any one of the following conditions is satisfied. 
     (1) Before the start of the LCP, the vehicle speed exceeds the speed limit. 
     (2) Before the start of the LCP, the driver keeps the hands on the handle and depresses the accelerator pedal. 
     (3) After the start of the LCP, a predetermined time is exceeded (after the start of the LCP, the LCM cannot be started within a predetermined time). 
     (4) Before the start of the LCM, the vehicle speed falls below a predetermined speed (for example, about 50 km/h). 
     (5) After the start of the LCP and before the start of the LCM, there is no space in an adjacent lane necessary for the lane change. 
     (6) Before the start of the LCM, the lane marker is not detected. 
     (7) Before the start of the LCM, it is determined that there is no adjacent lane in the lane change direction, or that there is no adjacent lane within a certain distance ahead. 
     (8) Before the start of the LCM, it is determined that there is a curve having a predetermined curvature radius (for example, a curvature radius R is 250 m or less) within a certain distance ahead. 
     (9) Before the start of the LCM, it is determined that there is a section within a certain distance ahead where a type of the lane marker indicates that a lane change to an adjacent lane in the lane change direction is prohibited. 
     (10) Before the start of the LCM, the radar  12  detects shielding, radio interference, or an axial deviation. 
     (11) The hands-on alert is issued. 
     (12) The driver turns off the direction indicator. 
     When any one of these conditions is satisfied, the controller  8  stops the lane change control. The hands-on alert is issued when the driver does not keep the hands on the handle within a predetermined time of several seconds after the lane change assistance switch  67  is pressed. 
     On the other hand, when the lane change stop conditions are not satisfied, in step S 508 , the controller  8  controls the operation of the actuator  7  (in particular, the steering actuator  73 ) to laterally move the host vehicle  10  and then executes the lane change manoeuvre (LCM). 
     The lane change stop conditions are examples, are not limited to these conditions, and can be appropriately set such that the lane change is stopped if certain predetermined conditions are satisfied depending on the situation. 
     Upon completion of the LCM, the controller  8  turns off the direction indicator  44  and ends LCP in step S 509 . The direction indicator  44  may be turned off by a driver operation. 
     Upon completion of LCP, the controller  8  returns to step S 501 , and determines, based on the traveling information of the host vehicle  10 , whether or not the lane change is further necessary for traveling according to the route. If the lane change is further necessary, the control of step S 502  to step S 510  is performed again. On the other hand, if no further lane change is necessary, the lane change control is stopped. 
     Although the vehicle speed limit control function and the steering control function provided in the driving assistance system  100  have been described above, these are examples of the autonomous driving control and are not necessarily limited to the above configurations and flows. That is, the driving assistance system in which control of notification operations of various alters described below is used is not limited to the driving assistance system that executes the above autonomous driving control. 
     Next, the control of the notification operations of various alters will be described. 
     The warning information notified to the driver by the presentation device  5  includes a forward looking alert, a primary hands-on alert, a secondary hands-on alert, a steering control release alert, a vehicle speed control/steering control release alert, an approach alert, a collision alert, a lane departure alert, and the like. 
     The forward looking alert is notified when it is determined that the driver is not looking forward. The primary hands-on alert is notified when it is detected that the driver releases the hands from the handle in the hands-on mode. The secondary hands-on alert is notified when the driver is not looking forward for a predetermined time after the forward looking alert is notified, or when the driver does not hold the handle for a predetermined time after the primary hands-on alert is notified. The steering control release alert is notified when the steering control is released during the hands-off mode and prompts the driver to perform a handle operation. The vehicle speed control/steering control release alert is notified when the vehicle speed control and the steering control is released during the hands-off mode and prompts the driver to perform a handle operation and a pedal operation. The approach alert is notified when it is determined that there is a risk of collision with a preceding vehicle. The collision alert is notified when an emergency brake (automatic brake control) is operated. The lane departure alert is notified when it is determined that the vehicle approaches a lane marker on a right side or a left side of the traveling lane. 
     Next, the details of the forward looking alert will be described. 
