Patent Publication Number: US-2015083921-A1

Title: Approaching vehicle detection apparatus and method

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is based on Japanese Patent Applications No. 2013-199677 filed on Sep. 26, 2013 and No. 2013-226888 filed on Oct. 31, 2013, disclosures of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure relates to an approaching vehicle detection apparatus and an approaching vehicle detection method for detecting approach of a vehicle in multiple vehicles. 
     BACKGROUND 
     There is a conventional art (e.g., Patent Document 1) relating to a collision prediction apparatus. The collision prediction apparatus receives a light beam spot emitted from a different vehicle to measure an angle between a subject vehicle and the different vehicle and determine a collision probability based on an angle of a heading direction of the subject vehicle with respect to a heading direction of the different vehicle. 
     With a simple configuration, the collision prediction apparatus of the conventional art can predict a possibility of collision between the subject vehicle and the different vehicle in advance and improve vehicle safety. 
     Patent Document 1: JP-2010-13012A 
     The needs of equipment for detecting the approach of a vehicle in multiple vehicles are increasing year by year. This is largely due to increasing needs for a pre-clash safety system, which predict a collision in advance and takes preliminary measures against it, and advancing concepts for a driverless vehicle transportation system. 
     In this relation, in order to detect approaching of vehicles each other, it is conceivable to apply a method of detecting a distance between a vehicle and an obstacle by transmitting a reflective media such as an ultrasonic wave or the like and using the reflective media reflected by the obstacle outside the vehicle. Specifically, each vehicle transmits the reflective medium, and detects a distance to a difference vehicle based on the reflective medium reflected by the different vehicle. 
     However, when respective vehicles transmit the reflective mediums in order to detect the distance between vehicles, the reflective mediums transmitted from respective vehicles interfere with each other, making precise distance detection impossible. Specifically, when the reflective medium transmitted from the subject vehicle interferes with the reflective medium from a different vehicle, the reflective medium reflected may not be correctly incident on the subject vehicle or the reflective medium from the different vehicle may be wrongly detected as the reflective medium having being transmitted from the subject vehicle. When precise distance detection between vehicles cannot be performed, it may lead to reduction in accuracy of vehicle-to-vehicle approach detection. No measures against this difficultly are addressed in the above-mentioned conventional art. 
     Moreover, in order to implement a driverless vehicle transportation system, it is necessary to transmit information through vehicle-to-vehicle communication in addition to detecting the distance to a different vehicle. In some cases, when the communication is performed between nearby vehicles, it may be preferable to use infrared light instead of radio wave. Specifically, as compared with the radio wave, the infrared light tends not to spread and not travel a far distance, and thus, the infrared light is resistant to noise. 
     Nevertheless, since an infrared amount in environments affects an infrared light, an infrared light from a distant light source may arrive and unnecessary information may be also received when the infrared amount in environments is small. When the unnecessary reception increases, a function of a communication device may be saturated or a processor may use much memory for information processing, and as a result, a controller may not function normally. 
     SUMMARY 
     In view of the foregoing, it is an object of the present disclosure to provide an approaching vehicle detection apparatus and an approaching vehicle detection method. 
     According to a first example of the present disclosure, an approaching vehicle detection apparatus mounted to a subject vehicle to detect a different vehicle approaching the subject vehicle comprises a ranging device, a communication device, and a ranging synchronization device. The ranging device detects a distance to the different vehicle based on receipt of an infrared light that is transmitted toward the different vehicle and reflected by the different vehicle. The communication device performs transmission and receipt of a signal of infrared light between the subject vehicle and the different vehicle to exchange information when the different vehicle approaches the subject vehicle within a first vehicle-to-vehicle distance. The ranging synchronization device makes transmission timing of the infrared light differ from the subject vehicle to the different vehicle, based on the signal of the communication device. 
     According to the above configuration, because the approaching vehicle detection apparatus makes the infrared light transmission timing differ from the different vehicle based on the signal of the communication device, interference of the infrared light transmitted between the vehicles can be prevented, and the infrared light reflected by the different vehicle can be incident on the subject vehicle correctly. For this reason, accurate distance detection between vehicles can be realized and the detection accuracy of the approach of the different vehicle can improve. Moreover, interference between the infrared light for the communication and the infrared light for the ranging can be prevented, and reliability of the communication between vehicles can improve. Moreover, since infrared light is resistant to disturbance, stable distance detection between vehicles can be realized at low cost. 
     According to a second aspect of the present disclosure, an approaching vehicle detection method comprises a ranging process, a communication process and a ranging synchronization process. The ranging process includes transmitting an infrared light between a subject vehicle and a different vehicle and detecting a distance to the different vehicle based on receipt of the infrared light that is reflected by the different vehicle. The communication process includes transmission and receipt of a signal of infrared light signal between the subject vehicle and the different vehicle to exchange information when the different vehicle approaches the subject vehicle within a first vehicle-to-vehicle distance. The ranging synchronization process includes making transmission timing of the infrared light differ from the subject vehicle to the different vehicle based on the signal transmitted and received in the communication process. 
     According to a third aspect of the present disclosure, an approaching vehicle detection apparatus mounted to a subject vehicle to detect a different vehicle approaching the subject vehicle comprises a ranging device, a communication device, a received strength detection device, an adjustment amount transmission device, and a strength adjustment device. The ranging device detects a distance to the different vehicle based on receipt of an infrared light that is transmitted toward the different vehicle and reflected by the different vehicle. The communication device performs transmission and receipt of a signal of infrared light between the subject vehicle and the different vehicle to exchange information when the different vehicle approaches the subject vehicle within a first vehicle-to-vehicle distance. The received strength detection device detects an infrared strength of the received signal from the different vehicle. The adjustment amount transmission device calculates an adjustment amount with regard to the received signal from the different vehicle based on the infrared strength of the received signal from the different vehicle and the distance to the different vehicle, and transmits the calculated adjustment amount to the different vehicle. The strength adjustment device adjusts an infrared strength of the transmitted signal, which is transmitted to the different vehicle, based on an adjustment amount of an infrared strength received from the different vehicle. 
     In the above approaching vehicle detection apparatus, because the infrared strength of the transmitted signal is adjusted based on the adjustment amount of the infrared strength received from the different vehicle, the infrared strength of the transmitted signal can be appropriately set and unnecessary reception in the vehicles can be reduced. 
     According to a fourth aspect of the present disclosure, an approaching vehicle detection method comprises a ranging process, a communication process, a received strength detection process, an adjustment amount transmission process, and a strength adjustment process. The ranging process includes transmitting an infrared light between a subject vehicle and a different vehicle and detecting a distance to the different vehicle based on receipt of the infrared light that is reflected by the different vehicle. The communication process includes performing transmission and receipt of a signal of infrared light between the subject vehicle and the different vehicle to exchange information when the different vehicle approaches the subject vehicle within a first vehicle-to-vehicle distance. The received strength detection process includes detecting an infrared strength of the received signal from the different vehicle in the communication process. The adjustment amount transmission process includes calculating an adjustment amount with regard to the received signal from the different vehicle based on the infrared strength of the received signal from the different vehicle and the distance to the different vehicle, and transmitting the adjustment amount to the different vehicle. The strength adjustment process includes adjusting an infrared strength of the transmitted signal, which is transmitted to the different vehicle, based on an adjustment amount of an infrared strength received from the different vehicle. 
