Patent Publication Number: US-2019168610-A1

Title: Vehicular display control apparatus and vehicle driving assistance system

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of International Patent Application No. PCT/JP2017/022095 filed on Jun. 15, 2017, which designated the United States and claims the benefit of priority from Japanese Patent Application No. 2016-158319 filed on Aug. 11, 2016. The entire disclosures of all of the above application are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a vehicular display control apparatus and a vehicle driving assistance system including the same. 
     BACKGROUND 
     There is a head-up display (referred to as an HUD) that projects a display image on a projection member that transmits a front scenery in a subject vehicle to virtually display the display image visually recognizably by a user. An HUD is widely used, for example, in a vehicle driving assistance system that assists driving of a subject vehicle by a user. 
     SUMMARY 
     The present disclosure provides a vehicular display control apparatus that, in a subject vehicle equipped with a head-up display that projects a display image on a projection member that transmits a front scenery to virtually display the display image visually recognizably by a user, controls the virtual image display. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is an interior view illustrating the inside of a cabin of a subject vehicle which is equipped with a vehicle driving assistance system according to an embodiment; 
         FIG. 2  is a block diagram illustrating the vehicle driving assistance system according to the embodiment; 
         FIG. 3  is a structural diagram schematically illustrating a detailed configuration of an HUD of  FIGS. 1 and 2 ; 
         FIG. 4  is a front view illustrating a virtual image display state by the HUD of  FIGS. 1 to 3 ; 
         FIG. 5  is a front view illustrating a virtual image display state by the HUD of  FIGS. 1 to 3 ; 
         FIG. 6  is a front view illustrating a virtual image display state by the HUD of  FIGS. 1 to 3 ; 
         FIG. 7  is a front view illustrating a virtual image display state by the HUD of  FIGS. 1 to 3 ; 
         FIG. 8  is a flowchart illustrating a display control flow by an HCU of  FIG. 2 ; 
         FIG. 9  is a schematic diagram for describing the details of processes in S 101  and S 107  of  FIG. 8 ; 
         FIG. 10  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 11  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 12  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 13  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 14  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 15  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 16  is a flowchart illustrating a modification of  FIG. 8 ; 
         FIG. 17  is a flowchart illustrating a modification of  FIG. 8 ; and 
         FIG. 18  is a front view illustrating a modification of  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     Hereinbelow, an embodiment of the present disclosure will be described with reference to the drawings. 
     A vehicle driving assistance system  1  of an embodiment to which the present disclosure is applied is mounted on a subject vehicle  2  as illustrated in  FIGS. 1 and 2  and, in particular, capable of assisting driving of the subject vehicle  2  when the subject vehicle passes through a tunnel  7 . 
     As illustrated in  FIG. 2 , the vehicle driving assistance system  1  includes a surroundings monitoring system  3 , a vehicle control system  4 , and a display system  5 . The surroundings monitoring system  3 , the vehicle control system  4 , and the display system  5  are connected to each other through an in-vehicle network  6  such as a local area network (LAN). 
     The surroundings monitoring system  3  includes an external sensor  30  and a surroundings monitoring electronic control unit (ECU)  31 . The external sensor  30  detects an obstacle which is present outside the subject vehicle  2  and may collide with the subject vehicle  2 , such as a different vehicle, an artificial structure, a human, or an animal, and a traffic-related object which is present outside. The external sensor  30  is, for example, one or more of the followings: a sonar; a radar; a light detection and ranging/laser imaging detection and ranging (LIDAR); and a camera. 
     Specifically, the sonar is an ultrasonic sensor which is installed, for example, in a front part or a rear part of the subject vehicle  2 . The sonar receives a reflected wave of an ultrasonic wave transmitted to a detection area outside the subject vehicle  2  to detect an obstacle inside the detection area and outputs a detection signal. The radar is a millimeter wave sensor or a laser sensor which is installed, for example, in the front part or the rear part of the subject vehicle  2 . The radar receives a reflected wave of a millimeter wave or a sub-millimeter wave transmitted to the detection area outside the subject vehicle  2  to detect an obstacle inside the detection area and outputs a detection signal. The LIDAR is a laser scanner which is installed, for example, in a roof part of the subject vehicle  2 . The LIDAR receives a reflected wave of laser light transmitted to the detection area outside the subject vehicle  2  to detect an obstacle inside the detection area and outputs a detection signal. The camera is a monocular or compound-eye camera which is installed, for example, in a room mirror or a door mirror of the subject vehicle  2 . The camera captures an image of the detection area outside the subject vehicle  2  to detect an obstacle and a traffic-related object inside the detection area and outputs an image signal. 
     The surroundings monitoring ECU  31  is mainly composed of a microcomputer which includes a processor and a memory. The surroundings monitoring ECU  31  is connected to the external sensor  30  and the in-vehicle network  6 . The surroundings monitoring ECU  31  acquires obstacle information such as a type of an obstacle (including a vehicle type when the obstacle is a different vehicle) and a relative position of the obstacle with respect to the subject vehicle  2  on the basis of an output signal of the external sensor  30 . In addition, the surroundings monitoring ECU  31  acquires sign information such as a speed limit sign and a lane sign, traveling road information such as a white line and a road surface condition, and tunnel information relating to the tunnel  7  on the basis of an output signal of the external sensor  30 . 
     The vehicle control system  4  includes a vehicle state sensor  40 , an occupant sensor  41 , and a vehicle control ECU  42 . The vehicle state sensor  40  is connected to the in-vehicle network  6 . The vehicle state sensor  40  detects a traveling state of the subject vehicle  2 . The vehicle state sensor  40  is, for example, one or more of the followings: a wireless communication device  40   a;  a vehicle speed sensor; an engine speed sensor, a steering angle sensor; a fuel sensor; a water temperature sensor; and a global positioning system (GPS) receiver. In the present embodiment, the vehicle state sensor  40  includes at least the wireless communication device  40   a.    
     Specifically, the wireless communication device  40   a  receives an electric wave transmitted by wireless communication from one or more of communication devices outside the subject vehicle  2  such as a different-vehicle transmitter or a portable terminal for vehicle-to-vehicle communication and a road side device for road-to-vehicle communication to generate various types of information indicated by the electric wave. In particular, the wireless communication device  40   a  of the present embodiment generates tunnel internal information Ii which indicates an internal condition of the tunnel  7  and tunnel external information Io which indicates external condition of the tunnel  7  as tunnel internal and external information I which indicates internal and external conditions of the tunnel  7  through which the subject vehicle  2  passes (e.g., refer to  FIG. 8 ). As the tunnel internal information Ii, at least illuminance information, traffic information, traveling road information, and tunnel information are generated. On the other hand, as the tunnel external information Io, at least illuminance information, traffic information, traveling road information, and meteorological information are generated. 
     The illuminance information is physical information which indicates an external illuminance of the subject vehicle  2 . The traffic information is, for example, one or more of the followings: obstacle information such as following distance information between a different vehicle as an obstacle and the subject vehicle  2 ; and congestion information, disabled vehicle information, accident vehicle information, and fallen object information ahead of the subject vehicle  2 . In the present embodiment, the traffic information includes at least the obstacle information. The tunnel information is, for example, one or more of the followings: arrival distance information to an entrance  7   a  and an exit  7   b  of the tunnel  7  (e.g., refer to  FIGS. 1, 4 to 7, and 9 ); and structure information of the entrance  7   a  and the exit  7   b,  and includes at least the arrival distance information. The traveling road information is, for example, one or more of the followings: road surface information which indicates a road surface condition such as an icy condition; and shape information which indicates a road shape such as a sloping road or a curve. In the present embodiment, the traveling road information includes at least the road surface information. The meteorological information is, for example, two or more of the followings: weather information which indicates a weather condition such as fine weather, rainy weather, cloudy weather, or snowfall; external light information which indicates an external light state such as a solar radiation direction; and temperature information which indicates an outside air temperature of the subject vehicle  2 . In the present embodiment, the meteorological information includes at least the weather information and the temperature information. 
