Abstract:
A field watch apparatus for use in a subject vehicle acquires a road image from a roadside capture unit located around the subject vehicle on a road and, based on the acquired image, generates an overhead view of a field around the subject vehicle in an intuitively recognizable manner by including a distant surrounding vehicle in the view, thereby providing a precisely understandable view of the field around the subject vehicle for the ease of satisfactorily identifying the distant surrounding vehicle without being compromised by the position of the roadside capture unit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority of Japanese Patent Application No. 2007-26951 filed on Feb. 6, 2007, the disclosure of which is incorporated herein by reference. 
     FIELD OF THE DISCLOSURE 
     The present disclosure generally relates to a field watch/observation apparatus for use in a vehicle. 
     BACKGROUND INFORMATION 
     In recent years, various techniques for watching a field around an automobile are disclosed. For example, an apparatus described in a republished patent document WO00/64175 (U.S. Pat. No. 7,161,616) has plural cameras on a subject vehicle that capture a field image around the subject vehicle in various angles, compose those images by shifting viewpoints for generating an overhead view of the subject vehicle, and displays the overhead view on the apparatus. Further, Japanese patent documents such as JP-A-2003-312415, JP-A-2005-5978, JP-A-2002-232948, JP-A-2006-107521, JP-A-2006-282111 disclose a field watch apparatus for monitoring surrounding vehicles condition around a traveling subject vehicle in the overhead view by applying a technique disclosed in the first patent document. 
     The field watch apparatuses disclosed in the second to fifth patent documents in the above description use only on-board image capture devices disposed on the subject vehicle for generating the overhead view, thereby making it difficult for a distant vehicle to be precisely displayed on a screen, due to smallness of a vehicle in the captured image. In particular, a vehicle approaching to the subject vehicle at a high speed may not be identified as a warning object until a remaining time to have a close contact is too small, due to difficulty of finding the approaching vehicle in the overhead view. 
     SUMMARY 
     In view of the above and other problems, the present disclosure provides a field watch apparatus that displays a distant vehicle in an intuitively recognizable manner, thereby providing field watch information regarding a subject vehicle precisely in a form of larger aerial picture. 
     The field watch apparatus of the present invention includes a road image acquisition unit capable of acquiring a road image that includes another vehicle around the subject vehicle from a roadside capture unit disposed in surroundings of the subject vehicle while the subject vehicle is traveling on a road; an overhead view generation unit capable of generating an overhead view of the subject vehicle with a viewpoint from above the subject vehicle; and a display unit disposed in the subject vehicle and capable of displaying the overhead view of the subject vehicle. The overhead view of the subject vehicle is generated as a view of a real image of a nearby area of the subject vehicle by composing the plural road images respectively captured in different perspectives of the plural roadside capture units with viewpoint shifting, and a position of the subject vehicle is represented in an identifiable manner in the overhead view. 