     The forward looking alert is an alert that requests the driver to look forward by voice and display when it is detected that the driver is not looking forward (a traveling direction of the host vehicle  10 ) during the travelling of autonomous driving (during the steering control operation). For example, a warning screen as shown in  FIGS. 10A and 10B  is displayed on the meter display  51  and the head-up display  52 , and a warning is issued to the driver by voice from the speaker  53 .  FIG. 10A  is an example of a warning screen displayed on the head-up display  52 , and  FIG. 10B  is an example of a warning screen displayed on the meter display  51 . 
     Although only the forward looking alert is notified during the hands-on mode, a secondary hands-on alert described later is notified when the driver does not look forward even after a predetermined time (for example, about 5 seconds) elapses after the forward looking alert is notified during the hands-off mode. 
     Here, if a warning is issued to the driver at all times when the driver is not facing forward, for example, for example, in a case where the driver is facing a lane direction when a lane change is planned, or in a case where the driver faces left or right for checking only, a warning will also be notified. That is, an unnecessary warning is issued. 
     Therefore, in this embodiment, first, even when the driver is not facing forward, for example, when the driver is facing the lane change direction, the notification (warning) of the forward looking alert is prohibited. That is, in this embodiment, during the lane change control, when the driver is not looking forward but is facing the lane change direction, the forward looking alert on the presentation device  5  is not notified. As a result, an unnecessary warning is reduced. 
     As for the lane change direction, when the direction indicator  44  is operating, a lighting (operation) direction of the direction indicator  44  is estimated to be the lane change direction. In this way, it is possible to determine whether or not prohibiting a warning is necessary based on only the information on the lighting direction of the direction indicator  44  and the orientation of the face of the driver, and no complicated processing is required. 
     In a case where the driving assistance system  100  proposes a lane change, it is conceivable that the driver checks a lane change direction before the direction indicator  44  is operated. Therefore, in this embodiment, in the case where the driving assistance system  100  proposes the lane change, when the driver is facing the direction of the lane change proposed by the system even before the direction indicator  44  is operated, the notification of the forward looking alert is prohibited. As a result, an unnecessary warning is further reduced. 
     In addition, in this embodiment, the forward looking alert is not notified until the time during which the driver is not facing forward exceeds a predetermined time. That is, in this embodiment, in a case where the time when the driver is not facing forward, such as when the driver is facing left and right for checking only, is a short time, that is, as long as the time during which the driver is not facing forward is not continued for a predetermined time, the forward looking alert is not notified. As a result, an unnecessary warning is further reduced. 
       FIG. 11  is a flowchart illustrating control of a notification operation of the forward looking alert. The following control of the notification operation is repeatedly executed by the controller  8 . 
     During the travelling of autonomous driving (the driving assistance control), in step S 601 , based on facial recognition information acquired by the driver monitoring camera  21 , the controller  8  determines whether or not the eyes of the driver are closed for a predetermined time (for example, about 5 seconds). 
     When the controller  8  determines that the eyes of the driver are closed continuously for a predetermined time, the controller  8  estimates that the driver is not looking forward and proceeds to processing of step S 608 . In step S 608 , the controller  8  notifies the driver of the forward looking alert by the presentation device  5 . The forward looking alert is notified by the display on the meter display  51  and the head-up display  52  and the voice from the speaker  53 . 
     On the other hand, when the controller  8  determines that the eyes of the driver are opened or the time during which the eyes are closed does not exceeds a predetermined time in step S 601 , the controller  8  proceeds to processing of step S 602 . 
     In step S 602 , based on the facial recognition information acquired by the driver monitoring camera  21 , the controller  8  determines whether or not the orientation of the face of the driver does not change and the eyes are not open or closed in a predetermined time (for example, about 60 seconds). When the orientation of the face of the driver does not change and the eyes are not opened or closed for a predetermined time, the controller  8  estimates that the driver is not looking forward, and proceeds to processing of step S 608 . In step S 608 , the controller  8  notifies the driver of the forward looking alert by the display and the voice of the presentation device  5 . 
     On the other hand, when the driver changes the orientation of the face within a predetermined time or when the driver opens or closes the eyes within a predetermined time, the controller  8  proceeds to processing of step S 603 . 