     The vehicle speed sensor detects a vehicle speed of the subject vehicle  2  and outputs a vehicle speed signal corresponding to the detection. The engine speed sensor detects an engine speed of an internal combustion engine in the subject vehicle  2  and outputs an engine speed signal corresponding to the detection. The steering angle sensor detects a steering angle of the subject vehicle  2  and outputs a steering angle signal corresponding to the detection. The fuel sensor detects a remaining amount of fuel in a fuel tank of the subject vehicle  2  and outputs a fuel signal corresponding to the detection. The water temperature sensor detects a cooling water temperature in the internal combustion engine of the subject vehicle  2  and outputs a water temperature signal corresponding to the detection. The GPS receiver receives an electric wave transmitted from a GPS satellite to acquire positional information relating to a traveling position of the subject vehicle  2 . 
     The occupant sensor  41  is connected to the in-vehicle network  6 . The occupant sensor  41  detects a state or an operation of a user inside a cabin  2   a  of the subject vehicle  2  illustrated in  FIG. 1 . The occupant sensor  41  is, for example, one or more of the followings: a power switch; a user state monitor; a display setting switch; a turn switch; and an automatic control switch. In the present embodiment, the occupant sensor  41  includes at least the power switch. 
     Specifically, the power switch is operated by the user inside the cabin  2   a  for starting the internal combustion engine or a motor generator of the subject vehicle  2  to output a power signal corresponding to the operation. The user state monitor captures an image of a user state on a driver&#39;s seat  20  inside the cabin  2   a  using an image sensor to detect the user state and outputs an image signal. The display setting switch is operated by the user for setting a display state inside the cabin  2   a  to output a display setting signal corresponding to the operation. The turn switch is operated by the user inside the cabin  2   a  for operating a blinker of the subject vehicle  2  to output a turn signal corresponding to the operation. The automatic control switch is operated by the user inside the cabin  2   a  for instructing automatic control with respect to a traveling state of the subject vehicle  2  to output an automatic control signal corresponding to the operation. 
     The vehicle control ECU  42  illustrated in  FIG. 2  is mainly composed of a microcomputer which includes a processor and a memory. The vehicle control ECU  42  is connected to the in-vehicle network  6 . The vehicle control ECU  42  is one or more of the followings: an engine control ECU; a motor control ECU; a brake control ECU; a steering control ECU; and an integrated control ECU. 
     Specifically, the engine control ECU controls the operations of a throttle actuator and a fuel injection valve of the internal combustion engine in accordance with an operation of an accelerator pedal  26  inside the cabin  2   a  illustrated in  FIG. 1  or automatically to increase or reduce the vehicle speed of the subject vehicle  2 . The motor control ECU controls the operation of the motor generator in accordance with an operation of the accelerator pedal  26  inside the cabin  2   a  or automatically to increase or reduce the vehicle speed of the subject vehicle  2 . The brake control ECU controls the operation of a brake actuator in accordance with an operation of a brake pedal  27  inside the cabin  2   a  or automatically to increase or reduce the vehicle speed of the subject vehicle  2 . The steering control ECU controls the operation of an electric power steering in accordance with an operation of a steering wheel  24  inside the cabin  2   a  or automatically to adjust the steering angle of the subject vehicle  2 . The integrated control ECU synchronously controls the operations of ECUs other than the integrated control ECU in the vehicle control ECU  42  on the basis of, for example, output signals of the sensors  40  and  41 , acquired information of the surroundings monitoring ECU  31 , and control information of the other ECUs. 
     As illustrated in  FIGS. 1 and 2 , the display system  5  is mounted on the subject vehicle  2  for visually presenting information. The display system  5  includes an HUD  50 , a multi-function display (MFD)  51 , a combination meter  52 , and a human machine interface control unit (HCU)  54 . 
     The HUD  50  is installed in an instrument panel  22  inside the cabin  2   a  illustrated in  FIGS. 1 and 3 . The HUD  50  forms a display image  56  which represents predetermined information by a projector  50   a  of a liquid crystal type or a scanning type. The HUD  50  projects the display image  56  formed in this manner on a front windshield  21  as a “projection member” in the subject vehicle  2  through an optical system  50   b  such as a concave mirror. The front windshield  21  is formed of a light-transmissive glass so as to transmit a front scenery  8  which is present ahead of the subject vehicle  2  outside the cabin  2   a.  At this time, a beam of the display image  56  reflected by the front windshield  21  and a beam from the front scenery  8  transmitted through the front windshield  21  are perceived by the user on the driver&#39;s seat  20 . As a result, a virtual image of the display image  56  which is formed at a position ahead of the front windshield  21  is displayed in a superimposed manner on a part of the front scenery  8  as illustrated in  FIG. 1 , so that the virtual image is visually recognizable by the user on the driver&#39;s seat  20 . 
     In the present embodiment, two kinds of areas A 1  and A 2  are set in a display area as a range in which the display image  56  is virtually displayed. The first display area A 1  is set for superimposing the display image  56 , which is virtually displayed, in association with a specific object in the front scenery  8 . The first display area A 1  for the association is adjusted by the HUD  50  so that the display image  56  is formed at a far position which is, for example, approximately 15 m away forward from the subject vehicle  2  in the front scenery  8 . The second display area A 2  is set for virtually displaying the display image  56  whose association with a specific object in the front scenery  8  is eliminated. The second display area A 2  for the elimination of association is adjusted to a smaller size than the first display area A 1  below the first display area A 1  by the HUD  50  so that the display image  56  is formed at a close distance position which is, for example, approximately 2 m away forward from the subject vehicle  2  in the front scenery  8 . That is, the display image  56  virtually displayed in the second display area A 2  is formed at the position that is closer to the subject vehicle  2  than the position of the display image  56  in the first display area A 1  is. 
     The projector  50   a  and the optical system  50   b  which constitute the HUD  50  may be disposed for each of the display areas A 1  and A 2  as illustrated in  FIG. 3 , or either the projector  50   a  or the optical system  50   b  may be disposed commonly to the display areas A 1  and A 2  (not illustrated). Although not illustrated, as long as at least one type of display image  56  is associated with a specific object in the front scenery  8 , a display image  56  having no such association may be virtually displayed in the first display area A 1 . Further, although not illustrated, as long as association of at least one type of display image  56  with a specific object in the front scenery  8  is eliminated, a display image  56  having such association may be virtually displayed in the second display area A 2 . 
     As the display image  56  which is virtually displayed in either the display area A 1  or the display area A 2  by the HUD  50  in this manner, multiple types of images as illustrated in  FIGS. 4 to 7  are prepared for assisting driving of the subject vehicle  2  by the user. Specifically, the multiple types of display images  56  include at least a following distance image  560 , an exit distance image  561 , a road surface image  562 , a weather image  563 , a temperature image  564 , a navigation image  565 , a vehicle speed image  566 , and an engine speed image  567 . 
