Patent Publication Number: US-2023141817-A1

Title: Collision avoidance system and vehicle equipped with it

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The disclosure of Japanese Patent Application No. 2021-182267 filed on Nov. 9, 2021, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND 
     The present invention relates to a collision avoidance system and a vehicle equipped with it, and for example, a collision avoidance system including a periphery monitoring system which detects a vehicle in proximity to a subject vehicle by monitoring a periphery of the subject vehicle by use of a sensor and an approaching vehicle notifying system which detects a vehicle in proximity to the subject vehicle by use of vehicle-to-vehicle communication, and a vehicle equipped with it. 
     There are disclosed techniques listed below. 
     [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2012-226635 
     For example, Patent Document 1 discloses a technology of preventing a collision of a vehicle. Patent Document 1 discloses an anticollision safety device capable of saving power consumption without impairing an anticollision function. 
     The periphery monitoring system uses a radar using a camera or a radio wave, for example, and receives a video of a vehicle approaching a subject vehicle or a reflected radio wave, thereby detecting a presence of a vehicle in proximity to the subject vehicle, and a distance between the subject vehicle and the vehicle in proximity thereto to notify a driver of the subject vehicle of these pieces information. Also, the approaching vehicle notifying system performs a wireless communication (vehicle-to-vehicle communication) between a subject vehicle and another vehicle, for example, communicating their locations with each other, calculating a distance between the subject vehicle and a vehicle in proximity thereto on the basis of the obtained location information, and then notifying the driver of the subject vehicle of the calculated distance. 
     The periphery monitoring system can sense a vehicle only in a case in which the vehicle is present in a range in which a sensor can sense the vehicle. Conversely, in the periphery monitoring system, it is difficult to sense a vehicle that is present outside the range in which the sensor can sense the vehicle. Moreover, a vehicle is detected by processing a video captured by the sensor or processing a reflected wave received by the sensor, thereby requiring time for detection of a vehicle. 
     In contrast, the approaching vehicle notifying system performs vehicle-to-vehicle communication between the subject vehicle and vehicles in and out of the range in which the sensor can sense the vehicles, thereby calculating a distance between the subject vehicle and a vehicle that is present in or beyond a line of sight of the subject vehicle, so that the approaching vehicle notifying system can notify the driver of the calculated distance. However, the approaching vehicle notifying system has an upper limit of the number of devices (the number of vehicles) capable of establishing vehicle-to-vehicle communication. Accordingly, for example, in a case in which the number of vehicles exceeding the upper limit is present around the subject vehicle, a vehicle approaching the subject vehicle from a range in which the sensor cannot sense may not be detected in a period of time in which collision avoidance is possible. As a result, the notifying the driver of the presence of the approaching vehicle may be delayed. 
     In view of this, both the periphery monitoring system and the approaching vehicle notifying system have difficulty in avoiding a collision smoothly. Patent Document 1 described above neither states nor recognizes such problems. 
     SUMMARY 
     The typical ones of the inventions disclosed in the present application will briefly be described as follows. 
     Specifically, a collision avoidance system according to an embodiment includes a periphery monitoring system which detects vehicles in proximity to a subject vehicle by use of a sensor, an approaching vehicle notifying system which communicates with another vehicle in proximity to the subject vehicle in vehicle-to-vehicle communication, and a detected vehicle comparison/determination system which is connected to the periphery monitoring system and the approaching vehicle notifying system, determines common vehicles detected by both the periphery monitoring system and the approaching vehicle notifying system, and controls the periphery monitoring system and the approaching vehicle notifying system. 
     Other objects and novel features will become apparent from the description of the present specification and the accompanied drawings. 
     According to one embodiment, it is possible to provide a collision avoidance system capable of enhancing an accuracy of detecting a vehicle that needs collision avoidance. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG.  1    is a block diagram showing a configuration of a collision avoidance system according to a first embodiment. 
         FIG.  2    is a diagram describing the collision avoidance system according to the first embodiment. 
         FIG.  3    is a diagram describing the collision avoidance system according to the first embodiment. 
         FIG.  4    is a diagram describing the collision avoidance system according to a modification of the first embodiment. 
         FIG.  5    is a diagram showing a state of communication packets received from and transmitted to other vehicles in vehicle-to-vehicle communication. 
         FIG.  6    is a diagram showing a configuration of communication packets according to the first embodiment. 
         FIG.  7    is a block diagram showing a configuration of a collision avoidance system according to a second embodiment. 
         FIG.  8    is a diagram describing the collision avoidance system according to the second embodiment. 
         FIG.  9    is a diagram showing a communication packet according to the second embodiment. 
         FIG.  10    is a diagram describing the collision avoidance system according to the second embodiment. 
         FIG.  11    is a diagram describing the collision avoidance system according to the second embodiment. 
         FIG.  12    is a block diagram showing a configuration of a collision avoidance system according to a third embodiment. 
         FIG.  13    is a diagram describing the collision avoidance system according to the third embodiment. 
         FIG.  14    is a diagram describing the collision avoidance system according to the third embodiment. 
         FIG.  15    is a diagram showing a configuration of a communication packet according to the third embodiment. 
         FIG.  16    is a diagram describing the collision avoidance system according to the third embodiment. 
         FIG.  17    is a cross-sectional view schematically showing a vehicle equipped with the collision avoidance system according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. Note that this disclosure is an example only and suitable modifications which can be easily conceived by those skilled in the art without departing from the gist of the present invention are included within the scope of the invention as a matter of course. 
     In addition, in this specification and the respective drawings, the same components described in the drawings which have been described before are denoted by the same reference signs, and detailed description thereof may be omitted as needed. 
     First Embodiment 
     In the following description, a collision avoidance system mounted in a vehicle will be described by way of example, but the present invention is not limited to this.  FIG.  17    is a cross-sectional view schematically showing a vehicle equipped with the collision avoidance system according to the first embodiment. In  FIG.  17   , “ 100 ” denotes a vehicle. As the vehicle, automobile is taken as an example, but the present invention is not limited to this. Also, “ 1 ” denotes a collision avoidance system mounted in the vehicle  100 . The collision avoidance system  1  detects a vehicle in proximity to the vehicle  100  and notifies the driver (not illustrated) of the vehicle  100  of the detected vehicle. The driver who has received the notification operates the vehicle  100  in such a manner as to avoid a collision with the vehicle having been notified to the driver, for example. In this case, an example in which the collision avoidance system  1  notifies the driver of the subject vehicle of a vehicle in proximity to the subject vehicle will be described, but the present invention is not limited to this. For example, it may be configured such that notification from the collision avoidance system  1  is given to the vehicle  100  and the vehicle  100  can act in such a manner as to avoid a collision automatically. 
     Configuration of Collision Avoidance System 
       FIG.  1    is a block diagram showing a configuration of a collision avoidance system according to the first embodiment.  FIG.  1    shows the collision avoidance system  1 . The collision avoidance system  1  includes various systems, and in  FIG.  1   , systems required in the following description only are depicted. 
