Patent Publication Number: US-2022234620-A1

Title: Vehicle control system and collision avoidance assistance apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2021-010597, filed Jan. 26, 2021, the contents of which application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     Field 
     The present disclosure relates to a vehicle control system having a collision avoidance assistance function and a collision avoidance assistance apparatus. 
     Background Art 
     Various techniques have been proposed for avoiding a collision between a vehicle and an obstacle. For example, JP2010-070069A discloses a prior art relating to control of a vehicle in the case where there is a risk on both the left side and the right side in front of the vehicle. According to the prior art disclosed in JP2010-070069A, when the vehicle detects an oncoming vehicle approaching from the right front and detects a parked vehicle in the left front while traveling in a straight lane, a first risk of the vehicle colliding with the parked vehicle and a second risk of the vehicle colliding with the oncoming vehicle are calculated. Then, a first control threshold is set on the left side of the vehicle based on the first risk, a second control threshold is set on the right side of the vehicle based on the second risk, and actuators are controlled so that the vehicle travels between the first control threshold and the second control threshold. 
     However, the prior art disclosed in JP2010-070069A is directed to a specific situation in which there is a risk on both the left side and the right side in front of the vehicle, and does not correspond to a situation to which the present disclosure is directed. As references showing the state of the art in the technical field to which the present disclosure belongs, for example, JP2009-137385A, JP2019-043405A, and JP2017-224163A can be cited in addition to JP2010-070069A. 
     SUMMARY 
     The present disclosure is directed to a situation in which an ego-vehicle travels in an area in which either one of an ego-lane and an opposing lane adjacent to the ego-lane is not travelable. When the ego-lane is not travelable, the ego-vehicle must enter the opposite lane. And, when the opposite lane adjacent to the ego-lane is not travelable, there is a possibility that an oncoming vehicle enters the ego-lane. In these situations, the risk of collision of the ego-vehicle with an oncoming vehicle increases. 
     The present disclosure has been made in view of the above-described problems. A first object of the present disclosure is to provide a vehicle control system capable of reducing a risk of collision of an ego-vehicle with an oncoming vehicle when the ego-vehicle must enter an opposite lane, or when the opposite lane is not travelable and an oncoming vehicle may enter an ego-lane. A second object of the present disclosure is to provide a collision avoidance assistance apparatus capable of reducing a risk of collision of an ego-vehicle with an oncoming vehicle when the ego-vehicle must enter an opposite lane, or when the opposite lane is not travelable and an oncoming vehicle may enter an ego-lane. 
     A first vehicle control system according to the present disclosure comprises at least one memory storing at least one program and at least one processor coupled with the at least one memory. The at least one program is configured to cause the at least one processor to execute following processing. The processing includes autonomous driving in which an ego-vehicle is autonomously driven based on necessary information for traveling including map information and information on a surrounding environment of the ego-vehicle. Also, the processing includes collision avoidance assistance in which the ego-vehicle is operated to avoid a collision in response to that a risk of collision of the ego-vehicle with a forward obstacle exceeds a threshold. Furthermore, the processing includes lowering the threshold when the ego-vehicle travels, by the autonomous driving, in an area in which either one of an ego-lane and an opposite lane adjacent to the ego-lane is not travelable. Through the above processing, it is possible to reduce a risk of collision of the ego-vehicle with an oncoming vehicle by making the collision avoidance assistance easy to be activated when the ego-lane is not travelable and the ego-vehicle must enter the opposite lane. In addition, even when the opposite lane is not travelable and there is a possibility that an oncoming vehicle enters the ego-lane, it is possible to reduce the risk of collision of the ego-vehicle with an oncoming vehicle by making the collision avoidance assistance easy to be activated. 
     In the first vehicle control system according to the present disclosure, the autonomous driving may include causing the vehicle to travel along a target trajectory determined based on the necessary information. Furthermore, the lowering the threshold may include lowering the threshold when the ego-lane is not travelable and the target trajectory protrudes into the opposite lane. When the risk of collision is increased depending on a positional relationship between the target trajectory and the opposite lane, the risk of collision of the ego-vehicle with an oncoming vehicle can be reduced by making the collision avoidance assistance easy to be activated. 