     In the above approaching vehicle detection method, because the infrared strength of the transmitted signal is adjusted based on the adjustment amount of the infrared strength received from the different vehicle, the infrared strength of the transmitted signal can be appropriately set and unnecessary reception in the vehicles can be reduced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present disclosure will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
         FIG. 1  is a block diagram illustrating an approaching vehicle detection apparatus of a first embodiment; 
         FIG. 2  is a diagram illustrating a principle of a ranging method by an optical device of an approaching vehicle detection apparatus; 
         FIG. 3  is a plan view illustrating a distance measurable area and a communicable area of an approaching vehicle detection apparatus; 
         FIG. 4  is a timing chart illustrating an approaching vehicle detection method in each vehicle; 
         FIG. 5  is a flow chart illustrating a collision avoidance method including approaching vehicle detection; 
         FIG. 6  is a block diagram illustrating a modification of the first embodiment; 
         FIG. 7  is a block diagram illustrating an approaching vehicle detection apparatus of a second embodiment. 
         FIG. 8  is a plan view illustrating a distance measurable area and a communicable area of an approaching vehicle detection apparatus; 
         FIG. 9  is a graph illustrating a relationship between an infrared strength and vehicle-to-vehicle distance; 
         FIG. 10  is a flowchart illustrating an infrared strength adjustment method performed by a controller of each vehicle; 
         FIG. 11  is a flowchart illustrating an infrared strength adjustment method of a first modification of the second embodiment; and 
         FIG. 12  is a flowchart illustrating an infrared strength adjustment method of a second modification of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     First Embodiment 
     An approaching vehicle detection apparatus  1  of a first embodiment will be described based on  FIG. 1  to  FIG. 5 . The approaching vehicle detection apparatus  1  of the present embodiment is mounted to a first vehicle VE 1  and a second vehicle VE 2  and has the same configuration in the first vehicle VE 1  and the second vehicle VE 2 . Therefore, in the below description based on  FIG. 1 , except cases where explanation on both of the first vehicle VE 1  and the second vehicle VE 2  is necessary, only explanation on a configuration of the first vehicle VE 1  may be given and explanation on a configuration of the second vehicle VE 2  may be omitted. In the second vehicle VE 2  of  FIG. 1 , the configuration except an optical device  2  is omitted. In the following, a vehicle VE 1 , VE 2  may be used as a generic term of the first vehicle VE 1  and the second vehicle VE 2 . Further, a subject vehicle VE 1 , VE 2  may be used as a generic term of the first vehicle VE 1  and the second vehicle VE 2 . A different vehicle VE 2 , VE 1  may be used as a generic term of a counterparty of the subject vehicle. 
     As shown in  FIG. 1 , the approaching vehicle detection apparatus  1  of the present embodiment includes two or more optical devices  2 . The optical device  2  corresponds to a ranging device, a ranging means, a communication device, and a communication means. Multiple optical devices  2  may not be necessarily required for each vehicle VE 1 , VE 2 . It may suffice that at least one optical device  2  is mounted to each vehicle VE 1 , VE 2 . The multiple optical devices  2  mounted to the first vehicle VE 1  are arranged on an outer peripheral surface of the first vehicle VE 1 . The optical device  2  includes a light emitter  21  capable of transmitting infrared light and a light receiver capable of receiving infrared light. The light emitter  21  includes a transmission driver for infrared light and a modulation circuit for a digital signal for communication. The light receiver  22  includes a peak hold circuit and a filter circuit for ranging, and a demodulator circuit for the digital signal for communication. 
     The light emitter  21  of the first vehicle VE 1  transmits the infrared light towards an outside of the first vehicle VE 1 . This infrared light is reflected by a body BD 2  of the second vehicle VE 2  and is incident on the light receiver  22 . In this way, the first vehicle VE 1  can detect the distance to the second vehicle VE 2  nearby. Similarly, the infrared light emitted from the light emitter  21  of the second vehicle VE 2  is reflected by a body BD 1  of the first vehicle VE 1  and is incident on the light receiver  22 . In this way, the second vehicle VE 2  can detect the distance to the first vehicle VE 1  (cf. the arrows with hatching in  FIG. 1 ). 
     The infrared light transmitted from the light emitter  21  of either one of the first vehicle VE 1  and the second vehicle VE 2  is incident on the light receiver  22  of the other of the first vehicle VE 1  and the second vehicle VE 2 , so that information exchange and communication can be performed between the first vehicle VE 1  and the second vehicle VE 2  (cf. the solid line or broken line arrows in  FIGS. 1 and 3 ). Specifically, in each of the vehicle VE 1 , VE 2 , a single optical device  2  including the infrared light emitter  21  and the light receiver  22  is made to serve a double purpose as a ranging device and a communication device. For this reason, the single optical device  2  may include multiple light emitters  21  for different roles, which are the ranging and the communication. In this case, each optical device  2  can perform infrared transmission and reception for both of the ranging and the communication. Alternatively, the single light emitter  21  may be used to perform time-sharing transmission, thereby enabling infrared transmission and reception for both of the ranging and the communication. Alternatively, part of a detection element of the light receiver  22  may be used for the ranging and the rest may be used for the communication, so that the single light receiver  22  can be used for both of the ranging and the communication. 
     The controller  3  is connected to the optical device  2 . The controller  3  is an electronic control unit with an I/O device, a processor, a storage etc. The controller  3  includes a ranging calculator  31 , a collision determinator  32 , and an avoid operation driver  33 . The ranging calculator  31  calculates the distance between the first vehicle VE 1  (subject vehicle) and the second vehicle VE 2  (different vehicle) based on a detection value concerning the ranging by the light receiver  22 . The collision determinator  32 , which corresponds to a collision determination device and a collision determination means, determines whether there is a possibility of collision between the first vehicle VE 1  and the second vehicle VE 2 , based on a calculation result of the ranging calculator  31 . When the collision determinator  32  determines that there is a possibility of collision between the first vehicle VE 1  and the second vehicle VE 2 , the avoid operation driver  33  operates a brake of the first vehicle VE 1  irrespective of a driver&#39;s operation, or operates a steering apparatus of the first vehicle VE 1  in order to avoid the collision. Additionally, the avoid operation driver  3  may warn the driver of the first vehicle VE 1 , or operate an airbag. 
     The controller  3  further includes a synchronization determinator  34 , which determines whether it is necessary to make the infrared light transmission timing differ between the first vehicle VE 1  and second vehicle VE 2 , based on the communication result by the light receiver  22 . The controller  3  further includes a synchronous driver  35 , which makes the infrared light transmission timing differ from the first vehicle VE 1  to the second vehicle VE 2  based on the determination result of the synchronization determinator  34 . The synchronization driver  35  pre-stores a program for making the infrared light transmission timing differ. The synchronization determinator  34  and the synchronization driver  35  correspond to a ranging synchronization device and ranging synchronization means. In the present disclosure, the term “synchronize” refers to making the infrared light transmission timing differ from the vehicle VE 1  to the vehicle VE 2 , so that the infrared lights for the ranging do not temporally overlap each other and that the infrared light for the ranging and the infrared light for the communication do not temporally overlap with each other. 
     Now, based on  FIG. 2 , explanation will be given on a specific structure of the optical device  2  and a principle of the ranging manner using the optical device  2  by a triangulation method. In  FIG. 2 , a travel direction of the infrared light from the light source  21   a  (a left-to-right direction in  FIG. 2 ) is a forward direction. The light emitter  21  of the optical device  2  includes a light source  21   a  formed with an infrared light emitting diode (IRLED). The light emitter  21  further includes the light transmission lens  21   b  located on a forward side of the light source  21   a.    
     The light receiver  22  includes a light receiving lens  22   a.  The light receiving lens  22   a  is separated C from the light transmission lens  21   b  in a direction perpendicular to the traveling direction of the infrared light from the light source  21   a.  The light receiver  22  includes a photo detector  22   b,  which is located on a back side of the light receiving lens  22   a  and separated a focal length from the light receiving lens  22   a.  The photo detector  22   b  includes a position sensitive detector (PSD) and is capable of detecting a light quantity centroid position of the incident light spot. 