     The collision avoidance system  1  in  FIG.  1    includes a periphery monitoring system  2 , an approaching vehicle notifying system  3 , and a detected vehicle comparison/determination system  4 . The periphery monitoring system  2  includes a sensor to be used for detecting a vehicle that is present in a detection range. In  FIG.  1   , as the sensor, a camera  2 _ 1  is illustrated as an example. The periphery monitoring system  2  performs image processing on a video captured by the camera  2 _ 1 , thereby detecting a vehicle that is present in the detection range to obtain information on the detected vehicle. The detected vehicle is a vehicle captured by the camera  2 _ 1 , and hereinafter, the vehicle detected by the periphery monitoring system  2  is referred to as a in-detection-range vehicle or a line-of-sight vehicle. Note that a vehicle that has not been captured by the camera  2 _ 1  is referred to as a out-of-detection range vehicle or a beyond-line-of-sight vehicle. The information on the detected vehicle includes, for example, distance information between the detected vehicle and the subject vehicle, a moving direction and an exterior shape of the detected vehicle, and the like, and is output to the detected vehicle comparison/determination system  4  as detected vehicle information  2 DB. 
     The approaching vehicle notifying system  3  includes Global Navigation Satellite System (hereinafter, also referred to as GNSS)  3 _ 1 , a vehicle information retaining unit  3 _ 2 , and a communication unit  3 _ 3 . The approaching vehicle notifying system  3  obtains location information on the subject vehicle  100  ( FIG.  17   ), topographical information on the surroundings of the subject vehicle, and the like, by the GNSS  3 _ 1 . In addition, the approaching vehicle notifying system  3  performs vehicle-to-vehicle communication between the subject vehicle and a vehicle that is present in a periphery of the subject vehicle via the communication unit  3 _ 3 . Then, the approaching vehicle notifying system  3  receives information on the vehicle that is present in the periphery of the subject vehicle, for example, information such as location information, a heading direction, and a vehicle type of the vehicle that is present in the periphery of the subject vehicle, and transmits information such as location information, a heading direction, and a vehicle type of the subject vehicle. The received (obtained) location information, heading direction, vehicle type of the vehicle, and the like are retained in the vehicle information retaining unit  3 _ 2 . Note that the information on the subject vehicle  100  obtained by the GNSS  31  may also be retained in the vehicle information retaining unit  3 _ 2 . The approaching vehicle notifying system  3  outputs the vehicle information retained in the vehicle information retaining unit  3 _ 2 , as detected vehicle information  3 DB, to the detected vehicle comparison/determination system  4 . 
     The detected vehicle comparison/determination system  4  compares the detected vehicle information  2 DB with the detected vehicle information  3 DB. That is, the detected vehicle comparison/determination system  4  compares vehicles indicated by the detected vehicle information  2 DB with vehicles indicated by the detected vehicle information  3 DB. As a result of this comparison, for example, common vehicles that have been detected by both the periphery monitoring system  2  and the approaching vehicle notifying system  3 , that is, common vehicles that are indicated by both pieces of the detected vehicle information  2 DB and  3 DB are determined. The detected vehicle comparison/determination system  4  according to the first embodiment extracts vehicles excluding the common vehicles from the vehicles detected by the approaching vehicle notifying system  3 , that is, the vehicles indicated by the detected vehicle information  3 DB, and on the basis of information regarding the extracted vehicles, controls the periphery monitoring system  2  and the approaching vehicle notifying system  3 . 
     More specifically, information on the vehicles obtained by excluding the common vehicles detected by both of the systems from the vehicles detected by the approaching vehicle notifying system  3 , that is, information on the vehicles that is not indicated by the detected vehicle information  2 DB but indicated only by the detected vehicle information  3 DB is notified, as detection area information  4 DA, to the periphery monitoring system  2 . In other words, the detection area information  4 DA includes information on vehicles that can perform vehicle-to-vehicle communication and are present beyond the line of sight of the subject vehicle. 
     Also, the detected vehicle comparison/determination system  4  notifies the approaching vehicle notifying system  3  of the information on the vehicles that are not indicated by the detected vehicle information  2 DB but are indicated only by the detected vehicle information  3 DB, as communication target information  4 CT. In other words, in the first embodiment, the detection area information  4 DA is notified, as the communication target information  4 CT, to the approaching vehicle notifying system  3 . As a matter of course, the present invention is not limited to this. For example, the information on the common vehicles detected by both systems may be configured to be output, as the communication target information  4 CT, by the detected vehicle comparison/determination system  4 . In this case, in the approaching vehicle notifying system  3 , from the information on the vehicles retained in the vehicle information retaining unit  3 _ 2 , the information on the common vehicles indicated by the communication target information  4 CT is eliminated. 
     The periphery monitoring system  2  is controlled on the basis of the detection area information  4 DA, and the approaching vehicle notifying system  3  is controlled on the basis of the communication target information  4 CT. Next, with use of a specific example, an operation of the collision avoidance system  1  will be described. 
     Operation of Collision Avoidance System 
     Example of Traveling State of Vehicle 
     An example when a plurality of vehicles are travelling on a road is indicated, and an operation of the collision avoidance system at this time will be described.  FIG.  2    is a diagram describing the collision avoidance system according to the first embodiment. In  FIG.  2   , reference signs  10 _ 1  to  10 _ 4  represent roads, and the roads  10 _ 1  to  10 _ 4  intersect with each other at a crossroads  10 . In addition, a reference sign  11  represents walls provided along the roads  10 _ 1  to  10 _ 4 . In  FIG.  2   , a home-plate-shaped pentagon represents a vehicle. Reference signs A, a 1  to a 9 , B, and b 1  to b 3  in the respective pentagons are reference signs for vehicles. Also, in each of the pentagons, a direction in which a vertex of a triangle formed by two long sides faces indicates a heading direction of each of the vehicles. 
     More specifically, on the road  10 _ 1 , the vehicles a 1  to a 3  and the vehicles b 1  and b 2  are present. The vehicles a 2  and a 3  travel in a direction heading to the crossroads  10 , and the vehicles a 1 , b 1 , and b 2  travel in a direction away from the crossroads  10 . Also, the vehicles A, a 9 , and b 3  and the vehicles a 6  to a 8  are present on the road  10 _ 2 . The vehicles A, a 9 , and b 3  travel in a direction  10 _ 2 A heading to the crossroads  10 , and the vehicles a 6  to a 8  travel in a direction away from the crossroads  10 . Also, on the road  10 _ 3 , the vehicle B is present and travels in a direction  10 _ 3 B heading to the crossroads  10 . The vehicles a 4  and a 5  are present on the road  10 _ 4  and travel in a direction away from the crossroads  10 . 