     The field watch apparatus of the present invention acquires road images including other vehicles around the subject vehicle from the roadside capture units disposed (in a detached manner relative to the subject vehicle) in a proximity of the subject vehicle on a currently traveling road, and generates the overhead view of a subject vehicle&#39;s vicinity. Therefore, the position of the roadside capture unit is not limited relative to the position of the subject vehicle, thereby enabling an acquisition of the road image that leads to generation of the overhead view being sufficiently precise for an identification of the another vehicle even when the another vehicle is distant from the subject vehicle. Further, freedom of perspective setting of the roadside capture unit is improved, thereby leading to the availability of the road image in an otherwise difficult perspective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings, in which: 
         FIG. 1  shows a block diagram of a field watch apparatus in an embodiment of the present invention; 
         FIG. 2  shows a perspective view of an arrangement of in-vehicle cameras with their fields of vision; 
         FIGS. 3A and 3B  show illustrations of other car detection by using a radar; 
         FIG. 4  shows an illustration of an arrangement of cameras on a road; 
         FIG. 5  shows a flowchart of a main process in the field watch apparatus; 
         FIG. 6  shows a flowchart of an overhead view generation process; 
         FIG. 7  shows a flowchart of an image composition process; 
         FIG. 8  shows another flowchart of the image composition process; 
         FIG. 9  shows a flowchart of a risk estimation process; 
         FIG. 10  shows a flowchart of a warning content determination process; 
         FIG. 11  shows an illustration of a screen of overhead view of a subject vehicle; 
         FIG. 12  shows an illustration of warnings of other cars; 
         FIG. 13  shows a diagram of composite warnings according to driver&#39;s conditions; and 
         FIG. 14  shows another illustration of a screen of overhead view of the subject vehicle. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram to show an example of the electric constitution of a vehicular field watch apparatus of the present exemplary embodiment. The vehicular field watch apparatus  1  mainly consists of an ECU that is in charge of total control, with an image ECU  50 , a risk estimate ECU  70  and a warning output control ECU  90  connected through a network in the present embodiment. Each of the ECU  50 ,  70 ,  90  is constituted of a CPU, a ROM that stores software being executed by the CPU, a RAM as a work memory, and input and output (I/O) portions respectively connected by a bus. That is, those ECUs are microcomputers of well-known types. 
     An in-vehicle camera group  11  is connected to the image ECU  50 . In addition, a road-vehicle-communication unit  40  including a radio transmitter-receiver is also connected to the ECU  50  through a communication interface (not illustrated). Furthermore, to the image ECU  50 , a GPS  4  for detecting a self car position as well as a radar  3  are respectively connected. In addition, in the image ECU  50 , another car information mapping unit  52  and a bird&#39;s-eye view generator  51  are provided as functional software implemented therein. Image data and position data from the road-vehicle-communication unit  40  and image data from the in-vehicle camera group  11  are transferred to the bird&#39;s-eye view generator  51  for generating a self car circumference overlook image  30  as shown in  FIG. 11 . In addition, other car detection data from the radar  3  and self car position data from the GPS  4  are transferred to the other car information mapping unit  52 . In addition, vehicle speed data from a vehicle speed sensor  76  is transferred to the other car information mapping unit  52 . 
     Further, a room camera  73  in a vehicle compartment photographing the face of the driver and a biometric sensor group  74  (e.g., a thermography imager, a temperature sensor, a sweat sensor, a skin resistance sensor, a heartbeat sensor and the like) acquiring various kinds of biometric information of the driver are connected to the risk estimate ECU  70 . In addition, an inter-vehicle communications unit  75  for acquiring image data, position data and the like from other cars by radio transmission is also connected to the ECU  70  through a communication interface. In addition, in the risk estimate ECU  70 , a closeness determination engine  72  and a risk assess engine  71  are provided as functional software implemented therein. These engines are used for considering both of distance factors and driver&#39;s conditions, that is, for example, providing risk estimation scheme of changing a threshold distance from other cars for risk assessment based on driver&#39;s conditions such as fatigue, drowsiness and the like. 
     The distance of the other car as well as a traveling direction and speed information derived from the radar  3  or the inter-vehicle communications unit  75  are transferred to the closeness determination engine  72  together with the speed information of the self car from the speed sensor  76 . In addition, the image data from the room camera  73  in the compartment and the driver&#39;s body state detection information from the biometric sensor group  74  are transferred to the risk assess engine  71 . 
     A warning output control ECU  90  has an image driver  92 , a sound driver  93  and a vibration driver  94  respectively connected thereto. In addition, the warning contents generator  91  is provided as functional software implemented in the ECU  90 . The self car circumference overlook image  30  from the image ECU  50  is transferred to the warning contents generator  91 . In addition, from the risk assess engine  71  in the ECU  70 , the degree of risk assessment result for an approach of other cars is also transferred to the warning contents generator  91 . The warning contents generator  91  processes, by referring to the risk assessment result, marking of the other car image for warning, and generates audio output data as well as control data of the vibration output to be respectively transmitted to the image driver  92 , the sound driver  93  and the vibration driver  94 . 