     As shown in  FIG. 2 , when the infrared light is sent out from the light source  21   a , the infrared light reaches a first detecting object  5   a  located L1 from the light transmission lens  21   b  on a forward side, and is reflected by the first detecting object  5   a.  The reflected infrared light passes through the light receiving lens  22   a  and is incident on the photo detector  22   b.  At this time, it is detected that the centroid position of the spot of the incident infrared light is displaced by Y1 in a direction perpendicular to an optical axis φ of the light receiving lens  22   a.  In this case, a triangle M1-Q-N formed by the first detecting object  5   a,  the light transmission lens  21   b  and the light receiving lens  22   a  has a similarity relation with a triangle N-q-m1 formed by the light receiving lens  22   a  and the photo detector  22   b.  Therefore, an expression (L1/C)=(f/Y1) is met. From an expression L1=(f·C/Y1), a distance L1 to the first detecting object  5   a  is obtained. 
     Let us assume that a second detecting object  5   b  is located L2 (L2&gt;L1) from the light transmission lens  21   b  in the forward direction. In this case, when the infrared light is sent out from the light source  21   a,  the infrared light is reflected by the second detecting object  5   b  and it is detected that the centroid position of the spot of the reflected infrared light is displaced by Y2 (Y2&lt;Y1) in a direction perpendicular to an optical axis φ of the light receiving lens  22   a.  In this case, like the above-mentioned example, a triangle M2-Q-N formed by the second detecting object  5   b,  the light transmission lens  21   b  and the light receiving lens  22   a  has a similarity relation with a triangle N-q-m2 formed by the light receiving lens  22   a  and the photo detector  22   b.  Thus, an expression (L2/C)=(f/Y2) is met. From an expression L2=(f·C/Y2), a distance L2 to the second detection target  5   b  is obtained. 
     An infrared output of the light emitter  21  and a detection sensitivity of the light receiver  22  are adjusted so that the distance measurable area  6  (shown as a blank portion in  FIG. 3 ) of the optical device  2  may be set to have a width Dd in the surrounding of the first vehicle VE 1 . Additionally, the infrared output of the light emitter  21  and the detection sensitivity of the light receiver  22  are adjusted so that an outer peripheral edge of the communicable area  7  is broader than an outer peripheral edge of the distance measurable area  6  by width Dc. In  FIG. 3 , the communicable area  7  is a sum total of the blank portion and the diagonal-line hatched portion. The above mentioned distance (Dc+Dd) or less corresponds to a first vehicle-to-vehicle distance or less. The distance Dd or less corresponds to a second vehicle-to-vehicle distance or less. Although not shown in the drawing, the distance measurable area  6  and the communicable area  7  are also set around the second vehicle VE 2 . 
     As mentioned above, the communicable area  7  of the present embodiment is set more widely than the distance measurable area  6 . Thus, when the second vehicle VE 2  approaches the first vehicle VE 1 , the communication function of the optical device  2  becomes usable earlier than the ranging function. It is conceivable that the communicable area  7  increases depending on environments of surroundings of the vehicles VE 1 , VE 2 , so that the communication is established with a vehicle located far away from the first vehicle VE 1 . In this case, the GPS-based position information of the counterparty vehicle is received, and the communication with the vehicle located far away may be cut off. 
     Next, based on  FIG. 4 , the time chart of the ranging method in the vehicle VE 1 , VE 2  will be explained. In  FIG. 4 , the horizontal axis is a time axis, in which the time passes in a right direction. The IRLED 11  RNG TRNS and the IRLED 21  RNG TRNS of  FIG. 4  show that the light is emitted during the rise of the waveform. The PSD 11  RNG RCV and a PSD 21  RNG RCV of  FIG. 4  show that the approach of the different vehicle is detected when the waveform rises. The distance to the different vehicle detected by the light receiver  22  is held as a peak hold value until next receipt of the infrared light. The IRLED 12  COMM TRNS, the PSD 12  COMM RCV, the IRLED 22  COM TRNS, and the PSD 22  COMM RCV of  FIG. 4  show that the transmission or reception is performed during the rise of the waveform. 
     In  FIG. 4 , a period after the time t 0  is a synchronization period of the ranging and the communication between the vehicle VE 1  and the vehicle VE 2 . A period before the time t 0  is an non-synchronization period. In  FIG. 4 , in an early stage of the non-synchronization period, the vehicle VE 2  does not enter into the communicable area  7  of the first vehicle VE 1 . Therefore, the communication is not established between the vehicles VE 1  and VE 2  and the ranging is performed irrespective of a different vehicle. Thus, the ranging light emission by the first vehicle VE 1  and the ranging light emission by the second vehicle VE 2  overlap temporally with other (shown as a period α). At this time, the second vehicle VE 2  does not enter into the distance measurable area  6  of the first vehicle VE 1 , and both the vehicles VE 1  and VE 2  do not perform the ranging light reception. Additionally, because the first vehicle VE 1  is communicating irrespective of the second vehicle VE 2 , the transmission of the first vehicle VE 1  (shown as S 0 ) temporally overlaps the ranging light emission of the second vehicle VE 2 . 
     At the time t 1 , when the second vehicle VE 2  enters into the communicable area  7  of the first vehicle VE 1 , the first vehicle VE 1  transmits a request message (S 1 ) and the second vehicle VE 2  receives the request message (S 2 ), and thereafter, the second vehicle VE 2  transmits a response message (S 3 ) and the first vehicle VE 1  receives the response message (S 4 ). Accordingly, the communication is established between the vehicles VE 1  and VE 2 . Through the request message and the response message, a method of synchronization of the ranging and the communication is determined between the vehicles VE 1  and VE 2 . 
     When the time enters the synchronization period, the rangings are performed time-divisionally based on the method of synchronization between the vehicles VE 1  and VE 2 . Therefore, after the second vehicle VE 2  enters into the distance measurable area  6  of the first vehicle VE 1  at the time t 2 , the period β for the ranging by the first vehicle VE 1  does not overlap the period γ for the ranging by the second vehicle VE 2 . 
     Moreover, the communication between vehicle VE 1  and VE 2  is also performed time-divisionally, the period δ for the transmission and reception (communication) between the vehicles VE 1  and VE 2  does not overlap the periods β, γ for the ranging by the vehicle VE 1 , VE 2 . 
     The synchronization driver  35  of each vehicle VE 1 , VE 2  pre-stores a program for stopping the light emitter  21  from sending out the infrared light, where the infrared light is sent out for detecting a distance to a different vehicle. For example, either one of the vehicles VE 1 , and VE 2  may stop sending out the infrared light for the ranging during the communication period δ in the synchronization period, so that the detection value of the ranging by the different vehicle is transmitted as a ranging data to the vehicle having stopped sending out the infrared light and becomes usable in both the vehicles VE 1  and VE 2 . 
     In the embodiment, as described above, the ranging may be performed in both vehicles VE 1  and VE 2 , and the detection values of the ranging may be exchanged through the communication and compared with each other, so that a more precise collision determination can be performed. 
     In the above, the explanation is given on cases where the vehicle El is a master vehicle, which carries forward the synchronization of the ranging and the communication. In this relation, when the vehicles VE 1 , VE 2  approach each other, each vehicle enters into the communicable area  7  of the counterparty vehicle. In such cases, the request message from the second vehicle VE 2  may be received by the first vehicle VE 1  early and the second vehicle VE 2  may act as the mater vehicle to carry forward the synchronization of the ranging and the communication. 
     In some cases, the request messages transmitted from the vehicles VE 1  and VE 2  may be simultaneously received by the counterparty vehicles. In such cases, in order to carry forward the synchronization of the ranging and the communication, the program may set the master vehicle in such manners that the master vehicle is set to a vehicle that receives the request message from its front part, or the mater vehicle is set, based on the vehicle position detected by GPS, to a most northerly or easterly vehicle among the vehicles simultaneously receiving the request messages. 