     As illustrated in  FIGS. 4 to 6 , the following distance image  560  is virtually displayed in association with a different vehicle as an obstacle ahead in the first display area A 1 . The following distance image  560  represents following distance information with the nearest vehicle which travels ahead of the subject vehicle  2  among pieces of the obstacle information as the traffic information. In the following distance image  560 , a standard display state as illustrated in  FIG. 4  and a simple display state having a limited display amount compared to a display amount in the standard display state as illustrated in  FIGS. 5 and 6  are set. The following distance image  560  in the standard display state includes a numeral and a character which indicate a value and a unit of the following distance, respectively, a character which indicates contents meant by the numeral, and a figure which indicates the nearest vehicle which is a target of the numeral. On the other hand, the display amount of the following distance image  560  in the simple display state is reduced by omitting the character indicating the contents. 
     As illustrated in  FIGS. 4 to 6 , the exit distance image  561  is virtually displayed in association with the exit  7   b  of the tunnel  7  in the first display area A 1 . The exit distance image  561  represents arrival distance information to the nearest exit  7   b  to which the subject vehicle  2  heads among pieces of the tunnel information. In the exit distance image  561 , a standard display state as illustrated in  FIG. 4  and a simple display state having a limited display amount compared to a display amount in the standard display state as illustrated in  FIGS. 5 and 6  are set. The exit distance image  561  in the standard display state includes a numeral and a character which indicate a value and a unit of the arrival distance, respectively, a character which indicates contents meant by the numeral, a figure which indicates the exit  7   b  which is a target of the numeral. On the other hand, the display amount of the exit distance image  561  in the simple display state is reduced by omitting the character indicating the contents. 
     As illustrated in  FIGS. 4 and 7 , the road surface image  562  is virtually displayed in association with a traveling road ahead of the subject vehicle  2  in the first display area A 1 . The road surface image  562  represents road surface information relating to the traveling road ahead of the subject vehicle  2  among pieces of the traveling road information. In the road surface image  562 , a standard display state as illustrated in  FIG. 4  and a simple display state having a limited display amount compared to a display amount in the standard display state as illustrated in  FIG. 7  are set. The road surface image  562  in the standard display state includes a figure which schematically indicates a road surface state and a character which indicates contents meant by the figure. On the other hand, the display amount of the road surface image  562  in the simple display state is reduced by omitting the character indicating the contents. 
     As illustrated in  FIGS. 4 and 7 , the weather image  563  is virtually displayed in association with a sky area ahead and above the subject vehicle  2  in the first display area A 1 . The weather image  563  represents weather information relating to a traveling environment of the subject vehicle  2  among pieces of the meteorological information. In the weather image  563 , a standard display state as illustrated in  FIG. 4  and a simple display state having a limited display amount compared to a display amount in the standard display state as illustrated in  FIG. 7  are set. The weather image  563  in the standard display state includes a figure which schematically indicates a weather condition and a character which indicates contents meant by the figure. On the other hand, the display amount of the weather image  563  in the simple display state is reduced by omitting the character indicating the contents. 
     As illustrated in  FIGS. 4 and 7 , the temperature image  564  is virtually displayed in association with a sky area ahead and above the subject vehicle  2  in the first display area A 1 . The temperature image  564  represents temperature information relating to the traveling environment of the subject vehicle  2  among pieces of the meteorological information. In the temperature image  564 , a standard display state as illustrated in  FIG. 4  and a simple display state having a limited display amount compared to a display amount in the standard display state as illustrated in  FIG. 7  are set. The temperature image  564  in the standard display state includes a numeral and a character which indicate a value and a unit of the outside temperature, respectively, and a character which indicates contents meant by the numeral. On the other hand, the display amount of the temperature image  564  in the simple display state is reduced by omitting the character indicating the contents. 
     As illustrated in  FIG. 4 , the navigation image  565  is virtually displayed in association with the traveling road ahead of the subject vehicle  2  in the first display area A 1 . The navigation image  565  represents navigation information for guiding a traveling route of the subject vehicle  2 . The navigation image  565  includes at least a figure which indicates a planned traveling route of the subject vehicle  2  among pieces of the navigation information. The navigation information can be acquired in the HCU  54 , which is described in detail below, on the basis of map information stored in a memory  54   b  and an output signal of the vehicle state sensor  40 . 
     As illustrated in  FIG. 4 , the vehicle speed image  566  is virtually displayed at a fixed position which is not associated with a specific object in the second display area A 2 . The vehicle speed image  566  represents vehicle speed information based on an output signal of the vehicle speed sensor of the vehicle state sensor  40 . The vehicle speed image  566  includes a numeral and a character which indicate a value and a unit of the vehicle speed, respectively. 
     As illustrated in  FIG. 4 , the engine speed image  567  is virtually displayed beside the vehicle speed image  566  at a fixed position which is not associated with a specific object in the second display area A 2 . The engine speed image  567  represents engine speed information based on an output signal of the engine speed sensor of the vehicle state sensor  40 . The engine speed image  567  includes a numeral and a character which indicate a value and a unit of the engine speed, respectively. 
     As the display image  56 , for example, an image related to music information or video information provided inside the cabin  2   a  of the subject vehicle  2  may be employed in addition to the above images  560 ,  561 ,  562 ,  563 ,  564 ,  565 ,  566 , and  567 . Further, virtual image display can be achieved also by projecting the display image  56  on a light-transmissive combiner which is disposed on the instrument panel  22  to transmit the front scenery  8  in cooperation with the front windshield  21 . 
     The MFD  51  is installed in a center console  23  inside the cabin  2   a  illustrated in  FIG. 1 . The MFD  51  displays a real image of an image which is formed to represent predetermined information in one or more liquid crystal panels visually recognizably by the user on the driver&#39;s seat  20 . As the real image display by the MFD  51 , display of an image representing one or more of the followings: the navigation information; the music information; and the video information is employed. 
     The combination meter  52  is installed in the instrument panel  22  inside the cabin  2   a.  The combination meter  52  displays vehicle information relating to the subject vehicle  2  visually recognizably by the user on the driver&#39;s seat  20 . The combination meter  52  is a digital meter which displays vehicle information by an image formed on a liquid crystal panel or an analog meter which displays vehicle information by indicating a scale number by an indicator. As the display by the combination meter  52 , for example, display representing one or more of the following pieces of information: the vehicle speed; the engine speed; the fuel remaining amount; the cooling water temperature; and information of the turn switch and the automatic control switch is employed. 
     As illustrated in  FIG. 2 , the HCU  54  is mainly composed of a microcomputer which includes a processor  54   a  and the memory  54   b.  The HCU  54  is connected to the display elements  50 ,  51 , and  52  of the display system  5  and the in-vehicle network  6 . The HCU  54  synchronously controls the operations of the display elements  50 ,  51 , and  52 . At this time, the HCU  54  executes the control of these operations in accordance with, for example, acquired information or output signals of the surroundings monitoring ECU  31  and the sensors  40  and  41 , control information of the vehicle control ECU  42 , stored information of the memory  54   b,  and acquired information of the HCU  54  itself. Each of the memory  54   b  of the HCU  54  and memories of the other various ECUs includes one or more storage media such as a semiconductor memory, a magnetic medium, and an optical medium. 
     The HCU  54  of the present embodiment functions as a “vehicular display control apparatus” so as to read the display image  56  which is stored in the memory  54   b  as data and virtually display the read display image  56  on the HUD  50 . Specifically, the HCU  54  executes a display control program by the processor  54   a  to functionally implement each step of a display control flow illustrated in  FIG. 8 . The display control flow is started in response to an ON operation of the power switch as the occupant sensor  41  and ended in response to an OFF operation of the power switch. In the display control flow, “S” refers to each step. An image storage unit which stores the display image  56  which is necessary in the display control flow may be implemented by any of memories of built-in ECUs of the display elements  50 ,  51 , and  52  or cooperation of multiple memories selected from the memories of the built-in ECUs of the display elements  50 ,  51 , and  52  and the memory  54   b  of the HCU  54 . 