     In step S 603 , based on the facial recognition information acquired by the driver monitoring camera  21 , the controller  8  determines whether or not the driver is not facing forward (a traveling direction of the host vehicle  10 ) continuously for a predetermined time (a first predetermined time, for example, about 5 seconds). Specifically, when the following conditions are satisfied, the controller  8  determines that the driver is not facing forward continuously for a predetermined time. 
     (1) The driver faces leftward or rightward at a predetermined angle (for example, about 20 degrees) or more with respect to the traveling direction of the vehicle for a predetermined time (for example, about 5 seconds).
 
(2) The driver faces upward or downward at a predetermined angle (for example, about 20 degrees) or more with respect to a horizontal direction of the vehicle for a predetermined time (for example, about 5 seconds).
 
     When any one of the conditions is satisfied, the controller  8  determines that the driver is not facing forward continuously for a predetermined time. 
     When the controller  8  determines that the driver is not facing forward continuously for a predetermined time in step S 603 , the controller  8  proceeds to processing of step S 604 . On the other hand, when the controller  8  determines that the driver is facing forward or the time during which the driver is not facing forward does not exceed the predetermined time, the forward looking alert is not notified, and the controller  8  ends the control of the notification operation of the forward looking alert. 
     The conditions used to determine whether the driver is not facing forward are examples, are not limited to these conditions, and can be appropriately set such that it is determined that the driver is not facing forward if certain predetermined conditions are satisfied depending on the situation. 
     In step S 604 , based on the traveling information of the host vehicle  10 , the controller  8  determines whether or not the lane change control is in progress. As described above, the lane change control is started when the driver performs a direction indicator operation or when the driving assistance system  100  proposes a lane change. Therefore, based on operation information of the in-vehicle device  4  among the traveling information of the host vehicle  10 , the controller  8  determines whether or not the lane change control is in progress depending on whether the direction indicator  44  operation is detected or whether the driving assistance system  100  proposes a lane change to the driver. 
     When the controller  8  determines that the lane change control is not in progress in step S 604 , the controller  8  proceeds to processing of step S 608  and notifies the driver of the forward looking alert by the display and voice of the presentation device  5 . 
     On the other hand, when the controller  8  determines that the lane change control is in progress in step S 604 , the controller  8  proceeds to processing of step S 605 . In step S 605 , the controller  8  determines whether or not the driver is facing the lane change direction based on the travel information of the host vehicle  10 . When the direction indicator  44  is operating, the controller  8  estimates that the lighting direction of the direction indicator  44  is the lane change direction. On the other hand, when the direction indicator  44  is not turned on, the controller  8  sets a direction toward a lane in a change destination proposed by the driving assistance system  100  as the lane change direction. The orientation of the face of the driver is determined based on the facial recognition information acquired by the driver monitoring camera  21 . 
     When the controller  8  determines that the driver is not facing the lane change direction in step S 605 , the controller  8  proceeds to processing of step S 608  and notifies the driver of the forward looking alert by the display and voice of the presentation device  5 . 
     On the other hand, when the controller  8  determines that the driver is facing the lane change direction in step S 605 , the controller  8  does not notify the driver of the forward looking alert and proceeds to processing of step S 606 . 
     In step S 606 , based on the traveling information of the host vehicle  10 , the controller  8  determines whether or not the driver is facing the lane change direction continuously for a predetermined time (a second predetermined time). When it is determined that the driver is facing the lane change direction continuously for a predetermined time, the controller  8  proceeds to processing of step S 608  and notifies the forward looking alert. This is because the driver is facing the lane change direction for more than a certain time, which is dangerous as in the case of the distracted driving. 
     The predetermined time (the second predetermined time) is set to be longer than the predetermined time (the first predetermined time, for example, about 5 seconds) in step S 603 . 
     When the controller  8  detects that the driver faces forward within the predetermined time (the second predetermined time) in step S 606 , the controller  8  prohibits the notification of the forward looking alert in step S 607 . 
     As described above, when the driver performs the direction indicator operation, the controller  8  detects that the direction indicator  44  is operated, and prohibits the notification (warning) of a forward looking alert when the driver is facing the lane change direction after the direction indicator  44  is operated. In addition, in a case where the controller  8  determines whether a lane change is necessary and notifies the driver of the lane change if it is determined that the lane change is necessary, that is, in a case where the driving assistance system  100  proposes a lane change, the notification (the warning) of the forward looking alert is prohibited when the driver is facing the lane change direction after the driver is notified of the lane change. 