     In the first vehicle control system according to the present disclosure, the autonomous driving may include temporarily stopping the vehicle before the vehicle enters the above area. Furthermore, the lowering the threshold may include lowering the threshold when or after the vehicle restarts after a temporary stop. Lowering the threshold in conjunction with temporarily stopping the ego-vehicle can reduce the risk of collision. 
     In the first vehicle control system according to the present disclosure, the at least one program may be configured to cause the at least one processor to execute the autonomous driving according to a stop instruction and a start instruction when the stop instruction and the start instruction are performed by a traffic signal or a guide person. Furthermore, the at least one program may be configured to cause the at least one processor to execute holding the threshold or reducing a lowering amount of the threshold when a stop instruction and a start instruction are performed by a traffic signal or a guide person. If a stop instruction and a start instruction are given by a traffic signal or a guide person, it is unlikely that the ego-vehicle will encounter an oncoming vehicle when traveling in the above area. In such a case, by holding the threshold or reducing a lowering amount of the threshold, it is possible to reduce that the collision avoidance assistance is unnecessarily activated by false detection. 
     A second vehicle control system according to the present disclosure comprises at least one memory storing at least one program and at least one processor coupled with the at least one memory. The at least one program is configured to cause the at least one processor to execute following processing. The processing includes autonomous driving in which a vehicle is caused to travel along a target trajectory determined based on necessary information for traveling including map information and information on a surrounding environment of the vehicle. Also, the processing includes collision avoidance assistance in which the ego-vehicle is operated to avoid a collision in response to that a risk of collision of the ego-vehicle with a forward obstacle exceeds a threshold. Furthermore, the processing includes lowering the threshold when the target trajectory protrudes into an opposite lane. Through the above processing, when the target trajectory protrudes into the opposite lane, for example, when the ego-vehicle protrudes into the opposite lane in order to overtake a vehicle ahead, it is possible to reduce the risk of collision of the ego-vehicle with an oncoming vehicle by making the collision avoidance assistance easy to be activated. 
     A collision avoidance assistance apparatus according to the present disclosure is an apparatus configured to operate in response to that a risk of collision of an ego-vehicle with a forward obstacle exceeds a threshold. The collision avoidance assistance apparatus according to the present disclosure is configured to lower the threshold when the ego-vehicle travels in an area in which either one of an ego-lane and an opposite lane adjacent to the ego-lane is not travelable. By such configuration, when the ego-lane is not travelable and the ego-vehicle must enter the opposite lane, the collision avoidance assistance apparatus becomes easy to be activated and the risk of collision of the ego-vehicle with an oncoming vehicle is reduced. Also, even when the opposite lane is not travelable and an oncoming vehicle may enter the ego-lane, the collision avoidance assistance apparatus becomes easy to be activated and the risk of collision of the ego-vehicle with an oncoming vehicle is reduced. 
     According to the present disclosure, the collision avoidance assistance can be activated easily to reduce the risk of collision of the ego-vehicle with an oncoming vehicle in the case where the ego-lane is not travelable and the ego-vehicle is unavoidable to enter the opposite lane, or in the case where the opposite lane is not travelable and an oncoming vehicle is likely to enter the ego-lane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for explaining an outline of vehicle control in a one-side traffic area according to an embodiment of the present disclosure. 
         FIG. 2  is a diagram for explaining an outline of vehicle control in a one-side traffic area according to the embodiment. 
         FIG. 3  is a diagram for explaining an outline of vehicle control in a one-side traffic area according to the embodiment. 
         FIG. 4  is a block diagram showing a configuration of a vehicle control system according to the embodiment. 
         FIG. 5  is a flowchart of vehicle control according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereunder, an embodiment of the present disclosure will be described with reference to the drawings. Note that when the numerals of numbers, quantities, amounts, ranges and the like of respective elements are mentioned in the embodiments shown as follows, the present disclosure is not limited to the mentioned numerals unless specially explicitly described otherwise, or unless the disclosure is explicitly designated by the numerals theoretically. Furthermore, structures and steps that are described in the embodiments shown as follows are not always indispensable to the disclosure unless specially explicitly shown otherwise, or unless the disclosure is explicitly designated by the structures or the steps theoretically. 