     In this case, although there is no particular limitation, all the vehicles A, a 1  to a 9 , B, and b 1  to b 3  are assumed to be equipped with the collision avoidance system  1  indicated in  FIG.  1   . Also, the subject vehicle (first vehicle) is assumed to be a vehicle denoted by the reference sign A. Since the walls  11  are provided along the roads  10 _ 2  and  10 _ 3 , a driver of the subject vehicle A cannot visually recognize the vehicle (second vehicle) B. That is, since the vehicle B is obscured by the walls  11 , the vehicle B is present beyond the line of sight of the subject vehicle A (out of the detection range of the sensor). Similarly, the subject vehicle A is present beyond the line of sight of the vehicle B. In  FIG.  2   , the vehicles that are present beyond the line of sight of the subject vehicle A are denoted by the reference signs b 1  to b 3 . Note that, since other vehicles are present between each of the vehicles b 1  to b 3  and the subject vehicle A, the driver of the subject vehicle A cannot visually recognize the vehicles b 1  to b 3 . Hence, the vehicles b 1  to b 3  are present beyond the line of sight of the subject vehicle A. In addition, in  FIG.  2   , the vehicles that are present in the line of sight of the subject vehicle A (in the detection range of the sensor) are denoted by the reference signs a 1  to a 9 . 
     A description will be given by taking the collision avoidance system  1  incorporated in the subject vehicle A as an example. In the example indicated in  FIG.  2   , as described above, the vehicles a 1  to a 5  are present in the line of sight of the subject vehicle A, and the vehicles B and b 1  to b 3  are present beyond the line of sight of the subject vehicle A. Accordingly, the detected vehicle information  2 DB output from the periphery monitoring system  2  includes vehicle information on the vehicles a 1  to a 5 . Meanwhile, the detected vehicle information  3 DB output from the approaching vehicle notifying system  3  includes the vehicle information on the vehicles a 1  to a 9 , B, and b 1  to b 3  that are present in the periphery of the subject vehicle A (in and beyond the line of sight of the subject vehicle A). 
     The detected vehicle comparison/determination system  4  determines common vehicles between pieces of the detected vehicle information  2 DB and  3 DB. Moreover, the detected vehicle comparison/determination system  4  generates detection area information  4 DA and communication target information  4 CT on the basis of the vehicle information excluding the common vehicle information from the vehicle information indicated by the detected vehicle information  3 DB. That is, the detected vehicle comparison/determination system  4  outputs the detection area information  4 DA and the communication target information  4 CT on the basis of the vehicle information detected only by the approaching vehicle notifying system  3 . Accordingly, in the example of  FIG.  2   , the detection area information  4 DA and the communication target information  4 CT that are generated on the basis of the vehicle information on the vehicles B and b 1  to b 3  are output to the periphery monitoring system  2  and the approaching vehicle notifying system  3 , respectively. 
     Periphery Monitoring System 
     The periphery monitoring system  2  determines the vehicles indicted by the received detection area information  4 DA as vehicles that need more attention. Moreover, the periphery monitoring system  2  determines the vehicle to be watched among the vehicles that need more attention, and performs a process according to the vehicle to be watched. 
     The periphery monitoring system  2  cannot detect the vehicles b 1  and b 2  since they are not visible behind the vehicle a 1 . Similarly, the periphery monitoring system  2  cannot detect the vehicle b 3  that is not visible behind the succeeding vehicle a 9 , and cannot detect the vehicle B that is present ahead to the left and is not visible behind the wall  11 . Accordingly, the periphery monitoring system  2  recognizes, on the basis of the detection area information  4 DA, presence of the vehicles b 1  to b 3 , and B that cannot be detected by the periphery monitoring system  2 , that is, presence of the beyond-line-of-sight vehicles, and determines that these beyond-line-of-sight vehicles need more attention. Further, the periphery monitoring system  2  can determine, from the detected vehicle information  2 DB detected by itself and the detection area information  4 DA notified from the detected vehicle comparison/determination system  4 , that there are present other vehicle a 1  and a 9  between the vehicles b 1 , b 2 , and b 3  that need more attention and the subject vehicle A. More specifically, the periphery monitoring system  2  can determine that there is another vehicle a 1  between the vehicles b 1  and b 2  that are located ahead of the subject vehicle A and the subject vehicle A and that there is another vehicle a 9  between the vehicle b 3  that is located in the rear of the subject vehicle A and the subject vehicle A. Hence, the periphery monitoring system  2  determines that these vehicles b 1 , b 2  and b 3  do not need to be watched immediately. In contrast, the periphery monitoring system  2  determines that, since there is no other vehicle between the vehicle B and the subject vehicle A, the vehicle B is a vehicle that needs to be watched for avoiding a possible collision. 
     Next, a method of detecting the vehicle B to be watched by the periphery monitoring system  2  will be described with reference to the drawing.  FIG.  3    is a diagram describing the collision avoidance system according to the first embodiment. Here, the camera  2 _ 1  indicated in  FIG.  1    is assumed to be a camera capturing a video of the front of the subject vehicle A. In  FIG.  3   , a reference sign  20  indicates a detection range of the periphery monitoring system  2 . 
     The periphery monitoring system  2  divides a video of the detection range  20  in a video obtained by capturing, into a plurality of processing ranges  20 _S (in  FIG.  3   , 7×3=21), and such image processing as to extract a moving object such as a vehicle is performed on each divided processing range  20 _S. Normally, divided processing ranges  20 _S are sequentially subjected to image processing. For example, in the detection range  20 , image processing is carried out in such a manner as to scan from the processing range  20 _S in the left uppermost stage to the processing range  20 _S in the right lowermost stage. Hence, a time is required for detection of an object in the detection range  20 . 
     According to the first embodiment, on the basis of the detection area information  4 DA, the information on the vehicle B is notified to the periphery monitoring system  2 . According to this notification, the periphery monitoring system  2  can recognizes that there is the vehicle B, though being invisible behind the wall  11 , heading toward the crossroads  10 . As a result, the periphery monitoring system  2  sets part of the video of the captured detection range  20 , for example, six of the processing ranges  20 _S indicated with oblique lines in  FIG.  3    to a focused detection range IMA_ 1 , and increases a frequency of image processing on the processing ranges  20 _S set as the focused detection range IMA_ 1 . As a result, it is possible to reduce a time required for detecting the vehicle B. 
     Note that the periphery monitoring system  2  may be configured to perform no image processing on a detection range other than the focused detection range IMA_ 1 , but the frequency of image processing on the detection range other than the focused detection range IMA_ 1  is desirably lowered. The frequency of image processing is set to be low, so that other vehicles than the vehicle B can also be detected by the periphery monitoring system  2 . 
     Modification 
       FIG.  4    is a diagram describing the collision avoidance system according to a modification of the first embodiment. In the modification, a collision avoidance system capable of further reducing a time required for detecting the vehicle B is provided. 