     The self car circumference overlook image  30  with a warning marking is output to the monitor  95  connected to the image driver  92 . The monitor  95  may be substituted by a display unit of a car navigation system  200  or an AV unit (at the center of an instrument panel or a center console of the automobile) as shown in  FIG. 11 , or may be built into a driving indicator  300 / 301  as shown in  FIG. 14 . In addition, as shown in  FIG. 11 , a back monitor image  212 ′ that combines images from the camera group  11  without viewpoint shift is output to the same monitor screen. 
     In addition, the warning sound by the voice output data is output from a speaker  96  (the speaker system of the AV unit can be used for substitution) connected to the sound driver  93 . In addition, the vibration driver  94  that has received control data of vibration output drives a vibrator  97  connected thereto. For example, the vibrator  97  may be incorporated in a steering wheel  100  or a seat  110  (a seat back portion or a sitting surface), for promoting warning recognition or awakening by transmitting warning vibration to the vehicle driver directly. 
     The in-vehicle camera group  11  constitutes a self car side and circumference photography device that photographs around a self car  12 , and the image from the camera group  11  is a field and road image in an immediate proximity of the self car  12  derived from plural cameras that respectively capture images in field of visions that continuously surround the self vehicle  12  in combination. The illustration in  FIG. 2  shows an example of arrangement of the camera group  11 . In this case, eight cameras  11   a - 11   h  are disposed on the body of the self car  12 . The cameras in the camera group  11  are, a front right camera  11   a , a front left camera  11   b , a rear right camera  11   c , a rear left camera  11   d , a right forward camera  11   e , a right backward camera  11   f , a left forward camera  11   g , a left backward camera  11   h  respectively. 
     For example, by the right forward camera  11   e , an image of a right front side of the self car  12  is captured as a field of vision Va, thus an image including a car  13   a  that is running in a right front of the self car  12  is captured together with a part of the body of the self car  12  included in the image. In addition, by the right backward camera  11   f , an image of the right rear side of the self car  12  is captured as a field of vision Vb, thereby capturing a car  13   b  that is running in a right side behind the self car  12  in the image, together with the part of the body of the self car  12  included therein. Further, a car  13   c  in front of the self car  12  as well as a car  13   d  can be photographed as an image by using the rest of the cameras. The image of the field and road from the cameras  11   a - 11   h  is converted three-dimensionally to serve as an overhead view that has a virtual viewpoint above the self car  12  (e.g., right above the center of the self car  12 ), with an integrated composition of overlaps of adjacent images in consideration, for providing the self car circumference overlook image  30  that continuously represents the area around the self car  12 . The viewpoint shift and integration method are disclosed in detail in, for example, the patent document JP-A-2003-312415 and the like. Therefore, description of the viewpoint shift and the integration method is omitted. In addition, the number of the cameras in the camera group  11  may be smaller than the one in the above description. For example, the number of cameras may be decreased to 4 to be disposed at a front, rear, right and left of the self vehicle. 
     The radar  3  of  FIG. 1  is constituted as an ACC radar measuring the distance from the front car and/or the speed of the front car with a laser and/or a millimeter wave, and is disposed in combination with the cameras  11   a - 11   d  mentioned above for distance/speed measurement of measured object in a front right side, front left side, rear right side, and rear left side of the self car  12 . In addition, the inter-vehicle communications unit  75  directly communicates with the other cars around the self car  12 , and exchanges other car information of the other cars (e.g., a vehicle size, speed, brakes, acceleration, position coordinates, a car model, a model number). 