     In S 101  of the display control flow, it is determined whether the timing of starting the execution of S 101  is a first timing T 1  when the traveling position of the subject vehicle  2  is before the entrance  7   a  of the tunnel  7 . At this time, as illustrated in  FIG. 9 , the first timing T 1  is adjusted to a timing when the traveling position of the subject vehicle  2  reaches a position before the entrance  7   a  by a set distance L 1 . The set distance L 1  which defines the first timing T 1  is variably set so as to secure the safety of the subject vehicle  2  on the basis of one or more of the followings: the vehicle speed information of the subject vehicle  2 ; the traffic information; the tunnel information; the traveling road information; and the meteorological information. For example, the set distance L 1  is set to 100 m when the subject vehicle  2  travels on an ordinary road where a legally permitted maximum speed is a predetermined speed or set to 200 m when the subject vehicle  2  travels on an expressway where a legally permitted maximum speed is higher than that in the ordinary road. 
     As illustrated in  FIG. 8 , when a negative determination is made in S 101 , the process shifts to S 102 . Normal control of virtual image display in each of the display areas A 1  and A 2  is executed in S 102 , and the process then returns to S 101 . Specifically, in the normal control, the following distance image  560 , the exit distance image  561 , the road surface image  562 , the weather image  563 , the temperature image  564 , and the navigation image  565  are virtually displayed as the display image  56  in the first display area A 1  as illustrated in  FIG. 4 . At this time, in the present embodiment, for example, the following distance image  560  and the road surface image  562  are displayed in bright red, the weather image  563  and the temperature image  564  are displayed in bright yellow, and the navigation image  565  are displayed in bright blue. In addition, in the normal control, as illustrated in  FIG. 4 , the vehicle speed image  566  and the engine speed image  567  are virtually displayed as the display image  56  in the second display area A 2 . At this time, for example, the vehicle speed image  566  and the engine speed image  567  are displayed in white. The brightness of each of the images  560 ,  561 ,  562 ,  563 ,  564 ,  565 ,  566 , and  567  can be adjusted, for example, by RGB gradation values in the projector  50   a  or light emission luminance values of a light source in the projector  50   a.  Virtual image display of the exit distance image  561  is started when the subject vehicle  2  passes a traveling position that is away from the entrance  7   a  of the tunnel  7  by a predetermined distance that is longer than the set distance L 1  outside the tunnel  7 . 
     On the other hand, as illustrated in  FIG. 8 , when an affirmative determination is made in S 101 , the process shifts to S 103 . In S 103 , each piece of information Ii, Io as the tunnel internal and external information I is acquired from the outside through the wireless communication device  40   a . At this time, at least illuminance information, traffic information, traveling road information, and tunnel information relating to an internal condition of the tunnel  7  ahead are acquired as the tunnel internal information Ii. Further, at this time, at least illuminance information, traffic information, traveling road information, and meteorological information relating to an external condition before the tunnel  7  ahead are acquired as the tunnel external information Io. 
     In the following S 104 , provision information which is provided to the user inside the tunnel  7  is limited to the tunnel internal information Ii among pieces of the tunnel internal and external information I acquired in S 103 . In other words, a display range and a display color are limited. Specifically, the provision information to the user is limited to following distance information and arrival distance information which are represented by the following distance image  560  and the exit distance image  561 , respectively, in the first display area A 1  as the tunnel internal information Ii necessary for driving of the subject vehicle  2 . Thus, information represented by the road surface image  562 , the weather image  563 , the temperature image  564 , and the navigation image  565  in the first display area A 1  and information represented by the vehicle speed image  566  and the engine speed image  567  in the second display area A 2  are not provided to the user. 
     The timing of starting the execution of S 104  can be regarded as the first timing T 1  before the entrance  7   a  of the tunnel  7 . Thus, the provision information in the first display area A 1  is limited to the following distance information and the arrival distance information substantially from the first timing T 1  to a second timing T 2  (described below), that is, from the position before the entrance  7   a  outside the tunnel  7  to a position before the exit  7   b  inside the tunnel  7 . The limiting process described above also limits the display range of the display image  56  only to the first display area A 1  and limits the display color of the display image  56  to the display color of the following distance image  560  and the exit distance image  561  (e.g., red similar to the color during normal control). 
     In the following S 105 , the brightness of the display image  56  which represents the tunnel internal information Ii limited in S 104  is adjusted according to the tunnel internal information Ii acquired in S 103 . Specifically, the brightness of the following distance image  560  and the exit distance image  561  which represent the following distance information and the arrival distance information, respectively, as the limited tunnel internal information Ii is adjusted to a brightness according to illuminance information inside the tunnel  7  ahead among pieces of the tunnel internal information Ii. At this time, in particular, the brightness of the following distance image  560  and the exit distance image  561  is adjusted to a brightness darker than the brightness in the normal control in S 102  according to the illuminance information inside the tunnel  7  ahead. As a result, in the present embodiment, for example, the display color of the following distance image  560  and the exit distance image  561  is set to dark red. 
     The timing of starting the execution of S 105  can be also regarded as the first timing T 1  before the entrance  7   a  of the tunnel  7 . Thus, the brightness of the images  560  and  561  displayed in the first display area A 1  is adjusted according to the illuminance information substantially from the first timing T 1  to the second timing T 2  (described below), that is, from the position before the entrance  7   a  outside the tunnel  7  to the position before the exit  7   b  inside the tunnel  7 . Accordingly, in the present embodiment, the first timing T 1  corresponds to an “entrance adjustment timing”. The brightness of each of the images  560  and  561  can be adjusted by RGB gradation values in the projector  50   a  or light emission luminance values of the light source in the projector  50   a.    
     In the following S 106 , the HUD  50  virtually displays the display image  56  having the information Ii limited in S 104  and the brightness adjusted in S 105 . Specifically, both of the following distance image  560  and the exit distance image  561  having a reduced brightness compared to the brightness in the normal control in S 102  are virtually displayed in a simple display state in the first display area A 1  as illustrated in  FIGS. 5 and 6 . In other words, the display amount is limited. At this time, the virtual image display of the road surface image  562 , the weather image  563 , the temperature image  564 , and the navigation image  565  in the first display area A 1  and the virtual image display of the vehicle speed image  566  and the engine speed image  567  in the second display area A 2  disappear. 
     The timing of starting the execution of S 106  can be also regarded as the first timing T 1  before the entrance  7   a  of the tunnel  7 . Thus, the display amount by the display image  56  in the first display area A 1  is limited to a display amount by the images  560  and  561  in the simple display state substantially from the first timing T 1  to the second timing T 2  (described below), that is, from the position before the entrance  7   a  outside the tunnel  7  to the position before the exit  7   b  inside the tunnel  7 .  FIG. 5  illustrates a display state from the first timing T 1  to the timing when the subject vehicle  2  enters the entrance  7   a  of the tunnel  7  outside the tunnel  7 . On the other hand,  FIG. 6  illustrates a display state from the timing when the subject vehicle  2  enters the entrance  7   a  of the tunnel  7  to the second timing T 2  inside the tunnel  7 . According to the above description, in the present embodiment, the first timing T 1  corresponds also to an “entrance limitation timing”. 