     In the collision avoidance system  1  shown in  FIG.  1   , information regarding a vehicle type of the vehicle B that is received in vehicle-to-vehicle communication is also stored in the vehicle information retaining unit  3 _ 2 . In the modification, the detected vehicle comparison/determination system  4  generates detection area information  4 DA including vehicle type information of the vehicle B, and outputs the generated information to the periphery monitoring system  2 . The periphery monitoring system  2  grasps the exterior shape of the vehicle B in advance from the vehicle type information of the vehicle B included in the detection area information  4 DA, and sets the focused detection range IMA_ 2  according to the exterior shape of the vehicle B. In  FIG.  4   , a case in which the vehicle type information of the vehicle B indicates that the vehicle B is a vehicle having a low total height is indicated. Although  FIG.  4    is similar to  FIG.  3   , since the total height of the vehicle B is low, as indicated with oblique lines, the processing ranges  20 _S on the left lower side of the detection range  20  are set as the focused detection range IMA_ 2 . Accordingly, it is possible to make the focused detection range smaller, and detecting the vehicle B in a much shorter time is enabled. 
     Approaching Vehicle Notifying System 
     1.  FIG.  5    is a diagram showing a communication state of communication packets received from and transmitted to other vehicles in vehicle-to-vehicle communication. In  FIG.  5   , as time elapses, receivable periods PA to PD periodically set to the subject vehicle are shown. Here, a case in which respective pieces of information on five vehicles are received in one receivable period is shown. For example, in the receivable period PA, in the order of the vehicles a 6 , a 7 , a 9 , b 3 , and a 8 , vehicle information is received from each vehicle. 
     In the receivable period, in a case in which the order of the vehicles performing vehicle-to-vehicle communication is set starting from the vehicle closer to the subject vehicle A to the vehicle farther from the subject vehicle A, the information on the vehicle B to be watched is received in the second place of the receivable period PB, and is next received in the fifth place of the receivable period PD. As a result, a communication interval with the vehicle B to be watched, as indicated in  FIG.  5    becomes CD_ 1 . That is, a reception interval of the information on the vehicle B to be watched becomes longer by a period of time according to the number of vehicles that perform vehicle-to-vehicle communication, in a case in which the vehicles detected by the approaching vehicle notifying system  3  are not changed. 
     The detected vehicle comparison/determination system  4  according to the first embodiment outputs, as described above, the information on the vehicle not indicated by the detected vehicle information  2 DB but indicated only by the detected vehicle information  3 DB, as the communication target information  4 CT, to the approaching vehicle notifying system  3 . In the example shown in  FIG.  2   , the communication target information  4 CT is information on the vehicle B, b 1  to b 3  other than the vehicles a 1  to a 9 . 
     The approaching vehicle notifying system  3  performs vehicle-to-vehicle communication with the vehicles indicated by this communication target information  4 CT.  FIG.  6    is a diagram showing the receivable periods according to the first embodiment. Similarly to  FIG.  5   ,  FIG.  6    shows the receivable periods PA to PD set to the subject vehicle A. Also in this case, a case in which respective pieces of information on five vehicles can be received in one receivable period is indicated. 
     According to the communication target information  4 CT, the approaching vehicle notifying system  3  performs vehicle-to-vehicle communication with four vehicles B and b 1  to b 3 . Accordingly, respective pieces of information on four vehicles B and b 1  to b 3  are received in one receivable periods, and for each receivable period, it is possible to obtain pieces of information on the vehicles including the vehicle B to be watched for collision avoidance. In addition, information on the vehicle B to be watched is first stored in the second place of the receivable period PA, and then, is stored in the second place of the receivable period PB. That is, a communication interval between the subject vehicle A and the vehicle B to be watched is a period CD_ 2  as shown in  FIG.  6   , and the subject vehicle A can obtain the information on the vehicle B in a short cycle. For example, the subject vehicle A can obtain location information on the vehicle B transmitted by a communication packet from the vehicle B to be watched, in a short interval. As a result, the subject vehicle A can have the location information on the vehicle B with high accuracy. 
     Note that, in  FIG.  6   , each of the receivable periods PA to PD includes a communication period for a reserve. For example, when there is a vehicle that the periphery monitoring system  2  has not been able to detect, this reserve communication period is used for vehicle-to-vehicle communication between the vehicle and the subject vehicle A. Accordingly, it is possible to restart vehicle-to-vehicle communication between the vehicle that periphery monitoring system  2  has not been able to detect, in other words, a vehicle other than the common vehicles detected by both systems and the subject vehicle A. 
     According to the collision avoidance system  1  of the first embodiment, in the detected vehicle comparison/determination system  4 , the detected vehicle information  2 DB from the periphery monitoring system  2  and the detected vehicle information  3 DB from the approaching vehicle notifying system  3  are compared with each other, and the common vehicles in both systems are determined, and a vehicle to be detected is identified for each system. For example, in the example shown in  FIG.  2   , the detected vehicle comparison/determination system  4  identifies the vehicle to be detected by the periphery monitoring system  2  according to the detection area information  4 DA and identifies the vehicle to be detected by the approaching vehicle notifying system  3  according to the communication target information  4 CT. Since a vehicle not identified according to the detection area information  4 DA and the communication target information  4 CT is a vehicle that is not a detection target or a vehicle that does not need attention, in a different perspective, it can be considered that the detected vehicle comparison/determination system  4  is determining which vehicle does not need to be detected for each system. 
     In addition, according to the collision avoidance system  1 , since the vehicle as a detection target is identified for each system, it is possible to obtain the information on the vehicle that needs collision avoidance in a short period of time, allowing for smooth operation for collision avoidance. 
     Second Embodiment 
     In the second embodiment, the information on the vehicle detected by the periphery monitoring system  2  is transmitted to other vehicles, as information on the vehicle that is present beyond the line of sight (beyond-line-of-sight vehicle information), through vehicle-to-vehicle communication. By adopting such configuration, the other vehicles can avoid a possible collision with use of the beyond-line-of-sight vehicle information received through vehicle-to-vehicle communication. 
       FIG.  7    is a block diagram showing a configuration of a collision avoidance system according to a second embodiment. Since  FIG.  7    is similar to  FIG.  1   , differences will mainly be described. Main differences are as follows: the detected vehicle comparison/determination system  4  in  FIG.  7    includes a beyond-line-of-sight vehicle determining unit  4 _ 1  that outputs beyond-line-of-sight vehicle information  4 OT, and the approaching vehicle notifying system  3  includes a beyond-line-of-sight vehicle information retaining unit  3 _ 4  that retains the beyond-line-of-sight vehicle information  4 OT. 
     The beyond-line-of-sight vehicle determining unit  4 _ 1  receives the detected vehicle information  2 DB output from the periphery monitoring system  2  and notifies the approaching vehicle notifying system  3  of information on the vehicle obtained from the detected vehicle information  2 DB, for example, information such as a vehicle type, a distance, a heading direction, a speed, an exterior shape of the vehicle, as the beyond-line-of-sight vehicle information  4 OT. The approaching vehicle notifying system  3  retains the notified beyond-line-of-sight vehicle information  4 OT in the beyond-line-of-sight vehicle information retaining unit  3 _ 4 . 