     In FIGS.  3 A/ 3 B, a method to calculate the position (distance/direction measurement) of the cars (cars A, B, C, D, E, F) in the surroundings (i.e., a circumference vehicle hereinafter) is explained in the following. At first, in the measurement of the distance to the circumference vehicle, transmission electricity power from the inter-vehicle communications unit  75  of the self car  12  is changed, regularly for roughly detecting the distance based on a detection of non-transmission electricity threshold that defines the level of non-transmission of the inter-vehicle communication. In this case, the inter-vehicle communications unit  75  disposed at the center of the backward lamps, for example, makes it easier to accurately measure the inter-vehicle distance based on the communication with the front vehicle, due to the ease of detection result matching between the detection result of the inter-vehicle communication and the detection result of the ACC radar  3 . When the ACC radar  3  is compared with the communications unit  75 , a measurement range V of the ACC radar  3  is longer than a communication range P of the communications unit  75 , thereby first detecting and determining the distance/direction of the inter-vehicle communication by the radar  3  as a preparation for the actual inter-vehicle communication by the communications unit  75 . In this manner, the transmission electricity power and the communication direction (Q) of the inter-vehicle communications unit  75  can be set. (“Pd” represents a pedestrian on a sidewalk.) 
     Plural cameras  21  are, as shown in  FIG. 4 , disposed at a predetermined interval as an infrastructure of photographing on a road  20  that is being traveled by the self car  12 . The road  20  in  FIG. 4  has plural lanes  20   a ,  20   b  in one traffic direction and lanes  20   c ,  20   d  in the other direction, with the cameras  21  for capturing the road and field image for the respective lanes. The cameras  21  may be disposed on a fence, a railing or the like for capturing the vehicle image from a front-sideward of the vehicles, or may be disposed on a post for capturing the vehicle image from above the vehicles. Further, cameras  21  have road side communications units  321  respectively disposed thereon for transmitting the road and field image from the cameras  21  together with the position of photographing to the road-vehicle-communication unit  40  on the vehicle. 
     The cameras  21  are used for capturing the road and field image including the cars around the self car  12 , or more practically, a far front and/or a far rear field of the self car  12  relative to the field of vision by the camera group  11 . The position of the self car  12  is detected by the above-mentioned GPS in a timely manner, and data from the cameras  21  is selectively received by identifying the cameras  21  within a predetermined frontward/backward range through the road-vehicle-communication unit  40 . The received data of road and field image undergoes viewpoint shift processing by the bird&#39;s-eye view generator  51 , thereby being combined to generate the self car circumference overlook image  30  as shown in  FIG. 11 . That is, more practically, the image  30  in  FIG. 11  is composed by supplementing an image  251  from the camera group  11  with an image  252  from the camera  21  at a far front or a far rear area at a predetermined distance from the self car  12 . More practically, the self car circumference overlook image  30  is generated by including an image  213  of another car traveling behind the self car  12  by at least the length of the self car  12  distant from the self car  12  in the self car traveling lane  20   b  or in the adjacent lane  20   a.    
     In addition, as shown in  FIG. 4 , an in-vehicle camera  11 ′ disposed on the other car  13  (including  13 F,  13 B) that is traveling around the self car  12  is configured in the same manner as the camera group  11 , and the image from the camera  11 ′ is available to be transmitted through the communications unit  75  for composing the overlook image  30 . 
     Operation of the vehicular field watch apparatus  1  is described based on a flowchart as follows. That is, in  FIG. 5 , a flow of main processing that is performed by the cooperation of three ECU&#39;s  50 ,  70 ,  90  is shown. First, step S 1  is a step for generating of the self car circumference overlook image by the image ECU  50 , and step S 2  is a step for risk estimate processing (“risk assessment”) by the risk estimate ECU  70  carried out in parallel. Then, steps S 3  and S 4  are steps for warning contents determination processing by the warning output control ECU  90  and warning output processing respectively performed in accordance with the other car existence situation in the self car circumference overlook image  30  and a risk estimate result of step S 2 . The main processing is carried out repeatedly at a predetermined time interval while updating a processing result. In addition, it is possible to carry out the entire main processing integrally in one large scale ECU if the capacity of the ECU is sufficient. 