     1. Outline of Vehicle Control System 
     1-1. Autonomous Driving and Collision Avoidance Assistance 
     A vehicle control system according to the present embodiment is a system configured to be able to execute autonomous driving for autonomously driving an ego-vehicle and collision avoidance assistance for assisting avoidance of collision of the ego-vehicle with an obstacle. 
     The autonomous driving is carried out based on necessary information for traveling including map information and information on a surrounding environment of the ego-vehicle. Specifically, an optimum route to a destination is determined based on the map information. Then, a traveling plan is drafted to make the ego-vehicle travel safely along the optimum route while following traffic rules. The traveling plan includes operations such as maintaining a current traveling lane, performing a lane change, etc. 
     In the autonomous driving, the target trajectory is generated on the basis of the traveling plan. The target trajectory is a trajectory that the ego-vehicle should ultimately follow, and is determined after considering a collision of the ego-vehicle with all obstacles in front of the ego-vehicle obtained from the information on the surrounding environment of the ego-vehicle. The target trajectory includes a set of target positions of the ego-vehicle in the road on which the ego-vehicle travels, and a target speed for each target position. In the autonomous driving, in order to make the ego-vehicle follow the target trajectory, a deviation between the ego-vehicle and the target trajectory (lateral deviation, yaw angle deviation, speed deviation, etc.) is calculated, and then, the steering, braking, or driving of the ego-vehicle is controlled so that the deviation is reduced. 
     The collision avoidance assistance is a function to operate the ego-vehicle so as to avoid a collision or reduce damage caused by a collision when it is determined that there is a high possibility that an obstacle existing in front of the ego-vehicle will collide with the ego-vehicle. One example of the collision avoidance assistance is a PCS (Pre-Crash Safety). In the PCS, automatic braking by a braking actuator is used as a method of collision avoidance. Hereinafter, it is assumed that the collision avoidance assistance according to the present embodiment is the PCS. 
     A need to actuate the PCS is determined based on a relative relationship between the ego-vehicle and an obstacle. In the present embodiment, the lateral position of the obstacle with respect to the ego-vehicle and the TTC (Time to Collision) of the obstacle with respect to the ego-vehicle are used for determination. Specifically, the PCS is activated when an obstacle whose lateral position is within a predetermined range is certified as an actuation target of the PCS and the TTC of the obstacle certified as the actuation target falls below a predetermined limit time. 
     The magnitude of the risk of collision of the ego-vehicle with an obstacle becomes larger as the lateral position of the obstacle becomes closer to the ego-vehicle, and becomes larger as the TTC of the obstacle becomes smaller. In the present embodiment, the risk of collision is represented by a function using the lateral position and the TTC as parameters. The PCS is then activated when the risk of collision exceeds a predetermined threshold. The threshold of the risk of collision at which the PCS is activated (the activation threshold) is variable. If the activation threshold of the PCS is lowered, the PCS becomes easy to be activated with respect to an obstacle away from the ego-vehicle. Lowering the activation threshold of the PCS includes widening the range for the lateral position of the obstacle to be certified as the actuation target of the PCS and increasing the time limit of the TTC at which the PCS is actuated with respect to the obstacle certified as the actuation target. 
     1-2. Summary of Vehicle Control in One-Side Traffic Area 
     The vehicle control according to the present embodiment is characterized in that the vehicle control is performed in a one-side traffic area in which either one of the ego-lane and the opposite lanes adjacent to the ego-lane is not travelable. The one-side traffic area has two cases, that is, one case in which only the opposite lane is travelable and another case in which only the ego-lane is travelable. In the former case, a first situation occurs in which the ego-vehicle must enter the opposite lane. In the latter case, a second situation occurs in which an oncoming vehicle may enter the ego-lane. 
       FIG. 1  is a diagram for explaining an outline of the vehicle control in the first situation.  FIG. 2  is a diagram for explaining an outline of the vehicle control in the second situation. In each situation, a vehicle (ego-vehicle)  2  is traveling on an opposing two-lane road  100  in which an ego-lane  101  and an opposite lane  102  are separated by a center line  103 . The vehicle  2  is an autonomous driving vehicle in which a vehicle control system  10  capable of executing autonomous driving and collision avoidance assistance is mounted. Further, the vehicle  2  is also a remote supported vehicle that communicates with a remote monitoring center  4  and is operated by remote assistance received from the remote monitoring center  4 . 