     Next, based on  FIG. 5 , explanation will be given on a collision avoidance method including the approaching vehicle detection method, which is performed by the controller  3  of the first vehicle VE 1  to detect approach of the second vehicle VE 2  and avoid collision with the second vehicle VE 2 . First, during non-establishment of the communication with the second vehicle VE 2 , the controller  3  performs the ranging and the communication using the optical device  2  (S 101 : ranging process). When the second vehicle VE 2  enters into the communicable area  7  of the first vehicle VE 1 , the exchange of the request message and the response message is performed between the vehicles VE 1  and VE 2 , as mentioned above. Based on this, at Step S 102 , the controller  12  determines that the communication is established between the vehicles VE 1  and VE 2  (communication process). In this case, based on the signals in the communication between the vehicles VE 1  and VE 2 , the ranging and the communication are performed in the synchronization, in which the time is divided for the vehicles VE 1  and VE 2  (S 103 : ranging synchronization process). When it is determined at S 102  that the communication is not established between vehicles VE 1  and VE 2 , the ranging and the communication in the non-synchronization continues to be performed until the communication is established between vehicles VE 1  and VE 2 . 
     When the second vehicle VE 2  enters into the distance measurable area  6  of the first vehicle VE 1  after the ranging and the communication in the synchronization are started, the optical device  2  of the first vehicle VE 1  detects the approach of the second vehicle VE 2 . Based on the detection result, calculation of the distance between the vehicles VE 1  and VE 2  is started. When it is determined based on the calculation result that there is a possibility of a collision between the first vehicle VE 1  and the second vehicle VE 2  (S 104 ), the avoidance operation for avoiding the collision is performed with the avoid operation driver  33  of the first vehicle VE 1  (S 105 ). When the second vehicle VE 2  does enter into the distance measurable area  6  of the first vehicle VE 1  and the approach of the second vehicle VE 2  is not detected, or when the approach of the second vehicle VE 2  is detected but it is determined that there is no possibility of a collision between the first vehicle VE 1  and the second vehicle VE 2  at S 104 , the process returns to S 102 . 
     In the embodiment, after the second vehicle VE 2  enters into the communicable area  7  of the first vehicle VE 1  but before the second vehicle VE 2  enters into the distance measurable area  6  of the first vehicle VE 1 , the reclined seat may be returned toward its upright position or a headrest may be moved forward. 
     The approaching vehicle detection apparatus  1  of the present embodiment, which may be mounted to each of multiple vehicles VE 1  and VE 2 , includes the optical device  2 , the synchronization determinator  34 , and the synchronization driver  35 . The optical device  2  transmits the infrared light toward the different vehicle VE 2 , VE 1  and receives the infrared light reflected by the different vehicle VE 2 , VE 1 , thereby detecting the distance to the different vehicle VE 2 , VE 1 . When the different vehicle VE 2 , VE 1  approaches within the communicable area  7 , the optical device  2  performs transmission and reception of a signal of infrared light between the vehicle VE 1  and the vehicle VE 2  to exchange information. Based on the optical device  2 , the synchronization determinator  34  and the synchronization driver  35  make the infrared light transmission timing differ from different vehicle VE 2 , VE 1 . 
     According to the above configuration, because the infrared light transmission timing is changed to differ from the different vehicle VE 2 , VE 1  based on the signal of the optical device  2 , interference of the infrared light transmitted between the vehicles VE 1 , VE 2  can be prevented, and the infrared light reflected by the different vehicle VE 2 , VE 1  can be incident on the subject vehicle VE 1 , VE 2  correctly. For this reason, accurate distance detection between vehicles VE 1 , VE 2  can be realized and the detection accuracy of the approach of the different vehicle VE 2 , VE 1  can improve. Moreover, interference between the infrared light for the communication and the infrared light for the ranging can be prevented, and reliability of the communication between vehicles VE 1 , VE 2  can improve. Moreover, since infrared light is resistant to disturbance, stable distance detection between vehicles VE 1 , VE 2  can be realized at low cost. 
     Moreover, when the different-vehicle VE 2 , VE 1  approaches within the distance measurable areas  6  smaller than the communicable area  7  after the different-vehicle VE 2 , VE 1  approaches within the communicable area  7 , the optical device  2  detects the distance to the different-vehicle VE 2 , VE 1 . Therefore, at a time of starting detecting the distance to the different-vehicle VE 2 , VE 1 , the synchronization has already started. Accordingly, ranging can be performed with high detection accuracy. 
     Moreover, because the synchronization driver  35  of each vehicle VE 1 , VE 2  pre-stores a program for making the infrared light transmission timing differ between the vehicles VE 1 , VE 2 , it is possible to minimize an amount (volume) of information to be exchanged between the vehicles VE 1 , VE 2  for the synchronization. 
     Moreover, in the vehicle VE 1 , VE 2 , the single optical device  2  including the light emitter  21  and the light receiver  22  plays roles of both the ranging and the communication, the approaching vehicle detection apparatus  1  can be downsized and provided at low cost. 
     Moreover, the approaching vehicle detection apparatus  1  includes the collision determinator  32 , which determines a possibility of a collision with the different vehicle VE 2 , VE 1  based on the detection result of the distance to the different vehicle VE 2 , VE 1 . Therefore, the danger of the collision with the different-vehicle VE 2 , VE 1  can be avoided beforehand and the safety of the vehicle VE 1 , VE 2  can improve. 
     Moreover, the synchronization driver  35  pre-stores a program for stopping sending out the infrared light, wherein the infrared light is sent out for detecting the distance to the different vehicle VE 2 , VE 1 . Either one of the vehicle VE 1  and the vehicle VE 2  stops sending out the infrared light for the ranging during the communication period δ in the synchronization period, so that the detection value of the ranging by the different vehicle VE 2 , VE 1  is transmitted as a ranging data to the vehicle having stopped sending out the infrared light and becomes usable in both the vehicles VE 1  and VE 2 . 
     Moreover, an approaching vehicle detection method of the present embodiment includes a ranging process (S 101 ), a communication process (S 102 ), and a ranging synchronization process (S 103 ). In the ranging process, an infrared light is transmitted between a subject vehicle VE 1 , VE 2  and a different vehicle VE 2 , VE 1 , and a distance to the different vehicle VE 2 , VE 1  is detected based on receipt of the infrared light that is reflected by the different vehicle VE 2 , VE 1 . In the communication process, transmission and receipt of a signal of infrared light between the subject vehicle VE 1 , VE 2  and the different vehicle VE 2 , VE 1  is performed to exchange information when the different vehicle VE 2 , VE 1  approaches within the communicable area  7  of the subject vehicle VE 1 , VE 2 . In the ranging synchronization process, transmission timing of the infrared light differs from the different vehicle VE 2 , VE 1  based on the signal transmitted and received in the communication process. According to the above method, interference of the infrared light transmitted between the vehicles VE 1 , VE 2  can be prevented, and the infrared light reflected by the different vehicle VE 2 , VE 1  can be incident on the subject vehicle correctly. For this reason, accurate distance detection between vehicles can be realized and the detection accuracy of the approach of the different vehicle can improve. 
     &lt;Modifications of First Embodiment&gt; 
       FIG. 6  illustrates a modification of the above-mentioned first embodiment. In  FIG. 6 , only the first vehicle VE 1  is depicted and the second vehicle VE 2  is omitted. In  FIG. 1  and  FIG. 6 , like references are used to refer to like parts. In the approaching vehicle detection apparatus  1  of this modification, each vehicle VE 1 , VE 2  is equipped with an optical device  2   a  for ranging, and an optical device  2   b  for the communication. As for other points, this modification is the same as the above mentioned embodiment. Each of the optical device  2   a  for ranging and the optical device  2   b  for the communication includes the light emitter  21  and the light receiver  22  like those in the above mentioned embodiment. In the vehicle VE 1  and VE 2 , the ranging is performed with the optical device  2   a  for ranging to detect a distance to the different-vehicle VE 2 , VE 1 . The communication with the different-vehicle VE 2 , VE 1  is performed with the optical device  2   b  for the communication. 