     As illustrated in  FIG. 8 , after the execution of S 106  is finished, the process shifts to S 107 . In S 107 , it is determined whether the timing of starting the execution of S 107  is the second timing T 2  when the traveling position of the subject vehicle  2  is before the exit  7   b  of the tunnel  7 . At this time, as illustrated in  FIG. 9 , the second timing T 2  is adjusted to a timing when the traveling position of the subject vehicle  2  reaches a position before the exit  7   b  by a set distance L 2 . The set distance L 2  which defines the second timing T 2  is variably set in a manner similar to the set distance L 1 . For example, the set distance L 2  is set to 100 m when the subject vehicle  2  travels on an ordinary road where a legally permitted maximum speed is a predetermined speed or set to 200 m when the subject vehicle  2  travels on an expressway where a legally permitted maximum speed is higher than that in the ordinary road. 
     As illustrated in  FIG. 8 , when a negative determination is made in S 107 , the process returns to S 106  to continue the virtual image display of the display image  56  having the limited information and the adjusted brightness. On the other hand, when an affirmative determination is made in S 107 , the process shifts to S 108  to acquire each piece of information Ii, Io as the tunnel internal and external information I in a manner similar to S 103 . 
     In the following S 109 , provision information which is provided to the user before the exit  7   b  of the tunnel  7  is limited to the tunnel external information Io among pieces of the tunnel internal and external information I acquired in S 108 . In other words, the display range and the display color are limited. Specifically, the provision information to the user is limited to road surface information, weather information, and temperature information which are represented by the road surface image  562 , the weather image  563 , and the temperature image  564 , respectively, in the first display area A 1  as the tunnel external information Io necessary for driving of the subject vehicle  2 . Thus, information represented by the following distance image  560 , the exit distance image  561 , and the navigation image  565  in the first display area A 1  and information represented by the vehicle speed image  566  and the engine speed image  567  in the second display area A 2  are not provided to the user. 
     The timing of starting the execution of S 109  can be regarded as the second timing T 2  before the exit  7   b  of the tunnel  7 . Thus, the provision information in the first display area A 1  is limited to the road surface information, the weather information, and the temperature information substantially from the second timing T 2  to a third timing T 3  (described below), that is, from the position before the exit  7   b  inside the tunnel  7  to an exit from the exit  7   b.  The limiting process described above also limits the display range of the display image  56  only to the first display area A 1  and limits the display color of the display image  56  to the display color of the road surface information, the weather information, and the temperature information (e.g., yellow similar to the color during normal control). 
     In the following S 110 , the brightness of the display image  56  which represents the tunnel external information Io limited in S 109  is adjusted according to the tunnel external information Io acquired in S 108 . Specifically, the brightness of the road surface image  562 , the weather image  563 , and the temperature image  564  which represent the road surface information, the weather information, and the temperature information, respectively, as the limited tunnel external information Io is adjusted to a brightness according to illuminance information and meteorological information outside the exit  7   b  ahead among pieces of the tunnel external information Io. At this time, in particular, the brightness of the road surface image  562 , the weather image  563 , and the temperature image  564  is adjusted to a brightness higher than the brightness in the adjustment in S 105  according to the illuminance information and the meteorological information outside the exit  7   b  ahead. As a result, in the present embodiment, for example, the display color of the road surface image  562 , the weather image  563 , and the temperature image  564  is set to bright yellow. 
     The timing of starting the execution of S 110  can be also regarded as the second timing T 2  before the exit  7   b  of the tunnel  7 . Thus, the brightness of the images  562 ,  563 , and  564  displayed in the first display area A 1  is adjusted according to the illuminance information and the meteorological information substantially from the second timing T 2  to the third timing T 3  (described below), that is, from the position before the exit  7   b  inside the tunnel  7  to an exit from the exit  7   b.  Accordingly, in the present embodiment, the second timing T 2  corresponds to an “exit adjustment timing”. The brightness of each of the images  562 ,  563 , and  564  can be adjusted by RGB gradation values in the projector  50   a  or light emission luminance values of the light source in the projector  50   a.    
     In the following S 111 , the HUD  50  virtually displays the display image  56  having the information Io limited in S 109  and the brightness adjusted in S 110 . Specifically, all of the road surface image  562 , the weather image  563 , and the temperature image  564  having an increased brightness compared to the brightness in the adjustment in S 105  are virtually displayed in a simple display state in the first display area A 1  as illustrated in  FIG. 7 . In other words, the display amount is limited. At this time, the virtual image display of the following distance image  560 , the exit distance image  561 , and the navigation image  565  in the first display area A 1  and the virtual image display of the vehicle speed image  566  and the engine speed image  567  in the second display area A 2  disappear. 
     The timing of starting the execution of S 111  can be also regarded as the second timing T 2  before the exit  7   b  of the tunnel  7 . Thus, the display amount by the display image  56  in the first display area A 1  is limited to a display amount by the images  562 ,  563 , and  564  in the simple display state substantially from the second timing T 2  to the third timing T 3  (described below), that is, from the position before the exit  7   b  inside the tunnel  7  to an exit from the exit  7   b.  According to the above description, in the present embodiment, the second timing T 2  corresponds also to an “exit limitation timing”. 
     As illustrated in  FIG. 8 , after the execution of S 111  is finished, the process shifts to S 112 . In S 112 , it is determined whether the timing of starting the execution of S 112  is the third timing T 3  when the subject vehicle  2  makes an exit from the exit  7   b  of the tunnel  7 . As a result, the process returns to S 111  when a negative determination is made and, on the other hand, returns to S 101  when an affirmative determination is made. 
     In the HCU  54  which implements the display control flow in this manner, a function part that executes S 103  and S 108  corresponds to an “information acquisition section”, a function part that executes S 105  and S 110  corresponds to an “adjustment section”, and a function part that executes S 104 , S 106 , S 109 , and S 111  corresponds to a “limitation section”. 
     As the present embodiment described above, examples of actions and effects achieved by the HCU  54  and the vehicle driving assistance system  1  which includes the HCU  54  will be described below. 
     First, according to the present embodiment, the brightness of the display image  56  is adjusted according to the tunnel internal information Ii acquired by the HCU  54  at the first timing T 1  before the entrance  7   a  of the tunnel  7 . The tunnel internal information Ii which indicates an internal condition of the tunnel  7  is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication. Thus, the internal condition immediately before an entry of the subject vehicle  2  into the entrance  7   a  can be timely and accurately reflected in the tunnel internal information Ii. Thus, the adjustment according to the timely and accurate tunnel internal information Ii makes it possible to virtually display the display image  56  having a brightness that enables visual recognition readability to be ensured against the front scenery  8  serving as a background of the display in the subject vehicle  2  with the entry into the entrance  7   a.    
     At the first timing T 1  of the present embodiment, the brightness of the display image  56  is adjusted according to the illuminance information inside the tunnel  7  among pieces of the tunnel internal information Ii acquired by the HCU  54 . The illuminance information is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication as the tunnel internal information Ii which indicates an internal condition of the tunnel  7 . Thus, the external illuminance inside the tunnel  7  immediately before an entry into the entrance  7   a  can be timely and accurately reflected in the illuminance information. Thus, the adjustment according to the timely and accurate illuminance information makes it possible to virtually display the display image  56  having a brightness that enables visual recognition readability to be ensured against the external illuminance inside the tunnel  7  with the entry into the entrance  7   a.    
     Further, at the first timing T 1  of the present embodiment, in the first display area A 1  in which the display image  56  is associated with a specific object in the front scenery  8  and the second display area A 2  in which the association is eliminated, the brightness of the display image  56  in the first display area A 1  is adjusted. Accordingly, it is possible to preferentially adjust the brightness of the display image  56  which can be easily closely observed by the user due to the association with the specific object. Thus, the visual recognition readability against the front scenery  8  can be ensured with the entry into the entrance  7   a.    