     In addition, as has been described in the first embodiment, the approaching vehicle notifying system  3  obtains communication target information  4 CT as vehicle information detected only by the approaching vehicle notifying system  3 , from the detected vehicle comparison/determination system  4 . The approaching vehicle notifying system  3  in the second embodiment transmits the communication target information  4 CT to other vehicles via vehicle-to-vehicle communication, along with information on the subject vehicle. The approaching vehicle notifying system  3  of a vehicle that has received vehicle information on other vehicles, including the communication target information  4 CT, via vehicle-to-vehicle communication can visually recognize that there is a vehicle of which information can be obtained only through vehicle-to-vehicle communication, for the vehicle that has received the vehicle information. In view of this, the approaching vehicle notifying system  3  of the vehicle that has received the vehicle information determines whether the information on the vehicle indicated in the received communication target information  4 CT is retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  thereof. In a case in which the information on the vehicle indicated in the received communication target information  4 CT is retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  thereof, the approaching vehicle notifying system  3  transmits, along with the information on the subject vehicle, the beyond-line-of-sight vehicle information  4 OT via vehicle-to-vehicle communication. 
     Operation of Collision Avoidance System 
       FIG.  8    is a diagram describing the collision avoidance system according to the second embodiment.  FIG.  8    is similar to  FIG.  2   . A difference is that, in  FIG.  8   , the vehicles A and a 10 , and B only are indicated. The vehicles A and B are heading to the crossroads  10 , similarly to  FIG.  2   . The vehicle a 10  is passing the crossroads  10 , in such a manner as to head to the road  10 _ 1 . Each of the vehicles A, a 10  and B is assumed to be equipped with the collision avoidance system  1  shown in  FIG.  7   . Also, in the following description, the vehicle a 10  is also referred to as a first vehicle, and the vehicle A is also referred to as a second vehicle or another vehicle. 
     Since the first vehicle a 10  is passing the crossroads  10 , the vehicle B is present in a line of sight of the first vehicle a 10 . Accordingly, the camera  2 _ 1  mounted in the first vehicle a 10  captures a video of the vehicle B, and the vehicle B is detected from the captured video. The information on the vehicle B is notified, as the detected vehicle information  2 DB, to the detected vehicle comparison/determination system  4  of the first vehicle a 10 . The beyond-line-of-sight vehicle determining unit  4 _ 1  in the detected vehicle comparison/determination system  4  of the first vehicle a 10  notifies the approaching vehicle notifying system  3  of the first vehicle a 10  of the detected vehicle information  2 DB, as the beyond-line-of-sight vehicle information  4 OT. The beyond-line-of-sight vehicle information  4 OT includes information on the vehicle detected by the periphery monitoring system  2  of the first vehicle a 10 , for example, information such as a vehicle type of the vehicle B, a distance between the vehicle B and the first vehicle a 10 , a moving direction of the vehicle B, a speed of the vehicle B. Hereinafter, the information on the vehicle B included in the beyond-line-of-sight vehicle information  4 OT is referred to as information B_inf. The beyond-line-of-sight vehicle information  4 OT including the information B_inf is retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  in the approaching vehicle notifying system  3 . 
     Meanwhile, the periphery monitoring system  2  and the approaching vehicle notifying system  3  in the collision avoidance system  1  of the second vehicle A also output respective pieces of detected vehicle information  2 DB and  3 DB. The detected vehicle comparison/determination system  4  of the second vehicle A generates communication target information  4 CT, on the basis of the detected vehicle information  2 DB and  3 DB of the second vehicle A. The approaching vehicle notifying system  3  of the second vehicle A transmits the communication target information  4 CT on the second vehicle A along with the information on the subject vehicle (second vehicle A), to other vehicles, via the communication unit  3 _ 3 . That is, in a situation indicated in  FIG.  8   , the communication target information  4 CT of the second vehicle A includes the information on the vehicle B. Thus, the second vehicle A transmits the information on the subject vehicle and the information on the vehicle B that performs vehicle-to-vehicle communication but is present beyond the line of sight of the second vehicle A, as the vehicle information on the second vehicle A, to other vehicles including the first vehicle a 10 . 
     The first vehicle a 10  receives the vehicle information on the second vehicle A, via vehicle-to-vehicle communication. The approaching vehicle notifying system  3  of the first vehicle a 10  determines whether vehicle information included in the communication target information  4 CT of the received vehicle information on the second vehicle A is included in the beyond-line-of-sight vehicle information  4 OT retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4 . In  FIG.  8   , the beyond-line-of-sight vehicle information  4 OT retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  of the first vehicle a 10  includes the information on the vehicle B included in the received communication target information  4 CT of the second vehicle A. Accordingly, the first vehicle a 10  adds the beyond-line-of-sight vehicle information  4 OT (information B_inf on the vehicle B) retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  to the information on the first vehicle a 10  (information on the subject vehicle), and then transmits the beyond-line-of-sight vehicle information  4 OT to the second vehicle A travelling behind the first vehicle a 10  via the communication unit  3 _ 3 . That is, vehicle-to-vehicle communication between the first vehicle a 10  and the second vehicle A allows the information on the vehicle B detected by the first vehicle a 10  and the information on the first vehicle a 10  to be transmitted to the second vehicle A. 
     Note that, as has been described in  FIG.  2   , the vehicle B is present beyond the line of sight of the vehicle A, and accordingly, the vehicle B cannot be detected by the periphery monitoring system  2  of the vehicle A. Moreover, in  FIG.  8   , the information B_inf exemplifies the information on the vehicle B transmitted from the first vehicle a 10  to the second vehicle A through vehicle-to-vehicle communication. 
       FIG.  9    is a diagram showing a communication packet according to the second embodiment.  FIG.  9    shows a communication packet PE to be transmitted by the first vehicle a 10 . The communication packet PE includes information a 10 _ inf  on the first vehicle a 10 , the information B_inf on the vehicle B, information A_inf on the vehicle A, and a reserve region. The information B_inf on the vehicle B in the communication packet PE is information retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  of the first vehicle a 10 . 
     The collision avoidance system  1  mounted in the second vehicle A that has received the communication packet PE from the first vehicle a 10  obtains location information on the first vehicle a 10  or the like, on the basis of the information a 10 _ inf  on the first vehicle a 10  included in the communication packet PE, and obtains a vehicle type of the vehicle B being present beyond the line of sight of the second vehicle A, a distance between the vehicle B and the first vehicle a 10 , and a speed of the vehicle B, on the basis of the information B_inf on the vehicle B. 