     According to the approaching vehicle detection apparatus  1  this modification, because the vehicle VE 1 , VE 2  is equipped with the optical device  2   a  for ranging and the optical device  2   b  for the communication, the optical device  2   a  for ranging can be attached a part of the vehicle VE 1 , VE 2  most suitable to perform the ranging for the different vehicle VE 2 , VE 1 , and the optical device  2   b  for the communication can be attached to a part of the vehicle VE 1 , VE 2  most suitable to perform the communication with the different vehicle VE 2 , VE 1 . For this reason, in the approaching vehicle detection apparatus  1 , the ranging accuracy can improve and the performance of the communication with the different-vehicle VE 2 ,VE 1  can improve. 
     &lt;Further Modifications of First Embodiment&gt; 
     The above illustrated first embodiment and modifications thereof do not limit embodiments and can be modified and extended in various ways. 
     In place of IRLED, the light source  21   a  may include a laser light source or a filament type infrared light source. In place of PSD, the photo detector  22   b  may include a CMOS image sensor or a CCD sensor. The approaching vehicle detection apparatus  1  is applicable to a driverless vehicle as well as a manned vehicle. 
     The approaching vehicle detection apparatus and method may be applied to approaching vehicle detection in three or more vehicles. 
     A method for the synchronization driver  35  to make the ranging and communication timing differ between the vehicles VE 1 , VE 2  is not limited to performing the ranging and the communication in a time divisional manner, in which the time is divided in advance. In other embodiments, at each time when the ranging or the communication by one of the vehicles VE 1 , VE 2  is ended, the other of the vehicles VE 2 , VE 1  is notified of it. After being notified of it, the other of the vehicles VE 2 , VE 1  starts performing the ranging or the communication. 
     In other modifications, in determining whether or not there is a possibility of a collision between the first vehicle VE 1  and the second vehicle VE 2 , the collision determinator  32  may take into account not only the detected distance between the first vehicle VE 1  and the second vehicle VE 2  but also a speed, a steering wheel position, a gear shifter position, a heading direction or the like of each vehicle VE 1  and VE 2 . 
     Second Embodiment 
     Based on  FIG. 7  to  FIG. 10 , an approaching vehicle detection apparatus  1  of a second embodiment will be explained. The approaching vehicle detection apparatus  1  of the present embodiment is mounted to a first vehicle VE 1 , a second vehicle VE 2  and a third vehicle VE  3 , and has the same configuration among the first vehicle VE 1 , the second vehicle VE 2  and the third vehicle VE 3 . Therefore, in the below description based on  FIG. 7 , except cases where explanation on the first vehicle VE 1 , the second vehicle VE 2  and the third vehicle VE 3  is necessary, only explanation on a configuration of the first vehicle VE 1  may be given and explanation on a configuration of the second vehicle VE 2  and the third vehicle VE 3  may be omitted. In the second vehicle VE 2  of  FIG. 7 , the configuration except an optical device  2  is omitted. 
     In the following, an optical device  2   a,    2   b  may be used as a genetic term of a front optical device  2   a  and a rear optical device  2   b.  A vehicle VE 1 , VE 2 , VE 3  or a subject vehicle may be used as a genetic term of the first vehicle VE 1 , the second vehicle VE 2  and the third vehicle VE 3 . A different vehicle VE 3 , VE 2 , VE 1  may be used to refer to a vehicle other than the subject vehicle VE 1 , VE 2 , VE 3 . 
     As shown in  FIG. 7  and  FIG. 8 , each vehicle VE 1 , VE 2 , VE 3  is equipped with a forward optical device  2   a  and a rearward optical device  2   b . The forward optical device  2   a  and the rearward optical device  2   b  may have the same configuration. However, the forward optical device  2   a  and the rearward optical device  2   b  may not be necessarily required for each vehicle VE 1 , VE 2 , VE 3 . It may suffice that at least one of the forward optical device  2   a  and the rearward optical device  2   b  is mounted to each vehicle VE 1 , VE 2 , VE 3 . The forward optical device  2   a  mounted to the first vehicle VE 1 , which corresponds to a ranging means, a communication means, a forward ranging device and a forward communication device, is mounted to a front end part of the first vehicle VE 1  and includes a light emitter  21  capable of emitting infrared light and a light receiver  22  capable of receiving an infrared light. 
     The light emitter  21  includes a light source (not shown) formed with an infrared light emitting diode (IRLED). As shown in FIG., a communicable area  7  determined by a maximum communication distance (Dc+Dd) depends on light distribution characteristic of the light emitting diode and may have a cone shape that widens in a forward direction and a rearward direction of the first vehicle VE 1 . The light emitter  21  includes a transmission driver for infrared light and a modulation circuit for a digital signal for communication. 
     The light receiver  22  includes a photo detector including a PSD (Position Sensitive Detector) formed with a photo diode. The light receiver  22  detects a light amount centroid position of the incident light spot. The light receiver  22  includes a peak hold circuit and a filter circuit for the ranging, and a demodulator circuit for the digital signal for the communication. The rearward optical device  2   b  mounted to the first vehicle VE 1  (which corresponds to a ranging means, a communication means, a rearward ranging device, and a rearward communication device) is mounted to a rear end part of the first vehicle VE 1 , and includes a light emitter  21  and light receiver  22  like the forward optical device  2   a.    
     The light emitter  21  of the forward optical device  2   a  of the first vehicle VE 1  transmits the infrared light towards an outside of the first vehicle VE 1 . This infrared light is reflected by a body BD 2  of the second vehicle VE 2  and is incident on the light receiver  22 . In this way, the first vehicle VE 1  can detect the distance to the approaching second vehicle VE 2 . The detection of the distance to the second vehicle VE 2  is performed with a triangulation method. Similarly, the infrared light emitted from the light emitter  21  of the rearward optical device  2   b  of the second vehicle VE 2  is reflected by a body BD 1  of the first vehicle VE 1  and is incident on the light receiver  22 . In this way, the second vehicle VE 2  can detect the distance to the first vehicle VE 1  (cf. the arrows with hatching in  FIG. 7 ). 
     The infrared light transmitted from the light emitter  21  of either one of the forward optical device  2   a  of the first vehicle VE 1  and the rearward optical device  2   b  of the second vehicle VE 2  is incident on the light receiver  22  of the other of the first vehicle VE 1  and the second vehicle VE 2 , so that information exchange and communication can be performed between the first vehicle VE 1  and the second vehicle VE 2  (cf. the solid line and the broken line arrows in  FIGS. 7 and 8 ). Specifically, in each of the vehicles VE 1 , VE 2 , a single optical device  2   a,    2   b  including the infrared light emitter  21  and the light receiver  22  is made to serve a double purpose as a ranging device and a communication device. 
     Each optical device  2   a,    2   b  may include multiple light emitters  21  for different roles, which are the ranging and the communication. In this case, each optical device  2  can perform infrared transmission and reception for both of the ranging and the communication. Alternatively, the single light emitter  21  may be used to perform time-sharing transmission, thereby enabling infrared transmission and reception for both of the ranging and the communication. Alternatively, part of a detection element of the light receiver  22  may be used for ranging and the rest may be used for the communication, so that the single light receiver  22  can perform infrared transmission and reception for both of the ranging and the communication. 
     In the same way as described above, each of the rearward optical device  2   b  of the first vehicle VE 1  and the forward optical device  2   a  of the third vehicle VE 3  (cf.  FIG. 8 ) can function as both of the ranging device and the communication device. 