     After the forward looking alert is notified in step S 608 , the controller  8  determines whether or not the vehicle is in the hands-off mode in step S 609 . In the case of the hands-off mode, the controller  8  proceeds to control of the notification operation of the hands-on alert (step S 810 ) described later. On the other hand, in the case of the hands-on mode (the driver keeps the hands on the handle), the controller  8  stops the notification of the forward looking alert when the controller  8  detects that the driver looks forward in step S 610 . 
     In the control of the notification operation of the forward looking alert, the controller  8  determines whether the driver is looking forward based on whether the eyes of the driver are closed, whether or not the orientation of the face of the driver does not change and the eyes are not open or closed in a predetermined time, and whether the driver is facing forward. However, the conditions for determining whether or not the driver is looking forward are not limited thereto, and can be appropriately set such that it is determined that the driver is not looking forward if certain predetermined conditions are satisfied depending on the situation. For example, step S 601  (whether the eyes of the driver are closed) or step S 602  (whether or not the orientation of the face of the driver does not change and the eyes are not opened or closed in a predetermined time) may be omitted as appropriate, or the processing order of steps S 601 , S 602  and steps S 603  to S 606  may be changed. 
     Next, the details of the hands-on alert will be described. The hands-on alert is issued during the travelling of autonomous driving (during the steering control operation) and includes the primary hands-on alert and the secondary hands-on alert. 
     The primary hands-on alert is notified when it is detected that the driver releases the hands from the handle in the hands-on mode, and requests the driver to perform a handle operation by a display. The primary hands-on alert prompts the driver to perform a handle operation by displaying, for example, a warning screen as illustrated in  FIGS. 12A and 12B  on the meter display  51  and the head-up display  52 .  FIG. 12A  is an example of a warning screen displayed on the head-up display  52 , and  FIG. 12B  is an example of a warning screen displayed on the meter display  51 . 
     The secondary hands-on alert is notified when the driver is not looking forward continuously for a predetermined time (a fourth predetermined time, for example, about 5 seconds) after the forward looking alert is notified, or when the driver does not hold the handle continuously for the predetermined time (the fourth predetermined time, for example, about 5 seconds) after the primary hands-on alert is notified. The secondary hands-on alert requests the driver to perform a handle operation by a display, a voice, and a warning brake (slowly operating a brake for plural times in a short time). The driver is prompted to perform a handle operation by displaying, for example, warning screens as illustrated in  FIGS. 13A and 13B  on the meter display  51  and the head-up display  52 , and by voice from the speaker  53  and slow braking (the warning brake) in a short time.  FIG. 13A  is an example of a warning screen displayed on the head-up display  52 , and  FIG. 13B  is an example of a warning screen displayed on the meter display  51 . 
     If the secondary hands-on alert is always notified after the forward looking alert is notified, the secondary hands-on alert may also be notified in the hands-off mode when the driver looks forward immediately after the forward looking alert is notified. That is, an unnecessary warning is issued. Therefore, in this embodiment, in the hands-off mode, even after the forward looking alert is notified, when the driver looks forward within a predetermined time (s third predetermined time) shorter than the fourth predetermined time, the secondary hands-on alert is not notified. That is, as described above, when the forward looking alert is notified in the hands-off mode, if the driver looks forward within a predetermined time, the hands-off mode is not released. Thus, in this embodiment, in the hands-off mode, even after the forward looking alert is notified, if the driver looks forward within the predetermined time (the third predetermined time), the hands-off mode is not released and the secondary hands-on alert is not notified. As a result, an unnecessary warning is further reduced. 
       FIG. 14  and  FIG. 15  are flowcharts illustrating control of a notification operation of a hands-on alert. The control of the notification operation of the hands-on alert is started when a mode is switched to the hands-on mode (or the steering control is started and the hands-on mode is started) and when the forward looking alert is notified in the hands-off mode. 