     Next, according to the present embodiment, the brightness of the display image  56  is adjusted according to the tunnel external information Io acquired by the HCU  54  at the second timing T 2  before the exit  7   b  of the tunnel  7 . The tunnel external information Io which indicates an external condition of the tunnel  7  is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication. Thus, the external condition immediately before an exit of the subject vehicle  2  from the exit  7   b  can be timely and accurately reflected in the tunnel external information Io. Thus, the adjustment according to the timely and accurate tunnel external information Io makes it possible to virtually display the display image  56  having a brightness that enables visual recognition readability to be ensured against the front scenery  8  serving as a background of the display in the subject vehicle  2  with the exit from the exit  7   b.    
     At the second timing T 2  of the present embodiment, the brightness of the display image  56  is adjusted according to the illuminance information outside the tunnel  7  among pieces of the tunnel external information Io acquired by the HCU  54 . The illuminance information is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication as the tunnel external information Io which indicates an external condition of the tunnel  7 . Thus, the external illuminance outside the tunnel  7  immediately before an exit from the exit  7   b  can be timely and accurately reflected in the illuminance information. Thus, the adjustment according to the timely and accurate illuminance information makes it possible to virtually display the display image  56  having a brightness that enables visual recognition readability to be ensured against the external illuminance outside the tunnel  7  with the exit from the exit  7   b.    
     Further, at the second timing T 2  of the present embodiment, the brightness of the display image  56  is adjusted according to the meteorological information outside the tunnel  7  among pieces of the tunnel external information Io acquired by the HCU  54 . The meteorological information is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication as the tunnel external information Io which indicates an external condition of the tunnel  7 . Thus, a meteorological condition outside the tunnel  7  immediately before an exit from the exit  7   b  can be timely and accurately reflected in the meteorological information. Thus, the adjustment according to the timely and accurate meteorological information makes it possible to virtually display the display image  56  having a brightness that enables visual recognition readability to be ensured against the meteorological condition outside the tunnel  7  with the exit from the exit  7   b.    
     Further, also at the second timing T 2  of the present embodiment, in the first display area A 1  in which the display image  56  is associated with a specific object in the front scenery  8  and the second display area A 2  in which the association is eliminated, the brightness of the display image  56  in the first display area A 1  is adjusted. Accordingly, it is possible to preferentially adjust the brightness of the display image  56  which can be easily closely observed by the user due to the association with the specific object. Thus, the visual recognition readability against the front scenery  8  can be ensured with the exit from the exit  7   b.    
     Inside the tunnel  7  of the present embodiment, the provision information by the display image  56  is limited to the tunnel internal information Ii among pieces of the tunnel internal and external information I acquired by the HCU  54 . The tunnel internal information Ii which indicates an internal condition of the tunnel  7  is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication as the tunnel internal and external information I which indicates internal and external conditions of the tunnel  7 . Thus, the internal condition during traveling of the subject vehicle  2  can be timely and accurately reflected in the tunnel internal information Ii. Thus, the timely and accurate limitation of information type makes it possible to limit the amount of provision information to an information amount that enables visual recognition readability to be ensured against the front scenery  8  serving as a background of the display in the subject vehicle  2  and virtually display the display image  56  which represents the provision information during traveling inside the tunnel  7 . 
     Inside the tunnel  7  of the present embodiment, the provision information by the display image  56  is limited to the tunnel internal information Ii necessary for driving of the subject vehicle  2  among pieces of the tunnel internal and external information acquired by the HCU  54 . Accordingly, it is possible to provide the tunnel internal information Ii limited to an information amount necessary for driving of the subject vehicle  2  inside the tunnel  7  where it is hardly acceptable for the user to look aside. Thus, it is possible to ensure the visual recognition readability of the display image  56  against the front scenery  8  while giving the user a sense of security during traveling inside the tunnel  7 . 
     According to the present embodiment, the limitation to the tunnel internal information Ii is started from the first timing T 1  before the entrance  7   a  of the tunnel  7 . Accordingly, the provision information by the display image  56  can be reliably limited to the tunnel internal information Ii from an entry of the subject vehicle  2  into the entrance  7   a  inside the tunnel  7 . Thus, it is possible to ensure the visual recognition readability against the front scenery  8 . 
     Further, not only the provision information by the display image  56  is limited to the tunnel internal information Ii, but also the display color of the display image  56  which represents the tunnel internal information Ii is limited inside the tunnel  7  of the present embodiment. According to the information limitation and the display color limitation, the visual recognition readability which is necessary during traveling inside the tunnel  7  where it is hardly acceptable for the user to look aside can be reliably ensured against the front scenery  8 . 
     In addition, inside the tunnel  7  of the present embodiment, not only the provision information by the display image  56  is limited to the tunnel internal information Ii, but also the display range of the display image  56  which represents the tunnel internal information Ii is limited. According to the information limitation and the display range limitation, the visual recognition readability which is necessary during traveling inside the tunnel  7  where it is hardly acceptable for the user to look aside can be reliably ensured against the front scenery  8 . 
     In addition, inside the tunnel  7  of the present embodiment, not only the provision information by the display image  56  is limited to the tunnel internal information Ii, but also the display amount of the display image  56  which represents the tunnel internal information Ii is limited. According to the information limitation and the display amount limitation, the visual recognition readability which is necessary during traveling inside the tunnel  7  where it is hardly acceptable for the user to look aside can be reliably ensured against the front scenery  8 . 
     In addition, inside the tunnel  7  of the present embodiment, in the first display area A 1  in which the display image  56  is associated with a specific object in the front scenery  8  and the second display area A 2  in which the association is eliminated, the provision information in the first display area A 1  is limited to the tunnel internal information Ii. Accordingly, it is possible to preferentially reduce the amount of provision information by the display image  56  which can be easily closely observed by the user due to the association with the specific object. Thus, the visual recognition readability against the front scenery  8  can be ensured during traveling inside the tunnel  7 . 
     In particular, according to the present embodiment, the provision information by the display image  56  is limited to the tunnel external information Io instead of the tunnel internal information Ii among pieces of the tunnel internal and external information I acquired by the HCU  54  at the second timing T 2  before the exit  7   b  of the tunnel  7 . The tunnel external information Io which indicates an external condition of the tunnel  7  is transmitted from the outside of the subject vehicle  2  and acquired by wireless communication as the tunnel internal and external information I which indicates internal and external conditions of the tunnel  7 . Thus, the external condition immediately before an exit from the exit  7   b  can be timely and accurately reflected in the tunnel external information Io. Thus, the timely and accurate limitation of information type makes it possible to limit the amount of provision information to an information amount that enables visual recognition readability to be ensured against the front scenery  8  serving as a background of the display in the subject vehicle  2  and virtually display the display image  56  which represents the provision information with the exit from the exit  7   b.    
     At the second timing T 2  of the present embodiment, the provision information by the display image  56  is limited to the tunnel external information Io necessary for driving of the subject vehicle  2  among pieces of the tunnel internal and external information I acquired by the HCU  54 . Accordingly, it is possible to provide the tunnel external information Io limited to an information amount necessary for driving of the subject vehicle  2  immediately before an exit from the exit  7   b  where it is hardly acceptable for the user to look aside. Thus, it is possible to ensure the visual recognition readability of the display image  56  against the front scenery  8  while giving the user a sense of security with the exit from the exit  7   b.    
     Further, at the second timing T 2  of the present embodiment, the display color, the display range, and the display amount of the display image  56  which represents the tunnel external information Io are limited in a manner similar to the processes inside the tunnel  7  described above. Thus, the visual recognition readability which is necessary in an exit from the exit  7   b  where it is hardly acceptable for the user to look aside can be reliably ensured against the front scenery  8 . 