     Regardless of the present embodiment, when multiple optical devices  2   a,    2   b  are attached in an outer peripheral portion of the first vehicle VE 1 , an infrared output of the light emitter  21  and a detection sensitivity of the light receiver  22  are adjusted so that a distance measurable area  6  (shown as a blank in  FIG. 8 ) of the optical device  2   a,    2   b  is set to have a width of Dd (maximum range) in the surrounding of the first vehicle VE 1 . Additionally, the infrared output of the light emitter  21  and the detection sensitivity of the light receiver  22  are adjusted so that an outer peripheral edge of the communicable area  7  is broader than an outer peripheral edge of the distance measurable area  6  by width Dc. In  FIG. 7 , the communicable area  7  is a sum total of the blank portion and the diagonal-line hatched portion. The above mentioned distance (Dc+Dd) or less corresponds to a first vehicle-to-vehicle distance or less. The distance Dd or less corresponds to a second vehicle-to-vehicle distance or less. Although not shown in the drawing, the distance measurable area  6  and the communicable area  7  are also set around the second vehicle VE 2  and the third vehicle VE 3 . 
     As mentioned above, the communicable area  7  of the present embodiment is set wider than the distance measurable area  6 . Thus, when the second vehicle VE 2  or the third vehicle VE 3  approaches the first vehicle VE 1 , the communication function of the optical device  2   a,    2   b  becomes usable earlier than the ranging function. 
     It may be preferable that depending on environments of the vehicle VE 1 , VE 2 , VE, the maximum communicable distance (Dc+Dd) should be set each time. For example, when the vehicle VE 1 , VE 2 , VE 3  is running a highway, the maximum communicable distance (Dc+Dd) is set to a large distance. When the vehicle VE 1 , VE 2 , and VE 3  is running at a low speed, the maximum communication distance (Dc+Dd) is set to a relatively-small distance. 
     Moreover, it is conceivable that depending on infrared environments of the surrounding of the vehicle VE 1 , VE 2 , VE 3 , the communicable area  7  may increases, and the communication with a vehicle far away from the first vehicle VE 1  may be established. The present embodiment cuts off the communication with the far away vehicle. 
     Explanation returns to  FIG. 7 . The controller  3  is connected to the optical devices  2   a,    2   b.  The controller  3  includes an electronic control unit with an I/O device, a processor, a storage etc. The controller  3  includes a ranging calculator  31 , a collision determinator  32 , and an avoid operation driver  33 . The ranging calculator  31  calculates the distance between the first vehicle VE 1  (subject vehicle) and the second or third vehicle VE 2 , VE 3  (different vehicle) based on a detection value concerning the ranging by the light receiver  22 . The collision determinator  32  (corresponding to a collision determination means) determines whether there is a possibility of collision between the first vehicle VE 1  and the second or third vehicle VE 2 , VE 3 , based on a calculation result of the ranging calculator  31 . When the collision determinator  32  determines that there is a possibility of collision between the first vehicle VE 1  and the second or third vehicle VE 2 , VE 3 , the avoid operation driver  33  operates a brake of the first vehicle VE 1  irrespective of a driver&#39;s operation, or operates a steering apparatus of the first vehicle VE 1  in order to avoid the collision. Additionally, the avoid operation driver  33  may warn the driver of the first vehicle VE 1 , or operate an airbag. 
     The controller  3  further includes a received strength detector  36 . Based on a received signal received by the light receiver  22  from the different vehicle VE 2 , VE 3 , VE 1 , the received strength detector  36  detects a received signal strength (RSSI). An adjustment amount calculator  37 , which corresponds to an adjustment amount transmitter and an adjustment amount transmission means, is connected to the ranging calculator  31 , the received strength detector  36 , and the optical devices  2   a  and  2   b.  Based on the infrared strength of the received signal from the different vehicle VE 2 , VE 3 , VE 1  and the distance to the different vehicle VE 2 , VE 3 , VE 1 , the adjustment amount calculator  37  calculates an adjustment amount of the infrared strength with regard to the received signal from the different vehicle VE 2 , VE 3 , VE 1 , and transmits the adjustment amount to the different vehicle VE 2 , VE 3 , VE 1  through the optical device  2   a,    2   b.    
     Now, based on  FIG. 9 , explanation will be given on a method of calculating the adjustment amount of the infrared strength by the adjustment amount calculator  37  of the first vehicle VE 1 . For example, as shown in  FIG. 9 , the infrared strength Qss of the received signal received by the first vehicle VE 1  from the second vehicle VE 2  is in inverse proportion to the square of the distance Dss between the first vehicle VE 1  and the second vehicle VE 2 . 
     In  FIG. 9 , the solid line Ld shows an ideal line of the infrared strength Qss as a function of the vehicle-to-vehicle distance Dss. Specifically, when the first vehicle VE 1  and the second vehicle VE 2  approach each other and a distance therebetween reaches the maximum distance measurable distance Dd (outer edge of the measureable area  6 ), the infrared light emitted from the light emitter  21  of the first vehicle VE 1  is reflected by the second vehicle VE 2  and is incident on the light receiver  22 , so that the ranging starts. In this case, when the infrared strength of the received signal from the second vehicle VE 2 , which is received by the forward optical device  2   a  of the first vehicle VE 1 , is Qd, the infrared strength at a time when the distance between the first vehicle VE 1  and the second vehicle VE 2  is the maximum communicable distance (Dc+Dd) is a minimum receivable strength Qmin. For this reason, when the second vehicle VE 2  moves out of the communicable area  7  of the first vehicle VE 1  (Dss&gt;(Dc+Dd)), the first vehicle VE 1  is already prevented from receiving the infrared light from the second vehicle VE 2 . 
     As another situation, it is assumed that a relation between the infrared strength and the vehicle-to-vehicle distance becomes the line Ler (shown as the dashed-dotted line in  FIG. 9 ) because the infrared environment around the first vehicle VE 1  has changed and the infrared strength Qss with respect to the vehicle-to-vehicle distance Dss has increased. In this case, even if the second vehicle VE 2  is out of the communicable area  7  of the first vehicle VE 1 , the infrared strength of the received signal from the second vehicle VE 2  may become larger than the minimum intensity Qmin, and the reception from the second vehicle VE 2  occurs. 
     In this case, in the approaching vehicle detection apparatus  1  of the present embodiment, the adjustment amount calculator  37  of the first vehicle VE 1  generates the line Ler based on the infrared strength Qss with respect to the vehicle-to-vehicle distance Dss detected with received strength detector  36 , and detects that the line Ler is out of a region Ldth (hatched region in  FIG. 9 ), which has a predetermined width around the line Ld. The adjustment amount calculator  37  calculates an infrared strength difference ΔQdr between Qd 1  and Qd as the adjustment amount of the infrared strength, where the Qd 1  is the infrared strength on the line Ler at a time when the vehicle-to-vehicle distance is Dd. The adjustment amount of the infrared strength ΔQdr is transmitted to the second vehicle VE 2  via the optical device  2   a.    
     Explanation returns to  FIG. 7 . The controller  3  further includes a strength adjustment driver  38  connected to the optical devices  2   a  and  2   b.  The strength adjustment driver  38  (corresponding to a strength adjustment means) adjusts an infrared strength of the transmitted signal at the vehicle-to-vehicle of Dd based on the adjustment amount ΔQdr of the infrared strength received from the different vehicle VE 2 , VE 3 , VE 1 . 
     The controller  3  is connected to a pyroelectric sensor  4  (corresponding to an infrared detector and an infrared detection means). The pyroelectric sensor  4  detects an infrared amount in space of the surrounding of the vehicle VE 1 , VE 2 , VE 3 . 