     In the first situation shown in  FIG. 1 , there is an obstacle  110  in front of the ego-lane  101 . The obstacle  110  blocks the ego-lane  101  and makes the ego-lane  101  not travelable. Therefore, in order for the vehicle  2  to proceed forward, once the vehicle  2  must enter the opposite lane  102  so as to bypass the obstacle  110 . Examples of the obstacle  110  may include a construction site or a parked large vehicle such as a truck or bus. The obstacle  110  is comprehensively detected by external sensors mounted on the vehicle  2 , or is acquired from a road traffic information communication system. The road traffic information communication system provides road traffic information to vehicles by beacons installed on roads and FM multiplex broadcasting. 
     In the first situation, the vehicle control system  10  temporarily stops the vehicle  2  in front of the obstacle  110 . The stopping position L 10  of the vehicle  2  is determined by the distance to the obstacle  110 . After temporarily stopping, the vehicle control system  10  requests a start instruction from the remote monitoring center  4 . The vehicle control system  10  transmits to the remote monitoring center  4  an image of surroundings of the vehicle  2  captured by a camera of the vehicle  2  when requesting remote assistance from the remote monitoring center  4 . A remote operator in the remote monitoring center  4  checks a condition of the surroundings of the vehicle  2  from a camera image displayed on a display in the remote monitoring center  4 . If it is determined that there is no problem, the remote operator transmits the start instruction to the vehicle control system  10 . 
     Upon receiving the start instruction, the vehicle control system  10  starts the vehicle  2  and generates a target trajectory TR that passes through the opposite lane  102  temporarily so as to bypass the obstacle  110 . The target trajectory TR is generated so that the distance that the vehicle  2  passes through the opposite lane  102  is as short as possible and that the vehicle  2  can safely return to the ego-lane  101 . The vehicle  2  travels along the target trajectory TR and enters the opposite lane  102  beyond the center line  103 . Then, after passing beside the obstacle  110 , the vehicle  2  again returns to the ego-lane  101  beyond the center line  103 . 
     The risk of collision of the vehicle  2  with an oncoming vehicle increases from the time the vehicle  2  enters the opposite lane  102  beyond the center line  103  until it returns to the ego-lane  101  again, i.e., while the vehicle  2  is traveling on the opposite lane  102 . Therefore, the vehicle control system  10  lowers the activation threshold of the PCS below a normal value at the position L 11  where the vehicle  2  enters the opposite lane  102  beyond the centerline  103 . The position L 11  where the activation threshold of the PCS is lowered may, in particular, be a position where the target trajectory TR intersects the center line  103 . Then, at the position L 12  where the vehicle  2  returns from the opposite lane  102  to the ego-lane  101 , the vehicle control system  10  returns the activation threshold of the PCS to the normal value. The position L 12  where the activation threshold of the PCS is reset may, in particular, be a position where the target trajectory TR intersects the center line  103  again. 
     As described above, in the first situation, the vehicle control system  10  temporarily lowers the activation threshold of the PCS while the vehicle  2  is in the opposite lane  102 . This makes the PCS easy to be activated and reduces the risk of collision of the vehicle  2  with an oncoming vehicle. Incidentally, it is assumed that an oncoming vehicle appears during the vehicle  2  is traveling in the opposite lane  102  for bypassing the obstacle  110  and then the PCS is activated. If the vehicle  2  is stopped by actuation of the PCS, for example, the vehicle  2  may move backward by autonomous traveling, or the remote operator may remotely operate the vehicle to move backward. 
     In the second situation shown in  FIG. 2 , an obstacle  110  is in the opposite lane  102 . The obstacle  110  blocks the opposite lane  102  and makes the opposite lane  102  not travelable. For this reason, an oncoming vehicle  111  traveling in the opposite lane  102  must enter the ego-lane  101  once to bypass the obstacle  110 . 
     In the second situation, since the vehicle traveling in the ego-lane  101  has priority over the oncoming vehicle  111 , it is likely that the vehicle traveling in the ego-lane  101  does not anticipate a temporary stop of a preceding vehicle. If the vehicle  2  has a temporary stop unnecessarily, there is a possibility that the vehicle-to-vehicle distance between the vehicle  2  and the subsequent vehicle is adversely affected, that is, the operation of the subsequent vehicle is adversely affected. Therefore, in the second situation, the vehicle control system  10  makes the vehicle  2  pass beside the obstacle  110  without temporarily stopping the vehicle  2 . 