       FIG. 14  is a flowchart illustrating control of a notification operation of a hands-on alert that is started when the mode is switched to the hands-on mode. The following control of the notification operation is repeatedly executed by the controller  8 . 
     When the mode is switched to the hands-on mode (or the hands-on mode is started) during the autonomous driving, in step S 710 , the controller  8  determines whether or not the lane change control is in progress based on the traveling information. As described above, the lane change control is started when the driver performs an operation or when the driving assistance system  100  proposes a lane change. In a case where the lane change control is not in progress (that is, in a case where the lane center keeping control is in progress), the controller  8  proceeds to processing of step S 720 . 
     In step S 720 , based on the traveling information (driver state information acquired by the touch sensor  22 ), the controller  8  determines whether or not the driver keeps the hands on the handle within a predetermined time (for example, within about 5 seconds) in the hands-on mode. When the driver keeps the hands on the handle within the predetermined time, it is not necessary to notify a hands-on alert, and thus the controller  8  ends the control of the notification operation of the hands-on alert. 
     On the other hand, when the driver releases the hands from the handle continuously for a predetermined time in the hands-on mode, the controller  8  notifies the primary hands-on alert in step S 730 . The primary hands-on alert prompts the driver to perform a handle operation by the display on the presentation device  5 . 
     Next, in step S 740 , based on the traveling information of the host vehicle  10  (the driver state information acquired by the touch sensor  22 ), the controller  8  determines whether or not the driver keeps the hands on the handle within a predetermined time (for example, within about 5 seconds) after the notification of the primary hands-on alert. 
     In step S 740 , when the controller  8  determines that the driver keeps the hands on the handle within the predetermined time after the notification of the primary hands-on alert, the controller  8  stops the primary hands-on alert. 
     On the other hand, when the driver releases the hands from the handle continuously for a predetermined time in step S 740 , the controller  8  proceeds to processing of step S 750 , and the secondary hands-on alert is notified. The secondary hands-on alert is notified by the display, the voice, and the warning brake. That is, the controller  8  prompts the driver to perform a handle operation by notifying the driver by the display and the voice of the presentation device  5  and controlling the brake actuator  72  to operate the brake plural times. 
     Next, in step S 760 , based on the traveling information (the driver state information acquired by the touch sensor  22 ), the controller  8  determines whether or not the driver keeps the hands to the handle within a predetermined time (for example, within about 5 seconds). When the driver keeps the hands on the handle within the predetermined time (the handle operation is performed), the controller  8  stops the secondary hands-on alert. 
     On the other hand, when it is determined that the driver releases the hands from the handle continuously for a predetermined time in step S 760 , the controller  8  controls the brake actuator  72  to decelerate and forcibly stop the host vehicle  10  in step S 770 . 
     Next, in step S 804 , based on the traveling information (the driver state information acquired by the touch sensor  22 ), the controller  8  determines whether or not the driver keeps the hands to the handle within a predetermined time (a fifth predetermined time, for example, within about 5 seconds). When the driver keeps the hands on the handle within the predetermined time (the handle operation is performed), the controller  8  stops the secondary hands-on alert. 
     When the controller  8  determines that the lane change control is in progress in step S 710 , the controller  8  proceeds to processing of step S 721 . 
     In step S 721 , based on the traveling information, the controller  8  determines whether or not the driver keeps the hands on the handle within a predetermined time (for example, within about 2 seconds) after the start of the lane change procedure (LCP). When the driver keeps the hands on the handle within a predetermined time after the start of the LCP, the controller  8  ends the control of the notification operation of the hands-on alert. 
     On the other hand, when the driver releases the hands from the handle continuously for a predetermined time after the start of the LCP, in step S 722 , the controller  8  notifies the primary hands-on alert and stops the lane change control. 
     When the primary hands-on alert is notified in step S 722 , the controller  8  next proceeds to processing of step S 740 , and determines, based on the traveling information of the host vehicle  10 , whether or not the driver keeps the hands on the handle within a predetermined time (for example, within about 5 seconds) after the notification of the primary hands-on alert. When the driver releases the hands from the handle continuously for a predetermined time, the controller  8  proceeds to processing of step S 750 , and the secondary hands-on alert is notified. When the driver releases the hands from the handle continuously for a predetermined time even if the secondary hands-on alert is notified, the controller  8  forcibly stops the host vehicle  10  (step S 770 ). 