     Next, based on  FIG. 10 , explanation will be given on an infrared strength adjustment method performed by the controllers  3  of the first, second and third vehicle VE 1 , VE 2 , VE 3 . In the following explanation, it is assumed that the positional relationship among the vehicle VE 1 , VE 2 , and VE 3  is that shown in  FIG. 7 . In  FIG. 10 , a middle vehicle corresponds to the first vehicle VE 1 , and a forward vehicle corresponds to the second vehicle VE 2 , a rearward vehicle corresponds to the third vehicle VE 3 . First, it is determined (S 401 : ranging process) whether the ranging to measure a distance to the second vehicle VE 2  (corresponding to the forward vehicle) is started by the optical device  2   a  of the first vehicle VE 1 . When the ranging to measure a distance to the second vehicle VE 2  is not started, the determination at S 401  is repeated. 
     When the second vehicle VE 2  enters into the distance measurable area  6  of the first vehicle VE 1  and the ranging to measure the distance to the second vehicle VE 2  is started, the process proceeds to S 402 . Note that at this time, the communication between the vehicles VE 1  and VE 2  is already established. At S 402  (received strength detection process), the received strength detector  36  of the first vehicle VE 1  detects the infrared strength of the received signal from the second vehicle VE 2  (cf. S 421 ). At this time, if the first vehicle VE 1  has not received the signal from the second vehicle VE 2 , the transmission of the signal is requested to the second vehicle VE 2 . Next, at S 403  (adjustment amount transmission process), the adjustment amount calculator  37  calculates the adjustment amount of the infrared strength based on the infrared strength of the received signal from the second vehicle VE 2  and the distance to the second vehicle VE 2 , and transmits the adjustment amount of the infrared strength towards the second vehicle VE 2  through the forward optical device  2   a.    
     At S 422 , the second vehicle VE 2  receives the adjustment amount of the infrared strength from the first vehicle VE 1 , and the process proceeds to S 423  (adjustment amount transmission process). At S 423 , based on the distance to the first vehicle VE 1  and the infrared strength of the received signal from the first vehicle VE 1 , the second vehicle VE 2  calculates an adjustment amount of the infrared strength (not shown in  FIG. 10 ) and transmits the adjustment amount toward the first vehicle VE 1  through the rearward optical device  2  like the first vehicle VE 1  does. 
     At S 404 , the strength adjustment driver  38  of the first vehicle VE 1  receives the adjustment amount of the infrared strength from the second vehicle VE 2 . At S 405 , the strength adjustment driver  38  of the first vehicle VE 1  adjusts the infrared strength of the transmitted signal of the forward optical device  2   a  based on the received adjustment amount of the infrared strength. At S 422 , the strength adjustment driver  38  of the second vehicle VE 2  receives the adjustment amount of the infrared strength from the first vehicle VE 1 . 
     At S 424  (corresponding to the strength adjustment process), the strength adjustment driver  38  of the second vehicle VE 2  adjusts the infrared strength of the transmitted signal of the rearward optical device  2   b  based on the received adjustment amount of the infrared strength. 
     In the above-mentioned flow, S 402  to S 404  and S 421  to S 423  correspond to a communication process. 
     Thereafter, at S 406  (ranging process), it is determined whether the ranging to measure the distance to the third vehicle VE 3  (corresponding to the rearward vehicle) is started by the rearward optical device  2   b  of the first vehicle VE 1 . When the ranging to measure the distance to the third vehicle VE 3  (corresponding to the rearward vehicle) is not started, S 406  is repeated. 
     When the third vehicle VE 3  enters into the distance measurable area  6  of the first vehicle VE 1  and the ranging to measure the distance to the second vehicle VE 3  is started by the first vehicle VE 1 , the process proceeds to S 407 . At this time, the communication between the vehicles VE 1 , VE 3  is already established. At S 407  (received strength detection process), the received strength detector  36  of the first vehicle VE 1  detects the infrared strength of the received signal from the third vehicle VE 3 . At this time, if the first vehicle VE 1  has not received the signal from the third vehicle VE 3 , the transmission of the signal is requested to the third vehicle VE 3 . 
     Because the subsequent flow is the same as described above, explanation on S 408 , S 409 , S 431  to S 433  is omitted. 
     At S 410  (corresponding to the strength adjustment process), the first vehicle VE 1  receiving the adjustment amount of the infrared strength from the third vehicle VE 3  adjusts the infrared strength of the transmitted signal of the rearward optical device  2   b  based on the received adjustment amount. 
     At S 434  (corresponding to the strength adjustment process), the third vehicle VE 3  receiving the adjustment amount of the infrared strength from the third vehicle VE 1  adjusts the infrared strength of the transmitted signal of the forward optical device  2   b  based on the received adjustment amount. In the above-mentioned flow, S 407  to S 409  and S 431  to S 433  correspond to a communication process. 
     As can been seen the above, each time the optical device  2   a,    2   b  detects the distance to the different vehicle VE 2 , VE 3 , VE 1 , the strength adjustment driver  38  adjusts the infrared strength of the transmitted signal. 
     In the above, the explanation is given on cases where the first vehicle VE 1  is a master vehicle and the infrared strength is adjusted in each vehicle VE 1 , VE 2 , VE 3 . Incidentally, when the vehicles VE 1 , VE 2 , VE 3  approach each other and, the vehicles VE 1 , VE 2 , VE 3  enter into the distance measurable areas  6  of the counterparty vehicles. In this case, when the second vehicle VE 2  establishes the ranging to measure the distance to the first vehicle VE 1  earlier, the second vehicle VE 2  acts as a master vehicle and the adjustment of the infrared strength between the first and second vehicles VE 1  and VE 2  is performed. In another case, when the third vehicle VE 3  establishes the ranging to measure the distance to the first vehicle VE 1  earlier, the third vehicle VE 3  acts as a master vehicle and the adjustment of the infrared strength between the first and third vehicles VE 1  and VE 3  is performed. 
     In some cases, the vehicles VE 1 , VE 2  and VE 3  establish their ranging simultaneously. In this case, the adjustment of the infrared strength may be performed according to such a program that the master vehicle is set to a vehicle that receives the signal from its front side, or the mater vehicle is set, based on the vehicle position detected by GPS, to a most northerly or easterly vehicle among the vehicles. 
     The approaching vehicle detection apparatus  1  of the present embodiment adjusts the infrared strength of the transmitted signal based on the adjustment amount of the infrared strength received from the different vehicle VE 2 , VE 3 , VE 1 . Thereby, the infrared strength of the transmitted signal can be appropriately set and unnecessary reception in the vehicles VE 1 , VE 2 , VE 2  can be reduced. 
     Moreover, the strength adjustment driver  38  adjusts the infrared strength of the transmitted signal each time the optical device  2   a,    2   b  starts detecting the distance to the different vehicle VE 2 , VE 3 , VE 1 . Thereby, when the different vehicle VE 2 , VE 3 , VE 1  newly approaches, the infrared strength of the transmitted signal can be adjusted in a timely manner, and necessary communication with the approaching vehicle can be performed in a timely manner. 
     Moreover, when the different vehicle VE 2 , VE 3 , VE 1  approaches within the distance measurable area  6  smaller than the communicable area  7 , the optical device  2   a,    2   b  detects the distance to the different vehicle VE 2 , VE 3 , VE 1 . Therefore, because the signal from the different vehicle VE 2 , VE 3 , VE 1  is receivable at a time of the establishment of the ranging, it becomes possible to promptly detect its infrared strength. 
     Moreover, the optical devices  2   a,    2   b  include the forward optical device  2   a  and the rearward optical device  2   b.  The forward optical device  2   a  detects a distance to the second vehicle VE 2  in front of the first vehicle VE 1 , and performs information exchange with the second vehicle VE 2 . The rearward optical device  2   b  detects the distance to the third vehicle VE 3  in rear of the first vehicle VE 1 , and performs information exchange with the third vehicle VE 3 . The forward optical device  2   a,  which performs the ranging and the communication to the forward vehicle, is separated from the rearward optical device  2   b,  which performs the ranging and the communication to the rearward vehicle. Therefore, each ranging to measure the distance to the second vehicle VE 2  located in front of the first vehicle VE 1  and the third vehicle VE 3  located in rear of the first vehicle VE 1  can be performed with high accuracy. In addition, it becomes possible to improve quality of the communication with the second vehicle VE 2  located in front of the first vehicle VE 1  and quality of the communication with the third vehicle VE 3  located in rear of the first vehicle VE 1 . 