     However, there may be an oncoming vehicle  111  entering the ego-lane  101  from the opposite lane  102  when the vehicle  2  is passing beside the obstacle  110 . Therefore, the vehicle control system  10  temporarily lowers the activation threshold of the PCS than the normal value when the vehicle  2  passes beside the obstacle  110 . More specifically, from a trajectory tr of the oncoming vehicle  111  predicted from the position and size of the obstacle  110 , the position L 22  where the oncoming vehicle  111  enters the ego-lane  101  beyond the center line  103  and the position L 21  where the oncoming vehicle  111  returns from the ego-lane  101  to the opposite lane  102  are predicted. Then, the section including at least from the position L 21  to the position L 22  is a section where the activation threshold of the PCS is lowered. 
     As described above, in the second situation, the vehicle control system  10  makes the vehicle pass beside the obstacle  110  without temporarily stopping the vehicle  2  while temporarily lowering the activation threshold of the PCS. Thus, even when the oncoming vehicle  111  appears in the ego-lane  101 , the PCS can be easily activated and the risk of collision of the vehicle  2  with the oncoming vehicle  111  can be reduced. Incidentally, it is assumed that the oncoming vehicle  111  appears during the vehicle  2  is passing beside the obstacle  110  and then the PCS is activated. If the vehicle  2  is stopped by the activation of the PCS, for example, the vehicle  2  may advance in autonomous traveling after waiting for the oncoming vehicle  111  to move backward, or the remote operator may advance the vehicle  2  while monitoring backward movement of the oncoming vehicle  111 . 
       FIG. 3  shows a third situation to which the vehicle control according to the present embodiment is directed. In the third situation, as in the first situation, an obstacle  110  in front of the ego-lane  101  creates a one-side traffic area where only the opposite lane  102  is travelable. Therefore, in order for the vehicle  2  to proceed forward, once the vehicle  2  must enter the opposite lane  102  so as to bypass the obstacle  110 . However, in the third situation, traffic signals  121 ,  122  are installed in front of and behind the obstacle  110 . A traffic signal  121  in front of the obstacle  110  as viewed from the vehicle  2  is used for safety confirmation of the vehicle  2  passing through the ego-lane  101 , and a traffic signal  122  behind the obstacle  110  as viewed from the vehicle  2  is used for safety confirmation of the oncoming vehicle  111  passing through the opposite lane  102 . The obstacle  110  in the third situation is specifically a construction site. 
     In the third situation, the vehicle control system  10  recognizes the traffic signal  121  present in a foreword detection range SA by an external sensor mounted on the vehicle  2 . Then, the vehicle control system  10  determines whether a signal color of the traffic signal  121  is red or blue by an image recognition process for a camera image. If the signal color is blue, the vehicle control system  10  passes the vehicle  2  as it is. If the signal color is red, the vehicle control system  10  stops the vehicle  2  temporarily, and starts the vehicle  2  after the signal color changes to blue. 
     In the third situation, the oncoming vehicle  111  also follows the traffic signal  122  as the vehicle  2  follows the traffic signal  121 . While the vehicle  2  is traveling on the opposite lane  102 , the traffic light  122  is red so that the oncoming vehicle  111  is stopped behind the obstacle  110 . Thus, in the third situation, it is unlikely that the vehicle  2  will encounter the oncoming vehicle  111  when traveling on the opposite lane  102 . Thus, the vehicle control system  10  holds the activation threshold of the PCS at the normal value rather than lowering the activation threshold of the PCS as in the first situation. 
     Incidentally, the vehicle control in the third situation is also applied to a case where the traffic signals are installed in front of and behind the obstacle  110  in the opposite lane  102  as in the second situation. Again, the oncoming vehicle  111  also follows the traffic signal as well as the vehicle  2  follows the traffic signal. Therefore, rather than lowering the activation threshold of the PCS as in the second situation, the activation threshold of the PCS is held at the normal value. 