       FIG. 6  shows the details of a generation process of the self car circumference overlook images (S 1 ). At first, by processing by the other car information mapping unit  52  ( FIG. 1 ), the detection information of the other car by the radar  3  is acquired in step S 11  before acquiring the road and filed image, and the information of the existence direction of the other car  13  relative to the self car  12  as well as the distance information regarding the self car  12  and the other car  13  are acquired based on the analysis of the detection information in step S 12 . Then, in step S 13 , based on the distance information and the existence direction information mentioned above, mapping information of the other car  13  in the surroundings of the self car  12  is generated. More practically, the position of the other car is mapped on a polar coordinate plane that is defined in parallel to the road surface with the position of the self car  12  as its origin and a radius and a displacement angle of the other car respectively derived from the distance information and the existence direction information. 
     Then, in step S 14 , the viewpoint shift processing of well known type is performed after acquiring the images from the camera group  11  and the camera  21 , as well as the images from the camera  11 ′ on the other car  13  in the surroundings of the self car  12 . After the acquisition of the images, the image is analyzed by using a well known analysis method to extract an image of the other car in the viewpoint-shifted images in step S 15 . Then, if there is the other car in the image, the position of the extracted image of the other car relative to the self car  12  is detected by considering the photography direction and the field of vision of each of the cameras, and the detected position of the other car is compared with the mapping result of the other car&#39;s position in step S 16 . In step S 17 , the images after the viewpoint shift are, composed as the self car circumference overlook image  30  with the images of the “real” other cars that are identified as actually existing based on the comparison with the mapping result. 
       FIG. 7  shows an example of the flow of the self car circumference overlook image generation processing when the camera  21  is used. At first, in step S 51 , the image of the camera  21  that is positioned to capture the self car  12  in the image is acquired for position adjustment of the in-vehicle camera group  11  on the self car  12 , and a viewpoint of the image is converted to a bird&#39;s-eye view (the image always includes the image of the self car  12 : the viewpoint is shifted to a direct top of the self car (that is, the image is composed as a plan view)). Then, in step S 52 , the image of the front field or the rear field of the self car  12  in the field of view detached from the direct surrounding of the self car  12  is acquired from the camera  21  that is positioned to capture an appropriate image that can be seamlessly combined with the image acquired in step S 51  for viewpoint shift. Then, in step S 53 , both of the images acquired in steps S 51  and S 52  are composed as a composite image A. 
     Then, in step S 54 , each of the images from the in-vehicle camera group  11  is converted into the overhead view, or the bird&#39;s-eye view, successively, and the converted images are composed as a composite image B. In the image by the in-vehicle camera group  11  and its composite image B, the image of the self car  12  is only included partially. Therefore, in step S 55 , the viewpoint of the composite image A that includes the self car  12  image is aligned with the viewpoint of the composite image B that partially includes the self car  12  image in a position matching manner, and in step S 56 , the matched images are composed to include only the images of the matching other cars reflected therein. Finally, in step S 57 , the self car circumference overlook image  30  is completed. In this manner, the image of the self car  12  can be included as a real image from the camera  21  in the self car circumference overlook image  30 . However, alternatively, the image of the self car  12  may be composed in the self car circumference overlook image  30  by using a prefixed image of the self car  12  as a substitution. 
       FIG. 8  shows a flow of processing when the camera  11 ′ on the other car is used. In step S 61 , each of the images of the in-vehicle camera group  11  of the self car  12  is acquired, and viewpoint is converted into the bird&#39;s-eye view in step S 62 . Then, in step S 63 , the inter-vehicle communication is performed with the other cars in the surroundings that are discovered by the radar  3 , and the images from the camera group  11  on the other car  13  are acquired as the images after conversion to the plan view. In step S 64 , the bird&#39;s-eye view acquired from the other car  13  is combined with the bird&#39;s-eye view from the camera group  11  of the self car  12 , for composing the composite image A. Then, on the composite image A, the other car image identified in the mapping result is exclusively reflected for completing the self car circumference overlook image  30  (S 66 ). 