     Moreover, in the vehicle VE 1 , VE 2 , VE 3 , because a single optical device  2   a,    2   b  including the light emitter  21  and the light receiver  22  is made to serve a double purpose as the ranging and the communication, the approaching vehicle detection apparatus  1  can be downsized. 
     Moreover, the approaching vehicle detection apparatus  1  includes the collision determinator  32 , which determines a possibility of a collision with the different vehicle VE 2 , VE 3 , VE 1  based on the detection result of the distance to the different vehicle VE 2 , VE 1  by the optical device  2   a,    2   b.  Therefore, the danger of the collision with the different vehicle VE 2 , VE 1  can be avoided beforehand and the safety of the vehicle VE 1 , VE 2 , VE 3  can improve. 
     Moreover, in the approaching vehicle detection method of the present embodiment, the infrared strength of the transmitted signal is adjusted based on the adjustment amount of the infrared strength received from the different vehicle VE 2 , VE 3 , VE 1 . The infrared strength of the transmitted signal can be appropriately set and unnecessary reception in the vehicles VE 1 , VE 2 , VE 3  can be reduced. 
     &lt;First Modification of Second Embodiment&gt; 
     Next, based on  FIG. 11 , an adjustment method of the infrared strength of a first modification of the second embodiment will be described. A difference from the second embodiment will be mainly explained. In the following explanation, it is assumed that the positional relationship among. the vehicle VE 1 , VE 2 , and VE 3  is that shown in  FIG. 7 . In  FIG. 11 , a middle vehicle corresponds to the first vehicle VE 1 , and a forward vehicle corresponds to the second vehicle VE 2 , a rearward vehicle corresponds to the third vehicle VE 3 . In the present modification, it is determined at S 501  whether a timer tc inside the controller  3  of the first vehicle VE 1  becomes equal to or greater than a threshold ti. When the timer tc is less than the threshold ti, S 501  is repeated. When the timer tc becomes equal to or greater than the threshold ti, it is determined whether or not the ranging to measure the distance to the second vehicle Ve 2  is started by the optical device  2   a  of the first vehicle VE 1 . 
     Out of the subsequent flow, S 503  to S 511 , S 521  to S 524 , and S 531  to S 534  are the same as those in the second embodiment. Thus, explanation on it is omitted. 
     At S 511 , the controller  3  of the first vehicle VE 1  adjusts the infrared strength of the transmitted signal of the rearward optical device  2   b.  Thereafter, the process proceeds to S 512  at which the timer tc is reset. 
     At S 524 , the controller  3  of the second vehicle VE 2  adjusts the infrared strength of the transmitted signal of the rearward optical device  2   b.  Thereafter, the process proceeds to S 525  at which the timer tc is reset. 
     At S 534 , the controller  3  of the third vehicle VE 3  adjusts the infrared strength of the transmitted signal of the forward optical device  2   a.  Thereafter, the process proceeds to S 535  at which the timer tc is reset. 
     As described above, in the approaching vehicle detection apparatus  1  of the present modification, the strength adjustment driver  38  adjusts the infrared strength of the transmitted signal each time a predetermined time ti has elapsed. Therefore, it becomes possible to adjust the infrared strength in response to a surrounding environment change resulting from passage of time. 
     &lt;Second Modification of Second Embodiment&gt; 
     Next, based on  FIG. 12 , an adjustment method of the infrared strength of a second modification of the second embodiment will be described. A difference from the second embodiment will be mainly explained. In the following explanation, it is assumed that the positional relationship among the vehicle VE 1 , VE 2 , and VE 3  is that shown in  FIG. 8 . In  FIG. 12 , a middle vehicle corresponds to the first vehicle VE 1 , and a forward vehicle corresponds to the second vehicle VE 2 , a rearward vehicle corresponds to the third vehicle VE 3 . In the present modification, at S 601 , it is determined whether of not the change ΔQx in the infrared amount in the surrounding space detected with the pyroelectric sensor  4  of the first vehicle Ve 1  becomes greater than or equal to a change amount threshold Qa. When the change ΔQx in the infrared amount is less than the change amount threshold Qa, S 601  is repeated. When the change ΔQx in the infrared amount becomes equal to or greater than the change amount threshold Qa, the process proceeds to S 602 . At S 602 , it is determined whether the ranging to measure a distance to the second vehicle VE 2  is started by the forward optical device  2   a  of the first vehicle VE 1 . 
     Out of the subsequent flow, S 603  to S 611 , S 621  to S 624 , and S 631  to S 634  are the same as those in the second embodiment. Thus, explanation on it is omitted. 
     As described above, in the approaching vehicle detection apparatus  1  of the present modification, the strength adjustment driver  38  adjusts the infrared strength of the transmitted signal each time the change ΔQx in the infrared amount in the surrounding space becomes equal to or greater than the change amount threshold Qa. 
     In the present modification, the strength adjustment driver  38  adjusts the infrared strength of the transmitted signal each time the pyroelectric sensor  4  detects a certain change in the infrared amount in the surrounding space. Therefore, it becomes possible to adjust the infrared strength in response to the change in the infrared amount in the surrounding space. 
     &lt;Further Modifications of Second Embodiment&gt; 
     The above second embodiment and modification thereof do not limit embodiments and can be modified and extended in various ways. 
     In place of IRLED, the light source of the optical device  2   a,    2   a  may include a laser light source or a filament type infrared light source. In place of PSD, the photo detector of the optical device  2   a,    2   b  may include a CMOS image sensor or a CCD sensor. 
     The approaching vehicle detection apparatus  1  is applicable to a driverless vehicle as well as a manned vehicle. 
     Moreover, when the received strength detector  36  detects the infrared strength Qss of the received signal concerning the vehicle-to-vehicle distance, the infrared strength Qss at a time when the distance between the vehicles VE 1 , VE 2 , VE 3  is the maximum range Dd may not be necessarily detected. Alternatively, the infrared strength Qss at a time when the distance between the vehicles VE 1 , VE 2 , VE 3  is another distance may be detected. 
     Moreover, when the received strength detector  36  detects the infrared strength Qss of the received signal concerning the vehicle-to-vehicle distance Dss, the line illustrated in  FIG. 9  may be generated based on multiple infrared strengths Qss corresponding to multiple measured vehicle-to-vehicle distances Dss. 
     Moreover, when the adjustment amount calculator  37  calculates the adjustment amount of the infrared strength Qss, the adjustment amount of the infrared strength Qss at a time when the distance between the vehicles VE 1 , VE 2 , VE 3  is the maximum range Dd may not be necessarily calculated. Alternatively, the adjustment amount of the infrared strength Qss at a time when the distance between the vehicles VE 1 , VE 2 , VE 3  is another distance may be calculated. 
     Moreover, when the adjustment amount calculator  37  calculates the adjustment amount of the infrared strength Qss, the adjustment amount calculator  37  may calculate multiple adjustment amounts based on multiple infrared strengths Qss corresponding to multiple measured vehicle-to-vehicle distances Dss, and calculate an average value of the multiple adjustment amounts as a final adjustment amount. 
     Moreover, in place of the pyroelectric sensor  4 , the infrared detection device may be a photoelectric tube, a photo-conducted type element, a photo voltage type element, a thermocouple type element, or the like. 
     Embodiments of the present disclosure have been illustrated above. However, the above-illustrated embodiments do not limit embodiments of the present disclosure and can be variously modified without departing from the spirit of the present disclosure. For example, embodiments of the present disclosure include an embodiment provided by combining technical parts in different embodiments above and an embodiment provided as part of the embodiment above.