       FIG. 15  is a flowchart illustrating control of a notification operation of a hands-on alert that is started when the forward looking alert is notified in the hands-off mode. The case where the forward looking alert is notified in the hands-off mode is the case where it is determined in step S 609  of  FIG. 11  that the vehicle is in the hands-off mode. The following control of the notification operation is repeatedly executed by the controller  8 . 
     When the forward looking alert is notified in the hands-off mode, in step S 810 , the controller  8  determines, based on the facial recognition information acquired by the driver monitoring camera  21 , whether or not the driver looks forward (in the traveling direction of the host vehicle  10 ) within a predetermined time after the forward looking alert is notified. Whether the driver looks forward is determined depending on whether the driver changes the orientation of the face to the traveling direction of the vehicle or whether the eyes are not closed for a predetermined time or longer. When the driver changes the orientation of the face to the traveling direction and the eyes are not closed for a predetermined time or longer, the controller  8  determines that the driver looks forward. 
     When the driver looks forward within the predetermined time (the third predetermined time) in step S 810 , the controller  8  stops the forward looking alert and prohibits the notification of the hands-on alert, and the hands-off mode is continued. On the other hand, when the driver does not look forward continuously for a predetermined time in step S 810 , the controller  8  proceeds to processing of step S 820 . 
     In step S 820 , based on the facial recognition information acquired by the driver monitoring camera  21 , the controller  8  determines whether or not the driver looks forward within the predetermined time (the fourth predetermined time) after the third predetermined time elapses, after the forward looking alert is notified. When the driver looks forward within the fourth predetermined time, the controller  8  switches the mode to the hands-on mode and stops the notification of the forward looking alert in step S 851 . 
     On the other hand, when the driver does not look forward within the fourth predetermined time in step  820 , the controller  8  proceeds to processing of step S 830 . 
     In step S 830 , the controller  8  notifies the driver of the secondary hands-on alert. The secondary hands-on alert is notified by the display and the voice of the presentation device  5  and the warning brake. 
     Next, in step S 840 , based on the traveling information (the driver state information acquired by the touch sensor  22 ), the controller  8  determines whether or not the driver keeps the hands to the handle within the predetermined time (the fifth predetermined time, for example, within about 5 seconds). When the driver keeps the hands on the handle within the predetermined time (the handle operation is performed), the controller  8  stops the forward looking alert and the secondary hands-on alert, and continues the autonomous driving as the hands-on mode in step S 851 . 
     On the other hand, when it is determined that the driver releases the hands from the handle continuously for the predetermined time (the fifth predetermined time) in step S 840 , the controller  8  controls the brake actuator  72  to decelerate and forcibly stop the host vehicle  10  in step S 850 . 
     According to the driving assistance system  100  of the embodiment described above, the following effects can be obtained. 
     In the driving assistance system  100 , the controller  8  acquires the state information of the driver during the travelling of autonomous driving (during execution of the steering control), and notifies the driver of the forward looking alert (warning) when the driver is not facing forward based on the state information of the driver. However, when the driver is facing the lane change direction, the forward looking alert is prohibited. That is, in the case where the driver is not facing forward, for example, when the driver is facing the lane change direction, the driver is also not notified (warned) of the forward looking alert. As a result, an unnecessary warning is reduced. In addition, the controller  8  determines whether or not prohibiting the forward looking alert (warning) is necessary based on only the lane change direction and the state information of the driver (the orientation of the face of the driver), and no complicated processing is required to determine whether or not prohibiting the warning is necessary. Therefore, it is possible to provide a driving assistance method and a driving assistance system capable of reducing an unnecessary warning by a simple method. 
     In addition, when the driver is not facing forward and the lane change direction at the time of the lane change, the driving assistance system  100  issues the forward looking alert (warning). If all forward looking alerts (warnings) are prohibited when the driver is not facing forward at the time of the lane change, the warning is also prohibited when the driver is not facing forward and the lane change direction. On the other hand, the driving assistance system  100  of this embodiment also issues the forward looking alert (warning) for an act of distracted driving in which the driver does not face forward and the lane change direction at the time of the lane change. As a result, as compared with the case where all warnings are prohibited at the time of the lane change, it is possible to prevent the act of distracted driving in which the driver does not face forward and the lane change direction. 