       FIG. 9  shows a flow of the risk estimate processing (S 2  in  FIG. 5 ). In step S 21 , the process performs an other car detection process by the method that has already been explained with the radar  3 . The distance to the detected other car as well as the speed is acquired as information in step S 22 , and a relative approach speed of the self car toward the other car is calculated as a difference of the speed of the self car. Then, in step S 23 , a catch-up time of the other car is estimated based on the distance (i.e., a spatial distance in a narrow sense) from the other car and the relative approach speed when the relative approach speed has a positive value. 
     Then, in step S 24 , a physical condition of the driver is acquired as information from the biometric sensor group  74 . The estimation method of the driver&#39;s condition based on the biometric sensing is a well-known technique as disclosed in, for example, the patent document JP-A-2006-282111. Therefore, the technique is only briefly explained in the following. That is, the following sensors can serve as the biometric sensors:
         Infra-red sensor: a thermo-graphical image is captured by the infrared sensor about the face portion of the driver for detecting the body temperature based on the radiated infrared ray.   Face camera (the room camera  73 ): the camera is used to capture a facial expression of the driver sitting in the driver&#39;s seat. Further, the eye direction of the driver is utilized for detecting the level of attentiveness of the driver.   Microphone: the microphone is used to pick up the voice of the driver.   Pressure sensor: the pressure sensor is disposed at a position to be grasped by the driver on the steering wheel, the shift lever or the like for detecting the grasping force as well as a frequency of grip and release.   Pulse sensor: the pulse sensor as a reflective light sensor or the like is disposed at the grasping position on the steering wheel of the vehicle for detecting the blood stream of the driver that reflects the pulse.   Body temperature sensor: the temperature sensor is disposed at the grasping position on the steering wheel of the vehicle.       

     The driver&#39;s condition is determined, for example, based on the captured image in the following manner. That is, captured image of the driver&#39;s face (as a whole or as a part of the face such as an eye, a mouth or the like) from the face camera is compared with master images that templates various psychological conditions and/or physical conditions to determine that a user is in an anger/serenity, in a good temper (cheerfully)/in a bad temper (in disappointment or sorrow), or in anxiety/tension. Further, instead of applying a particular master image for respective users, extracting a facial outline, an eye shape, a mouth/nose position/shape as common facial characteristics for all users and the extracted characteristics may be compared with predetermined standard characteristics for the above determination. 
     The body movement may be detected based on the moving picture of the user captured by the face camera (e.g., a shivering movement, a frowning) and/or information from the pressure sensor or the like (e.g., a frequent release of the hand from the steering wheel, or the like) for determining that the user is irritated or not while he/she is driving the vehicle. 
     The body temperature is detected either by the body temperature sensor on the steering wheel or by the thermo-graphic image from the infra-red sensor. The body temperature may rise when the user&#39;s feeling is lifted, excited, or is offended, and may drop when the user&#39;s feeling is kept in calm. In addition, the strain and/or the excitement may be detected as the increase of pulse counts from the pulse sensor. 
     Further, the body temperature may rise when the user is in a physical condition such as being tired or in distraction regardless of the psychological condition. The cause of the temperature rise may be determined based on the combination of the facial expression (the face camera) or the body movement (the face camera/pressure sensor) with other information that represents the user&#39;s condition. Furthermore, the temperature rise due to the strain, excitement or the emotional response may be distinguished as a temporal temperature increase from the stationary fever due to a poor physical condition. In addition, when the user&#39;s normal temperature is being sampled and registered, the temperature shift from the registered normal temperature (e.g., a shift for higher temperature in particular) may enable a detection of a more subtle emotional change or the like. 