     Next, a description regarding location information according to the communication packet from the vehicle B and location information on the vehicle B according to the information B_inf included in the communication packet from the first vehicle a 10  will be given with reference to the drawing.  FIG.  10    is a diagram describing the collision avoidance system according to the second embodiment.  FIG.  10    is similar to  FIG.  8   . A difference between  FIG.  10    and  FIG.  8    is that each vehicle is depicted with a point in order to easily imagine a position of each vehicle in  FIG.  10   . Moreover, in  FIG.  10   , a circle B_G of a solid line surrounding the vehicle B indicates a range of a location of the vehicle B, the range obtained by adding error information to coordinates of the GNSS transmitted by the vehicle B in vehicle-to-vehicle communication. In contrast, a circle B_i of dashed lines surrounding the vehicle B indicates a range of a location of the vehicle B, the range obtained by adding error information to coordinates resulting from calculation based on a distance between the vehicle B and the subject vehicle (first vehicle a 10 ) detected by the periphery monitoring system  2  of the first vehicle a 10 . That is, the range of the circle of the dashed lines is a range of the location of the vehicle B which is represented by the information B_inf. In this manner, the range of the circle of the dashed lines is inside the range of the circle of the solid line. Hence, a detection accuracy of the location of the vehicle B by the periphery monitoring system  2  is higher than a detection accuracy of the location of the vehicle B obtained by the GNSS. Thus, the second vehicle A can recognizes the location of the vehicle B with the information B_inf included in the communication packet transmitted from the first vehicle a 10  with high accuracy. 
       FIG.  11    is a diagram describing the collision avoidance system according to the second embodiment. In  FIG.  11   , a detection range  20  of the periphery monitoring system  2  of the second vehicle A is shown.  FIG.  11    is similar to  FIG.  3   . A difference between  FIG.  11    and  FIG.  3    is that, in  FIG.  11   , a right end of the detection range  20  of the periphery monitoring system  2  is shifted in a direction indicated with an arrow  20 _SF on the right side. Note that, in  FIG.  11   , a one-dot chain line  3 LD indicates a left-end position of the detection range  20  shown in  FIG.  3   . 
     The second vehicle A can grasp the location of the vehicle B more accurately since the second vehicle A calculates the coordinates on the basis of the information B_inf of the vehicle B detected by the periphery monitoring system  2  of the first vehicle a 10 . Accordingly, the detection range  20  of the periphery monitoring system  2  of the second vehicle A can be shifted to the right side as shown in  FIG.  11   . That is, it is possible to shift in the rightward direction, a range in which image processing is carried out, the range of the video captured by the camera  2 _ 1  mounted in the periphery monitoring system  2  of the second vehicle A. By shifting the detection range  20  in the rightward direction, it is possible to detect a vehicle, a person, or the like entering from the right side and avoid a collision. That is, in the second vehicle A, it becomes possible to control the detection range of the periphery monitoring system  2  of the second vehicle A such that the second vehicle A detects not only the vehicle B but also other regions (region on the right side). 
     Moreover, since the information B_inf includes information regarding the vehicle type of the vehicle B, according to the information of the vehicle type, the focused detection range IMA_ 3  detected by the periphery monitoring system  2  of the second vehicle A may be changed. Further, the information B_inf includes distance information between the first vehicle a 10  and the vehicle B and information on the exterior shape of the vehicle B, which are obtained on the basis of the video captured by the camera  21  of the periphery monitoring system  2  of the first vehicle a 10 . Accordingly, for example, even if the vehicle B is a vehicle type that has newly been sold, or even if the vehicle B has a load ULD which is not included in the information on the vehicle type, the second vehicle A can set the focused detection range IMA_ 3  by use of the information on the exterior shape on the vehicle B and the distance information included in the information B_inf. In the example indicated in  FIG.  11   , a case in which the load ULD projecting further than the vehicle B is mounted on the vehicle B is shown. The periphery monitoring system  2  of the first vehicle a 10  detects a distance between the load ULD of the vehicle B and the subject vehicle (first vehicle a 10 ) and the information on the exterior shape of the vehicle B including the load ULD. The periphery monitoring system  2  of the second vehicle A can narrow down the focused detection range IMA_ 3  on the basis of the information A_inf ( FIG.  9   ) on the first vehicle a 10  transmitted from the first vehicle a 10  and the information B_inf on the vehicle B including the load ULD. The focused detection range IMA_ 3  shown in  FIG.  11    is made narrower than the focused detection range IMA_ 1  shown in  FIG.  3   , and set to the left upper side of the detection range  20  such that the load ULD projecting further than the vehicle B is focused on. By making the focused detection range IMA_ 3  narrower, a frequency of image processing to be carried out in the focused detection range IMA_ 3  is made higher, so that the vehicle B can be detected in a shorter time. 
     In this case, when a location accuracy of the GNSS mounted in the vehicle B is lower than a location accuracy of the vehicle B detected by the periphery monitoring system  2  of the first vehicle a 10 , a case in which the information regarding the vehicle B detected by the periphery monitoring system  2  of the first vehicle a 10  is used by the second vehicle A to avoid a collision has been described. However, even in a case in which the location accuracy of the GNSS mounted in the vehicle B is relatively high, it can be considered that, in a case in which there are high buildings, an overhead bridge, and the like in the vicinity of a crossroads such as the crossroads  10 , the location accuracy of the GNSS mounted in the vehicle B is deteriorated. Consequently, the location accuracy of the GNSS may be lowered without any relation to the location accuracy of the GNSS mounted in the vehicle B. 
     In this case, a map of an automotive navigation system mounted in the vehicle or a map that is in communication with the vehicle via the approaching vehicle notifying system  3  or the like is retained in the collision avoidance system  1  of the second vehicle A as peripheral information of the second vehicle A, and by making use of this map, the second vehicle A may change the focused detection range IMA_ 3 . That is, with reference to the retained map, the collision avoidance system  1  of the vehicle A determines that, in a case in which there is a crossroads or the like in the periphery thereof, the location accuracy of the GNSS becomes lower, and may make the focused detection range IMA_ 3  wider. In addition, as has been described in  FIG.  7    to  FIG.  10   , with reference to the retained map, when there is a crossroads or the like in the periphery of the vehicle A, the collision avoidance system  1  of the vehicle A may set the focused detection range IMA_ 3  in order to detect the vehicle B in a short time, with use of the information B_inf on the vehicle B transmitted from another vehicle. 
     In addition, a case in which the communication quality of vehicle-to-vehicle communication is deteriorated by being disturbed by the buildings or the like, making it difficult to transmit the location information of the GNSS in the vehicle-to-vehicle communication in a short time can also be considered. Also in this case, with reference to the retained map, at the crossroads or the like, for example, the focused detection range IMA_ 3  may be made wider. 
     In  FIG.  8    and  FIG.  10   , a case in which all the vehicles A, B, and a 10  have the collision avoidance system  1  shown in  FIG.  7    mounted therein has been described, but the present invention is not limited to this. For example, in a case in which there is a vehicle not having the collision avoidance system  1  mounted therein between the vehicle B and the first vehicle a 10 , this vehicle not having the collision avoidance system  1  mounted therein is detected by the periphery monitoring system  2  of the first vehicle a 10 . The first vehicle a 10  may notify the second vehicle A of information on the vehicle detected by the periphery monitoring system  2  thereof, as beyond-line-of-sight vehicle information  4 OT, via vehicle-to-vehicle communication, regardless of the communication target information  4 CT of the second vehicle A. This notification enables the second vehicle A to obtain the information on the vehicle not having the collision avoidance system  1  mounted therein, and further, the second vehicle A can set the focused detection range IMA_ 3 . 