     As described above, the vehicle control system  10  holds the activation threshold of the PCS to the normal value in a situation where the stop instruction and the start instruction are generated by the traffic signals  121 ,  122 . If the activation threshold of the PCS is lowered, the activation sensitivity of the PCS is increased, while the PCS is easily activated unnecessarily by false detection. In the first and second situations, since it is prioritized to reduce the risk of collision of the vehicle  2  with an oncoming vehicle, the activation threshold of the PCS is lowered. However, in the third situation, the risk of collision of the vehicle  2  with an oncoming vehicle is not as high as in the first and second situations. Therefore, the activation threshold of the PCS can be held, thereby reducing unnecessary activation of the PCS due to false detection. 
     2. Configuration and Function of Vehicle Control System 
     2-1. Configuration of Vehicle Control System 
       FIG. 4  is a diagram showing an example of a configuration of the vehicle control system  10  according to the present embodiment and the vehicle  2  to which the vehicle control system  10  is applied. The vehicle  2  comprises the vehicle control system  10 , on-board sensors  20  for inputting information to the vehicle control system  10 , and vehicle actuators  30  operated by a signal output from the vehicle control system  10 . In addition, the vehicle  2  comprises a communication device  40  that can be connected to a communication network  6  by mobile communication such as 4G or 5G. The vehicle control system  10  communicates with the remote monitoring center  4  via the communication device  40 . The vehicle control system  10 , the on-board sensors  20 , the vehicle actuators  30 , and the communication device  40  are connected by an in-vehicle network. 
     The on-board sensors  20  include an external sensor  21 , an internal sensor  22 , and a GPS receiver  23 . The external sensor  21  is a sensor for obtaining information on the surrounding environment of the vehicle  2 . The external sensor  21  includes a camera, a millimeter-wave radar, and a LiDAR. Based on the information obtained by the external sensor  21 , processing such as detection of an object existing around the vehicle  2 , measurement of a relative position and relative speed of a detected object with respect to the vehicle  2 , and recognition of a shape of the detected object is performed. The internal sensor  22  is a sensor for obtaining information on the movement of the vehicle  2 . The inner sensor  22  includes, for example, a wheel speed sensor, an acceleration sensor, a yaw rate sensor, and a steering angle sensor. The GPS receiver  23  is used to obtain information on the current position of the vehicle  2 . In addition to these, the vehicle  2  is also provided with a receiver for receiving information from the road traffic information communication system. 
     The vehicle actuators  30  includes a steering actuator  31  for steering the vehicle  2 , a drive actuator  32  for driving the vehicle  2 , and a braking actuator  33  for braking the vehicle  2 . The steering actuator  31  includes, for example, a power steering system, a steer-by-wire steering system, and a rear wheel steering system. The drive actuator  32  includes, for example, an engine, an EV system, and a hybrid system. The braking actuator  33  includes, for example, a hydraulic brake, and a power regenerative brake. 
     The vehicle control system  10  includes an autonomous driving apparatus  11  and a collision avoidance assistance apparatus  12 . As described above, the collision avoidance assistance apparatus  12  according to the present embodiment is a PCS. The autonomous driving apparatus  11  and the collision avoidance assistance apparatus  12  are independent ECUs (Electronic Control Unit). The autonomous driving apparatus  11  includes a processor  11   a  and a memory  11   b.  The collision avoidance assistance apparatus  12  also includes a processor  12   a  and a memory  12   b.  The memories  11   b,    12   b  store various programs and data. The memories  11   b,    12   b  described herein may include, in addition to a narrowly defined memory such as a RAM (Random Access Memory), a data storage device such as a magnetic disk such as an HDD, an optical disk such as a DVD, or a flash memory storage device such as an SSD. Necessary information is input and output between the autonomous driving apparatus  11  and the collision avoidance assistance apparatus  12  via, for example, CAN communication. 