     In the present embodiment, the physical/emotional condition detection result is used to classify the driver&#39;s condition into plural levels, that is, three levels of the normal, the medium-low, and the low in this case, as shown in  FIG. 13  for the risk estimation (S 25 ), and for providing a suitable warning for respective levels. More practically, the warning is provided only from the monitor when the user is classified as “Normal,” with the addition of voice warning when the user is in “Medium-Low,” and further with the vibration when the user is classified as in “Low.” In addition, the threshold of closeness of the other car in three levels of “Near (N)” “Medium-far (M-F)” and “Far (F)” reflected in the risk estimation is defined in a manner that reserves a longer time for the user in the lower condition toward the classification of “low.” That is, in other words, when the user&#39;s response is considered to be slower, the warning is provided earlier. (“R” “Y” “USU.” in the diagram of  FIG. 13  respectively represent “Red,” “Yellow,” and “As Usual.”) 
       FIG. 10  shows a flow of warning contents determination and output (in steps S 3 , S 4 ). The chart depicts an example of lane change by the driver of the self car. In step S 31 , an operation of the turn signal for a lane change is detected. Then, the existence of the other car is confirmed on the self car circumference overlook image  30  (i.e., a detection of surrounding cars) in step S 32 . In step S 33 , whether the other car is in front of the self car or in the rear of the self car is confirmed. When the other car is existing in front of the self car, whether a slower car in comparison to the self car is included in the other car is determined. When there is a slower car, the process proceeds from step S 35  to S 36  for providing information that includes a warning for the driver. On the other hand, when the other car is detected in the rear of the self car in step S 33 , a faster car detection is performed in step S 41  for finding a faster car that is traveling faster than the self car. When a faster car is found in the rear of the self car, the process proceeds from step S 42  to S 36  for providing information that includes the warning (If no slower/faster car is detected, the process concludes itself without performing any process). 
     In the following, individual warning processing for the other car depending on the distance is described. That is, when a threshold of the distance from the other car in the above description is defined as exceeding A seconds as “Far,” the process proceeds from step S 37  to S 43  for providing the driver with warning  1 . When a threshold of the distance from the other car is defined as exceeding B seconds and within or equal to A seconds as “Medium-Far,” the process proceeds from step S 38  to S 44  for providing the driver with warning  2 . Further, when a threshold of the distance from the other car is defined as within or equal to B seconds as “Near,” the process proceeds from step S 38  to S 39  for providing the driver with warning  3 .  FIG. 12  shows the example of the warnings  1 - 3  described above. That is, the warning of other car is provided in a form that the object of warning (i.e., the other car image  213 ) is marked in a predetermined color on the screen. More practically, marking frames  213   f ,  213   bf  for outlining the other car image  213  in the self car circumference overlook image  30  are distinguished for each of the warnings  1 - 3  depending on the distance. That is, when the other car is classified as “Far,” the marking frame  213   f  is displayed in yellow, and when the other car is classified as “Medium-Far,” the marking frame  213   f  is displayed in red. Further, when the other car is classified as “Near,” the red marking frame  213   bf  is displayed as a blinking frame. 
     In addition, the warning for the lowered driver&#39;s condition may be provided as a simple alarm sound, or may be vocally provided as concrete warning contents. For example, the vocal message may sound:
         “Dangerous vehicle from behind” (or “Warning: Faster car is catching up from behind.”);   “Dangerous vehicle ahead” (or “Warning: Slow car is in front.”);   “A vehicle is approaching. Stay on a current lane.”   “A vehicle is ahead. Stay on a current lane.”       

     Further, the warning may be provided depending on the spatial distance instead of the distance of the catch-up time of the other car, or may be provided depending on the speed of the other cars. Furthermore, the warning may be replaced with the figure of the spatial distance/the speed of the other car displayed on the screen.