     Further, at the second timing T 2  of the present embodiment, in the first display area A 1  in which the display image  56  is associated with a specific object in the front scenery  8  and the second display area A 2  in which the association is eliminated, the provision information in the first display area A 1  is limited to tunnel external information Io. Accordingly, it is possible to preferentially reduce the amount of provision information by the display image  56  which can be easily closely observed by the user due to the association with the specific object. Thus, the visual recognition readability against the front scenery  8  can be ensured with an exit from the exit  7   b.    
     As described above, according to the vehicle driving assistance system  1  which includes the HCU  54  of the present embodiment, it is possible to ensure the visual recognition readability of the display image  56 , which is virtually displayed, against the front scenery  8  inside and outside the tunnel  7 , the front scenery  8  serving as a background of the display. 
     Other Embodiments 
     While the embodiment of the present disclosure has been described above, the present disclosure is not limited to the described embodiment. The present disclosure may be applied to various embodiments within the gist of the present disclosure. 
     Specifically, as a first modification, as illustrated in  FIG. 10 , virtual image display similar to the virtual image display in the normal control in S 102  may be executed except that the brightness of each of the images  560 ,  561 ,  562 ,  563 ,  564 ,  565 ,  566 , and  567  is adjusted in a manner similar to S 105  by sequentially shifting to S 113  and S 114  from S 103 . In this case, in S 115  following S 114 , it is determined whether the timing of starting the execution of S 115  is a fourth timing T 4  when the subject vehicle  2  enters the entrance  7   a  of the tunnel  7 . As a result, the process returns to S 114  when a negative determination is made and, on the other hand, shifts to S 104  when an affirmative determination is made. 
     As a second modification, as illustrated in  FIG. 11 , limitation and virtual image display similar to S 104  and S 106  may be executed with a brightness similar to the brightness in the normal control in S 102  by sequentially shifting to S 116  and S 117  from S 103 . In this case, in S 118  following S 117 , it is determined whether the timing of starting the execution of S 118  is the fourth timing T 4  when the subject vehicle  2  enters the entrance  7   a  of the tunnel  7 . As a result, the process returns to S 117  when a negative determination is made and, on the other hand, shifts to S 104  when an affirmative determination is made. S 116  corresponds to limiting the display range and the display color. S 117  corresponds to limiting the display amount. 
     As a third modification, as illustrated in  FIG. 12 , S 119  may be executed instead of S 101  to determine whether the timing of starting the execution of S 119  is the fourth timing T 4  when the subject vehicle  2  enters the entrance  7   a  of the tunnel  7 . As a result, the process shifts to S 102  when a negative determination is made and, on the other hand, shifts to S 103  when an affirmative determination is made. 
     As a fourth modification, as illustrated in  FIG. 13 , virtual image display similar to the virtual image display in the normal control in S 102  may be executed except that the brightness of each of the images  560 ,  561 ,  562 ,  563 ,  564 ,  565 ,  566 , and  567  is adjusted in a manner similar to S 110  by sequentially shifting to S 120  and S 121  from S 108 . In this case, in S 122  following S 121 , it is determined whether the timing of starting the execution of S 122  is the third timing T 3  when the subject vehicle  2  makes an exit from the exit  7   b  of the tunnel  7 . As a result, the process returns to S 121  when a negative determination is made and, on the other hand, returns to S 101  when an affirmative determination is made. 
     As a fifth modification, as illustrated in  FIG. 14 , limitation and virtual image display similar to S 109  and S 111  may be executed with a brightness similar to the brightness in the adjustment in S 105  by sequentially shifting to S 123  and S 124  from S 108 . In this case, in S 125  following S 124 , it is determined whether the timing of starting the execution of S 125  is the third timing T 3  when the subject vehicle  2  makes an exit from the exit  7   b  of the tunnel  7 . As a result, the process returns to S 124  when a negative determination is made and, on the other hand, returns to S 101  when an affirmative determination is made. In other words, S 123  corresponds to limiting the display range and the display color, and S 124  corresponds to limiting the display amount. 
     As a sixth modification, as illustrated in  FIG. 15 , S 126  may be executed instead of S 107  to determine whether the timing of starting the execution of S 126  is the third timing T 3  when the subject vehicle  2  makes an exit from the exit  7   b  of the tunnel  7 . As a result, the process returns to S 106  when a negative determination is made and, on the other hand, shifts to S 101  when an affirmative determination is made. 
     As a seventh modification, any one of the first to third modifications may be combined with any one of the fourth to sixth modifications.  FIG. 16  illustrates a seventh modification that combines the first modification with the fourth modification.  FIG. 17  illustrates a seventh modification that combines the second modification with the fifth modification. As an eighth modification, as illustrated in  FIG. 18 , in the images  560 ,  561 , and  564  in a simple display state, a character which indicates a unit in a standard display state may be omitted in addition to or instead of a character which indicates the contents in the standard display state. 
     As a ninth modification, the brightness adjustment in S 105  may be executed according to, for example, traveling road information (specifically, a road surface condition) or external light information (specifically, a solar radiation direction) in addition to or instead of illuminance information. As a tenth modification, the brightness adjustment in S 110  may be executed according to, for example, traveling road information (specifically, a road surface condition) or external light information (specifically, a solar radiation direction) in addition to or instead of at least either illuminance information or meteorological information. 
     Information other than following distance information and arrival distance information, for example, congestion information may be employed as provision information limited in S 104  of an eleventh modification as long as the congestion information is tunnel internal information Ii necessary for driving of the subject vehicle  2 . Information other than the road surface information, weather information, and temperature information, for example, following distance information or congestion information may be employed as provision information limited in S 109  of a twelfth modification as long as the following distance information or the congestion information is tunnel external information Io necessary for driving of the subject vehicle  2 . 
     Information (e.g., a vehicle type as obstacle information) unnecessary for driving of the subject vehicle  2  may be employed as provision information of a thirteenth modification in addition to or instead of information necessary for the driving as long as the provision information is limited to tunnel internal information Ii in S 104 . Information (e.g., a vehicle type as obstacle information) unnecessary for driving of the subject vehicle  2  may be employed as provision information of a fourteenth modification in addition to or instead of information necessary for the driving as long as the provision information is limited to tunnel external information Io in S 109 . In these thirteenth and fourteenth modifications, the display image  56  which represents the employed provision information is virtually displayed in S 106  or S 111 . 
     As a fifteenth modification, provision information in each of the display areas A 1  and A 2  during normal control may not be limited in S 104 , but may all be employed so that, of the display color and the display range of the display image  56 , at least the latter is not limited. As a sixteenth modification, provision information in each of the display areas A 1  and A 2  during normal control may not be limited in S 109 , but may all be employed so that, of the display color and the display range of the display image  56 , at least the latter is not limited. In these fifteenth and sixteenth modifications, the display image  56  which represents the employed provision information is virtually displayed in S 106  or S 111 . 
     As a seventeenth modification, not a simple display state, but a standard display state may be employed in S 106  so as not to limit the display amount of the display image  56 . As an eighteenth modification, not a simple display state, but a standard display state may be employed in S 111  so as not to limit the display amount of the display image  56 . 
     As a nineteenth modification, at least one type of provision information in the second display area A 2  during normal control may not be limited, but may be employed in S 104 , and the brightness of the provision information in the second display area A 2  may be adjusted in S 105  in a manner similar to provision information in the first display area A 1 . As a twentieth modification, at least one type of provision information in the second display area A 2  during normal control may not be limited, but may be employed in S 109 , and the brightness of the provision information in the second display area A 2  may be adjusted in S 110  in a manner similar to provision information in the first display area A 1 . In these nineteenth and twentieth modifications, the display image  56  which represents the employed provision information is virtually displayed in S 106  or S 111 . 