     The autonomous driving apparatus  11  is responsible for the management of the autonomous driving of the vehicle  2  among the functions of the vehicle control system  10 . The memory  11   b  of the autonomous driving apparatus  11  stores an autonomous driving program executable by the processor  11   a  and various information related thereto. The information related to the autonomous driving program includes map information. The map information may be stored in advance in the memory  11   b,  or may be downloaded from an external server via the communication network  6 . Furthermore, the map information on the external server may be referred to. When the autonomous driving program is executed by the processor  11   a,  the processor  11   a  obtains sensor information from the external sensor  21 , recognizes the position of the vehicle  2  on the map, and recognizes the situation around the vehicle  2 . The processor  11   a  makes a traveling plan of the vehicle  2  during autonomous driving based on the position of the vehicle  2  on the map and the situation around the vehicle  2 . In addition, the processor  11   a  generates a target trajectory based on the traveling plan and operates the vehicle actuators  30  such that the vehicle  2  follows the target trajectory. 
     The collision avoidance assistance apparatus  12  is responsible for the management of the activation of the PCS among the functions of the vehicle control system  10 . The memory  12   b  of the collision avoidance assistance apparatus  12  stores a collision avoidance assistance program executable by the processor  12   a  and various information related thereto. By executing the collision avoidance assistance program by the processor  12   a,  the processor  12   a  detects an obstacle in front of the vehicle  2  by the external sensor  21  and calculates the risk of collision of the vehicle  2  with the obstacle based on the information obtained from the external sensor  21  and the internal sensor  22 . Then, if the risk of collision exceeds the activation threshold of the PCS, the processor  12   a  activates the PCS to avoid collision of the vehicle  2  with the obstacle by automatic braking by the braking actuator  33 . Further, as described with reference to  FIGS. 1 to 3 , when the vehicle  2  travels in an area where one of the ego-lane and the opposite lane is not travelable, the processor  11   a  changes the activation threshold of the PCS according to the situation in which the vehicle  2  is placed. 
     2-2. Procedure for Vehicle Control by the Vehicle Control System 
     Next, the procedure of the vehicle control by the vehicle control system  10 , in particular, the procedure of the vehicle control when there is a one-side traffic area in front of the vehicle  2  will be described with reference to a flowchart shown in  FIG. 5 . 
     First, the vehicle control system  10  determines whether there is a one-side traffic area in front of the vehicle  2  based on the information obtained from the external sensor  21  and the road traffic information communication system. When there is no one-side traffic area in front of the vehicle  2 , the vehicle control system  10  skips all processes in this flowchart and holds the activation threshold of the PCS at the normal value (step S 101 ). 
     When there is a one-side traffic area in front of the vehicle  2 , the vehicle control system  10  determines whether a traffic signal is provided in front of the one-side traffic area based on the information obtained by the external sensor  21  (step S 102 ). When a traffic signal is provided, the vehicle control system  10  determines whether the signal color of the traffic signal is blue or red based on a camera image (step S 111 ). When the signal color is red, the vehicle control system  10  stops the vehicle  2  temporarily in front of the traffic signal (step S 112 ). 
     When the signal color is blue, the vehicle control system  10  passes the vehicle  2  without temporarily stopping the vehicle  2 . Further, when the signal color is changed to blue in a situation where the vehicle  2  is stopped temporarily, the vehicle control system  10  restarts the vehicle  2  (step S 113 ). When a traffic signal is provided in the one-side traffic area, not only the vehicle  2  but also an oncoming vehicle follows the signal color of the traffic signal. Thus, the vehicle control system  10  holds the activation threshold of the PCS at the normal value while the vehicle  2  is passing through the one-side traffic area (step S 114 ). 
     When no traffic signal is provided in the one-side traffic area, the vehicle control system  10  determines which side of the ego-lane and the opposite lane is not travelable (step S 103 ). When the opposite lane is not travelable, the ego-lane is the priority lane. In this case, the vehicle control system  10  passes the vehicle  2  through the one-side traffic area without temporarily stopping the vehicle  2  (step S 115 ). 
     Although the vehicle  2  passes through the one-side traffic area while traveling in the ego-lane, there is a possibility that an opposing vehicle will move to the ego-lane from the opposite lane. Therefore, the vehicle control system  10  lowers the activation threshold of the PCS than the normal value (step S 108 ). The activation threshold of the PCS remains lowered until the vehicle  2  passes through the one-side traffic area (step S 109 ). Then, after the vehicle  2  passes through the one-side traffic area, the vehicle control system  10  returns the activation threshold of the PCS to the normal value (step S 110 ). 