     According to the collision avoidance system  1  of the second embodiment, the detected vehicle comparison/determination system  4  includes the beyond-line-of-sight vehicle determining unit  4 _ 1 , and transmits the beyond-line-of-sight vehicle information  4 OT to another vehicle, by use of the approaching vehicle notifying system  3 . Anther vehicle can grasp the location of the vehicle being present beyond the line of sight by the beyond-line-of-sight vehicle information  4 OT with high accuracy and can grasp the shape of the vehicle. As such, it is possible to detect a vehicle that needs to avoid a possible collision in a short time, so that an action for collision avoidance can smoothly be carried out. 
     Third Embodiment 
     In the third embodiment, the detected vehicle comparison/determination system of the periphery monitoring system includes a beyond-line-of-sight vehicle determining unit and a line-of-sight vehicle determining unit, and a collision avoidance system that transmits both beyond-line-of-sight vehicle information and line-of-sight vehicle information to other vehicles in vehicle-to-vehicle communication is provided. 
       FIG.  12    is a block diagram showing a configuration of a collision avoidance system according to the third embodiment. Since  FIG.  12    is similar to  FIG.  7   , differences between  FIG.  12    and  FIG.  7    will mainly be described. First difference is that, in  FIG.  12   , the detected vehicle comparison/determination system  4  includes not only the beyond-line-of-sight vehicle determining unit  4 _ 1 , but also a line-of-sight vehicle determining unit  4 _ 2 . Second difference is that, in  FIG.  12   , the approaching vehicle notifying system  3  includes not only the beyond-line-of-sight vehicle information retaining unit  3 _ 4 , but also a line-of-sight vehicle information retaining unit  3 _ 5 . The line-of-sight vehicle information retaining unit  3 _ 5  retains the line-of-sight vehicle information output from the detected vehicle comparison/determination system  4  and outputs disappearance information to the beyond-line-of-sight vehicle information retaining unit  3 _ 4 . 
     Information on a vehicle being present in a line of sight of the subject vehicle, for example, the information such as a vehicle type of the vehicle, a distance between the vehicle and the subject vehicle, and a speed of the vehicle, is supplied to the beyond-line-of-sight vehicle determining unit  4 _ 1  and the line-of-sight vehicle determining unit  4 _ 2  as the detected vehicle information  2 DB, from the periphery monitoring system  2 . As has been described in the second embodiment, the beyond-line-of-sight vehicle determining unit  41  outputs the information on the vehicle detected by the approaching vehicle notifying system  3 , as the beyond-line-of-sight vehicle information  4 OT. The line-of-sight vehicle determining unit  4 _ 2  outputs, as time passes, information on a vehicle that has not been detected by the periphery monitoring system  2 , that is, a vehicle that has disappeared in a line of sight of the subject vehicle, as line-of-sight vehicle information  4 IT. For example, in a case in which a vehicle that was present within a predetermined distance from the subject vehicle has disappeared in the line of sight of the subject vehicle, the line-of-sight vehicle determining unit  4 _ 2  outputs information on the vehicle as the line-of-sight vehicle information  4 IT. The beyond-line-of-sight vehicle information  4 OT is retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  of the approaching vehicle notifying system  3 , and the line-of-sight vehicle information  4 IT is retained in the line-of-sight vehicle information retaining unit  3 _ 5 . The information on the vehicle that has disappeared in the line of sight of the subject vehicle, the information being retained in the line-of-sight vehicle information retaining unit  3 _ 5 , is supplied to the beyond-line-of-sight vehicle information retaining unit  3 _ 4  and is retained also in the beyond-line-of-sight vehicle information retaining unit  3 _ 4 . Note that, in the following description, the information on the vehicle that has disappeared in the line of sight of the subject vehicle is referred to as disappearance information. 
     The approaching vehicle notifying system  3  performs vehicle-to-vehicle communication with the approaching vehicle notifying system  3  mounted in each of other vehicles. At this time, the approaching vehicle notifying system  3  adds the information on vehicles retained in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  to the information on the subject vehicle retained in the vehicle information retaining unit  3 _ 2  to transmit the information to the other vehicles. That is, the approaching vehicle notifying system  3  transmits the information obtained by adding the beyond-line-of-sight vehicle information  4 OT and the disappearance information to the information on the subject vehicle as the subject vehicle information. 
     Operation of Collision Avoidance System 
     Next, operation of the collision avoidance system according to the third embodiment will be described with reference to the drawings.  FIG.  13    and  FIG.  14    are diagrams each describing the collision avoidance system according to the third embodiment. 
     First, a travelling state to be used for description is provided. In  FIG.  13   , it is assumed that the vehicle A travelling from the road  10 _ 2  toward the crossroads  10  as the subject vehicle (first vehicle). In contrast, the vehicle a 2  and the vehicle a 3  (second vehicle or another vehicle) travel so as to head from the road  10 _ 1  toward the crossroads  10  in a platoon. Also, the vehicle B travels heading from the road  10 _ 3  to the crossroads  10 . 
     The vehicle B is obscured by the wall  11 , and accordingly, is not detected by the periphery monitoring system  2  of the first vehicle A. That is, the vehicle B is present beyond the line of sight of the first vehicle A. In contrast, the vehicle a 2  and the second vehicle a 3  are present in the line of sight of the first vehicle A, and accordingly, is detected by the periphery monitoring system  2  of the first vehicle A. Here, a range indicated by a reference signs SAE indicates an area being a shadow behind the vehicle a 2 , when the vehicle a 2  is viewed from the first vehicle A. In a state indicated in  FIG.  13   , part of the second vehicle a 3  is out of the shadow area SAE, and accordingly, the second vehicle a 3  is present in the line of sight of the first vehicle A. In such a state, the vehicle a 2  and the second vehicle a 3  are detected by the periphery monitoring system  2  of the collision avoidance system  1  of the first vehicle A. Information regarding the vehicle a 2  and the second vehicle a 3  thus detected are output to the approaching vehicle notifying system  3  as the beyond-line-of-sight vehicle information  4 OT, via the detected vehicle comparison/determination system  4 . The approaching vehicle notifying system  3  of the collision avoidance system  1  of the first vehicle A transmits the supplied information regarding the vehicle a 2  and the second vehicle a 3 , along with the information on the first vehicle A, to a vehicle being present in the periphery of the first vehicle A in vehicle-to-vehicle communication. 
     As has been described in  FIG.  6    of the first embodiment, the first vehicle A continues vehicle-to-vehicle communication, only with a vehicle being present beyond the line of sight (in  FIG.  13   , only the vehicle B). Since the vehicle-to-vehicle communication in this case is performed between the two vehicles, it becomes possible to keep the communication interval between the first vehicle A and the vehicle B short, and the information on the vehicle B that needs to avoid a possible collision can be detected in a short time. 