     As a twenty-first modification, virtual image display of the display image  56  in the second display area A 2  may not be employed. As a twenty-second modification, the display image  56  having information and brightness limited and adjusted by the display control flow may be virtually displayed in the second display area A 2  instead of the first display area A 1 . As a twenty-third modification, virtual image display of the display image  56  in the first display area A 1  may not be employed further in the twenty-second modification. 
     As a twenty-fourth modification, at least some of the steps of the display control flow may be implemented as hardware by one or more ICs instead of being functionally implemented by the HCU  54 . As a twenty-fifth modification, in addition to or instead of the HCU  54 , one or more of the followings: the surroundings monitoring ECU  31 ; and the display control ECUs of the display elements  50 ,  51 , and  52  may function as the “vehicular display control apparatus”. 
     Furthermore, for reference, a related technique may be described as follows. 
     A related art discloses a vehicular display control technique that changes the brightness of a display image on the basis of tunnel information before passage through an entrance or an exit of a tunnel to control virtual image display by an HUD. Positional information and shape information relating to the entrance or the exit of the tunnel are employed as the tunnel information. 
     However, in the technique disclosed in a related art, the tunnel information is mere map data which is stored in a navigation apparatus or a server apparatus which is communicable with the subject vehicle. Thus, it is assumed that the tunnel information does not timely and accurately reflect internal and external conditions of the tunnel. A display image which is virtually displayed on the basis of such tunnel information which is neither timely nor accurate may cause reduction in readability by visual recognition (referred to as visual recognition readability) against a front scenery serving as a background of the display in the subject vehicle. 
     Aspects of the disclosure described herein are set forth in the following clauses. 
     One example of the present disclosure may provide a vehicular display control apparatus and a vehicle driving assistance system that ensure the visual recognition readability of a virtually displayed display image against a front scenery inside and outside a tunnel, the front scenery serving as a background of the display in a subject vehicle. 
     One aspect of the present disclosure provides a vehicular display control apparatus that, in a subject vehicle equipped with a head-up display that projects a display image on a projection member that transmits a front scenery to virtually display the display image visually recognizably by a user, controls the virtual image display. The vehicular display control apparatus includes an information acquisition section that acquires tunnel internal information that is transmitted from outside of the subject vehicle by wireless communication and indicates an internal condition of a tunnel, and an adjustment section that adjusts a brightness of the display image according to the tunnel internal information acquired by the information acquisition section at an entrance adjustment timing before an entrance of the tunnel. 
     According to the one aspect, the brightness of the display image is adjusted according to the tunnel internal information acquired by the information acquisition section at the entrance adjustment timing before the entrance of the tunnel. The tunnel internal information which indicates an internal condition of the tunnel is transmitted from the outside of the subject vehicle and acquired by wireless communication. Thus, the internal condition immediately before an entry of the subject vehicle into the entrance of the tunnel can be timely and accurately reflected in the tunnel internal information. Thus, the adjustment according to the timely and accurate tunnel internal information makes it possible to virtually display the display image having a brightness that enables visual recognition readability to be ensured against the front scenery serving as a background of the display in the subject vehicle with the entry into the entrance of the tunnel. 
     Another aspect of the present disclosure provides a vehicular display control apparatus that, in a subject vehicle equipped with a head-up display that projects a display image on a projection member that transmits a front scenery to virtually display the display image visually recognizably by a user, controls the virtual image display. The vehicular display control apparatus includes an information acquisition section that acquires tunnel external information that is transmitted from outside of the subject vehicle by wireless communication and indicates an external condition of a tunnel, and an adjustment section that adjusts a brightness of the display image according to the tunnel external information acquired by the information acquisition section at an exit adjustment timing before an exit of the tunnel. 
     According to this aspect, the brightness of the display image is adjusted according to the tunnel external information acquired by the information acquisition section at the exit adjustment timing before the exit of the tunnel. The tunnel external information which indicates an external condition of the tunnel is transmitted from the outside of the subject vehicle and acquired by wireless communication. Thus, the external condition immediately before an exit of the subject vehicle from the exit of the tunnel can be timely and accurately reflected in the tunnel external information. Thus, the adjustment according to the timely and accurate tunnel external information makes it possible to virtually display the display image having a brightness that enables visual recognition readability to be ensured against the front scenery serving as a background of the display in the subject vehicle with the exit of the subject vehicle from the exit of the tunnel. 
     Another aspect of the present disclosure provides a vehicular display control apparatus that, in a subject vehicle equipped with a head-up display that projects a display image on a projection member that transmits a front scenery to virtually display the display image visually recognizably by a user, controls the virtual image display. The vehicular display control apparatus includes an information acquisition section that acquires tunnel internal and external information that is transmitted from outside of the subject vehicle by wireless communication and indicates internal and external conditions of a tunnel, and a limitation section that limits provision information provided to the user by the display image to tunnel internal information that indicates an internal condition of the tunnel among pieces of the tunnel internal and external information acquired by the information acquisition section inside the tunnel. 
     According to this aspect, inside the tunnel, the provision information by the display image is limited to the tunnel internal information among pieces of the tunnel internal and external information acquired by the information acquisition section. The tunnel internal information which indicates an internal condition of the tunnel is transmitted from the outside of the subject vehicle and acquired by wireless communication as the tunnel internal and external information which indicates internal and external conditions of the tunnel. Thus, the internal condition during traveling of the subject vehicle can be timely and accurately reflected in the tunnel internal information. Thus, the timely and accurate limitation of information type makes it possible to limit the amount of provision information to an information amount that enables visual recognition readability to be ensured against the front scenery serving as a background of the display in the subject vehicle and virtually display the display image which represents the provision information during traveling inside the tunnel. 
     Another aspect of the present disclosure provides a vehicle driving assistance system that assists driving of a subject vehicle. The vehicle driving assistance system includes the vehicular display control apparatus according to any one of the first to third disclosures, and the HUD. 
     The visual recognition readability of the display image, which is virtually displayed, can be ensured against the front scenery inside and outside the tunnel, the front scenery serving as a background of the display in the subject vehicle, by the vehicular display control apparatus according to any of the first to third disclosures. 
     It is noted that a flowchart or the processing of the flowchart in the present application includes multiple steps (also referred to as sections), each of which is represented, for instance, as S 101 . Further, each step can be divided into several sub-steps while several steps can be combined into a single step. 
     Control units, controllers and methods described in the present disclosure may be implemented by a special purpose computer created by configuring a memory and a processor programmed to execute one or more particular functions embodied in computer programs. Alternatively, the control units, the controllers and the methods described in the present disclosure may be implemented by a special purpose computer created by configuring a processor provided by one or more special purpose hardware logic circuits. Further alternatively, the control units, the controllers and the methods described in the present disclosure may be implemented by one or more special purpose computers created by configuring a combination of a memory and a processor programmed to execute one or more particular functions and a processor provided by one or more hardware logic circuits. The computer programs may be stored, as instructions being executed by a computer, in a tangible non-transitory computer-readable medium. 
     While various embodiments, configurations, and aspects of a vehicular display control apparatus and a vehicle driving assistance system including the same according to the present disclosure have been exemplified, the embodiments, configurations, and aspects of the present disclosure are not limited to those described above. For example, embodiments, configurations, and aspects obtained from an appropriate combination of technical elements disclosed in different embodiments, configurations, and aspects are also included within the scope of the embodiments, configurations, and aspects of the present disclosure.