     The driving assistance system  100  detects an operation state of the direction indicator  44 , and prohibits the forward looking alert (warning) when the driver is facing the lighting (operation) direction of the direction indicator  44 . In this way, the controller  8  determines whether or not prohibiting the forward looking alert (warning) is necessary based on only the lighting direction of the direction indicator  44  and the state information of the driver (the orientation of the face of the driver), and no complicated processing is required to determine whether or not prohibiting the warning is necessary. Therefore, an unnecessary warning can be reduced by a simple method. 
     When the driver is not facing forward and the lighting direction of the direction indicator  44  at the time of turning on the direction indicator  44 , the driving assistance system  100  issues the forward looking alert (warning). As a result, as compared with the case where all warnings are prohibited at the time of turning on the direction indicator  44 , it is possible to prevent the act of distracted driving in which the driver does not face forward and the lighting direction of the direction indicator. 
     The driving assistance system  100  determines whether or not a lane change of the vehicle is necessary, and proposes (notifies) the driver of a lane change when determining that the lane change is necessary. Then, after the driver is notified of the lane change, when the driver is facing the lane change direction, the forward looking alert (warning) is prohibited. In this way, in the case where the lane change is proposed by the driving assistance system  100 , when the driver is facing the lane change direction before the direction indicator  44  is turned on, the forward looking alert (warning) is also prohibited. Therefore, an unnecessary warning can be further reduced. 
     The driving assistance system  100  issues the forward looking alert (warning) to the driver when the driver drives the vehicle while not facing forward for the first predetermined time or longer. In this way, the forward looking alert is not notified until the time during which the driver is not facing forward is equal to or longer than the first predetermined time. Therefore, for example, it is possible to prevent a warning from being issued in a case where the time when the driver is not facing forward, such as when the driver is facing left and right for checking only, is a short time. That is, an unnecessary warning can be further reduced. In addition, when the driver is facing the lane change direction, the driving assistance system  100  does not notify the forward looking alert (prohibit the warning) until the time during which the driver is facing the lane direction exceeds the second predetermined time longer than the first predetermined time. In this way, when the driver is facing the lane change direction, the warning is prohibited until the second predetermined time longer than the first predetermined time is exceeded. Therefore, an unnecessary warning is reduced. Furthermore, when the driver is facing the lane change direction beyond the second predetermined time, the forward looking alert (warning) is issued. As a result, it is possible to prevent the driver from not looking forward for a long time at the time of the lane change. 
     The driving assistance system  100  includes a hands-off mode executed in a state where the driver releases the hands from the handle, and a hands-on mode executed in a state where the driver keeps the hands on the handle. Further, in the hands-off mode, after the driver is notified of the forward looking alert (warning), the hands-off mode is continued when the driver faces forward within a predetermined time. Further, when the driver faces forward after a predetermined time elapses, the travelling of autonomous driving is continued under a hands-on state. In this way, since the hands-off mode is continued when the driver faces forward within a predetermined time after the forward looking alert is notified, it is possible to reduce the notification of an unnecessary hands-on alert (a warning for prompting the driver to perform the handle operation). Therefore, an unnecessary warning can be further reduced. 
     The driving assistance system  100  operates the brake plural times to warn the driver when the driver is not facing forward even after the predetermined time after the forward looking alert (warning) is issued to the driver in the hands-off mode. In addition, after the brake is operated plural times to warn the driver, the brake is further operated to forcibly stop the vehicle when the driver does not hold the handle even after the predetermined time elapses. As a result, it is possible to prevent an accident that may occur due to the driver not looking forward. 
     Not all control flow steps described in this embodiment must be performed, and these control flow steps may be omitted as appropriate at the time of realizing the invention. The processing order can also be changed as appropriate. 
     Although the embodiment of the present disclosure has been described above, this embodiment is merely a part of application examples of the present disclosure and does not limit the technical scope of the present disclosure to a specific configuration of this embodiment.