     Next, a case in which the first vehicle A and the vehicle a 2  enter the crossroads  10  and each turn right will be described.  FIG.  14    shows the states at this time. That is, it is assumed that the vehicle a 2  turns right in a direction indicated with an arrow a 2 _R and the first vehicle A turns right in a direction indicated with an arrow A_R at the crossroads  10  and each of the vehicles a 2  and A then travels straight ahead. In the state shown in  FIG.  14   , the second vehicle a 3  is present at a left rearward position of the vehicle a 2 , and the entire body thereof is in the shadow area SAE of the vehicle a 2 . Hence, in a case in which the second vehicle a 3  is viewed from the first vehicle A, a state of the second vehicle a 3  is changed from a state to be present in the line of sight of the first vehicle A to a state to be present beyond the line of sight of the first vehicle A. That is, when the second vehicle a 3  is viewed from the periphery monitoring system  2  of the first vehicle A, the state of the second vehicle a 3  is changed from the line-of-sight state to a disappearing state. The line-of-sight vehicle determining unit  4 _ 2  of the first vehicle A compares the detected vehicle information  2 DB output by the periphery monitoring system  2  with the detected vehicle information  2 DB that has been output right before the output, to detect a vehicle that has changed its state from the line-of-sight state to the beyond-line-of-sight state. The line-of-sight vehicle determining unit  4 _ 2  of the first vehicle A determines whether or not a distance between the vehicle that has changed its state to the beyond-line-of-sight state when has been detected lastly in the line of sight of the first vehicle A and the first vehicle A is within a predetermined distance. Then, the line-of-sight vehicle determining unit  4 _ 2  of the first vehicle A identifies the second vehicle a 3  as a disappearing vehicle. Note that, in a case in which the line-of-sight vehicle determining unit  4 _ 2  of the first vehicle A detects a plurality of vehicles that change their states to the beyond-line-of-sight states, the vehicle having a shortest inter-vehicle distance when it has been in the line-of-sight state may be identified as the disappearing vehicle. The information on the second vehicle a 3  is retained, as disappearance information representing disappearance of the vehicle, in the beyond-line-of-sight vehicle information retaining unit  3 _ 4  of the first vehicle A. 
       FIG.  15    is a diagram showing a configuration of a communication packet according the third embodiment.  FIG.  15    is similar to  FIG.  9   .  FIG.  15    shows a communication packet PF transmitted by the first vehicle A, where A_inf indicates the information on the first vehicle A and S_inf indicates disappearance information. The disappearance information S_inf has, for example, a time at which the second vehicle a 3  has been detected lastly in the line of sight of the first vehicle A, a distance between the first vehicle A and the second vehicle a 3 , and a vehicle type (or a shape) of the second vehicle a 3  at this time included therein. 
     The second vehicle a 3  receives the communication packet PF transmitted by the first vehicle A. The approaching vehicle notifying system  3  of the collision avoidance system  1  of the second vehicle a 3  recognizes that the information of the first vehicle A includes the subject vehicle a 3  as the disappearance information and determines that the second vehicle a 3  becomes a vehicle that is beyond the line of sight of the first vehicle A. Accordingly, the second vehicle a 3  restarts vehicle-to-vehicle communication with the approaching vehicle notifying system  3  of the first vehicle A. Hence, the first vehicle A and the second vehicle a 3  can confirm the locations with each other in the respective approaching vehicle notifying systems  3 . In addition, the approaching vehicle notifying system  3  of the first vehicle A sets the second vehicle a 3  as the disappearing vehicle to be the most carefully watched vehicle of the beyond-line-of-sight vehicles, on the basis of the disappearance information retained in the line-of-sight vehicle information retaining unit  3 _ 5 , and a communication frequency with the second vehicle a 3  may be increased. 
     Also, as has been described in the first embodiment, the periphery monitoring system  2  of the first vehicle A can set the focused detection range by use of the detection area information  4 DA output by the detected vehicle comparison/determination system  4 . Accordingly, the detected vehicle comparison/determination system  4  focuses on the vehicle information of the second vehicle a 3  as the disappearing vehicle, of the vehicle information of the detected vehicle information  3 DB obtained by the approaching vehicle notifying system  3 , to generate the detection area information  4 DA. 
     An example of this setting will be described with reference to the drawing.  FIG.  16    is a diagram describing the collision avoidance system according to the third embodiment. As has been shown in  FIG.  16   , the collision avoidance system  1  of the first vehicle A sets a focused detection range IMA_ 4  indicated in oblique lines on the right side of the vehicle a 2  crossing in front of the first vehicle A, on the basis of the location information on the second vehicle a 3  obtained via vehicle-to-vehicle communication. Thus, in preparation for appearance of the second vehicle a 3 , the periphery monitoring system  2  of the first vehicle A is controlled, so that a detection range of appearance of the second vehicle a 3  can be processed in a short time. As a result, it is possible to detect the second vehicle a 3  in a short time. 
     In  FIG.  12    to  FIG.  16   , a case in which the vehicle (a 3 ) moves to the shadow area SAE of another vehicle (a 2 ), is then changed from the line-of-sight state to the beyond-line-of-sight state, and cannot be detected by the periphery monitoring system has been described, but the present invention is not limited to this. 
     For example, also in a case in which the sunlight in front of the first vehicle A lowers the detection sensitivity of the camera  2 _ 1  or a case in which presence of a vehicle or a building having a large area that interrupts a radar in a front direction causes deterioration of the detection sensitivity of the radar, it can be considered that the second vehicle a 3  cannot be detected by the periphery monitoring system  2 . Also in these cases, the second vehicle a 3  considers that the state thereof is changed from the line-of-sight of the subject vehicle A to the beyond-line of sight of the subject vehicle A, and vehicle-to-vehicle communication between the second vehicle a 3  and the first vehicle A may be restarted. Note that deterioration of the detection sensitivity due to the sunlight and the building constantly occurs regardless of presence of the vehicle a 2 , and hence, the map of the automotive navigation system mounted in the vehicle or the map that is in communication with other vehicles via the approaching vehicle notifying system or the like is retained as peripheral information in the approaching vehicle notifying system of the vehicle, so that it may be configured such that a vehicle passing a crossroads uses this map. 
     According to the third embodiment, the information on the disappearing vehicle is transmitted to another vehicle by use of the approaching vehicle notifying system, the location of the vehicle whose state is changed from the line-of-sight state to the beyond-line-of-sight state can be grasped with high accuracy. Hence, it is possible to detect the vehicle that needs collision avoidance in a short time and smoothly perform a collision avoidance action. 
     In the foregoing, the invention made by the inventor of the present invention has been concretely described based on the embodiments. However, it is needless to say that the present invention is not limited to the foregoing embodiments and various modifications and alterations can be made within the scope of the present invention.