     When no traffic signal is provided in the one-side traffic area and the ego-lane is not travelable, the vehicle control system  10  stops the vehicle  2  temporarily in front of the one-side traffic area (step S 104 ). Next, the vehicle control system  10  communicates with the remote monitoring center  4  using the communication device  40  to transmit the camera image of the surroundings of the vehicle  2  to the remote monitoring center  4  and request a start instruction from the remote monitoring center  4  (step S 105 ). The vehicle control system  10  continues the temporary stopping of the vehicle  2  until the start instruction is issued from the remote monitoring center  4  (step S 106 ). Then, when the start instruction is issued from the remote monitoring center  4 , the vehicle control system  10  starts the vehicle  2  (step S 107 ). 
     Although the situation of the opposite lane has been checked by the remote operator in the remote monitoring center  4 , there is a possibility that an oncoming vehicle appears in front of the vehicle  2  after the vehicle  2  enters the opposite lane. Therefore, the vehicle control system  10  lowers the activation threshold of the PCS than the normal value (step S 108 ). The activation threshold of the PCS remains lowered until the vehicle  2  passes through the one-side traffic area (step S 109 ). Then, after the vehicle  2  passes through the one-side traffic area and returns to the ego-lane, the vehicle control system  10  returns the activation threshold of the PCS to the normal value (step S 110 ). 
     According to the vehicle control according to the above-described procedure, it is possible to make the PCS easy to be activated in the case where the vehicle  2  must enter the opposite lane because the ego-lane is not travelable, and in the case where the opposite lane is not travelable and an oncoming vehicle may enter the ego-lane. As a result, it is possible to reduce the risk of collision of the vehicle  2  with an oncoming vehicle in the one-side traffic area. Further, when a traffic signal is provided in the one-side traffic area, it is unlikely that the vehicle will encounter an oncoming vehicle when the vehicle  2  travels in the one-side traffic area. In such a case, since the activation threshold of the PCS is held, it is possible to reduce unnecessary activation of the PCS due to false detection. 
     3. Other Embodiments 
     In the first situation shown in  FIG. 1 , when an oncoming vehicle traveling in the opposite lane is not detected by the external sensor  21 , the activation threshold of the PCS may be lowered below the normal value and the vehicle  2  may be allowed to pass through the one-side traffic area without temporarily stopping. Also, in the third situation shown in  FIG. 3 , the activation threshold of the PCS may be lowered, instead of being held, with a lowering amount of the activation threshold smaller than a lowering amount of the activation threshold in the first and second situations. In the third situation shown in  FIG. 3 , when there is a guide person instead of the traffic signal, the stop and the start of the vehicle  2  may be instructed according to the actions of the guide person. In this case, the actions of the guide person and the instruction contents are associated in advance. 
     The example in which the vehicle  2  enters the opposite lane is not limited to the case in which the ego-lane is not travelable and the vehicle  2  must enter the opposite lane. For example, the vehicle  2  may enter the opposite lane to overtake a preceding vehicle. In such a case as well as in the first situation, lowering the activation threshold of the PCS below the normal value may make the PCS easy to be activated and reduce the risk of collision of the vehicle  2  with an oncoming vehicle. Therefore, when the target trajectory determined based on the necessary information for traveling including the map information and the information on the surrounding environment of the vehicle  2  protrudes to the opposite lane beyond the center line, the vehicle control system  10  may lower the activation threshold of the PCS. 
     In the above-described embodiment, the PCS in which the collision avoidance is performed by braking is exemplified as the collision avoidance assistance, but the collision avoidance assistance by steering or the collision avoidance assistance by both braking and steering is also possible. For example, the collision avoidance assistance by steering is effective when the ego-vehicle returns to the ego-lane because an oncoming vehicle is found at the moment when the ego-vehicle tries to enter the opposite lane beyond the center line, or when the opposite lane including the road shoulder is sufficiently wide. In addition, even in the section where the ego-vehicle passes through an obstacle such as a construction site and returns to the ego-lane, collision of the ego-vehicle with an oncoming vehicle can be avoided by steering the ego-vehicle toward the ego-lane. 
     In the above embodiment, the function of the collision avoidance assistance is implemented in an ECU separate from the autonomous driving apparatus  11 , the function of the collision avoidance assistance can also be incorporated in the autonomous driving apparatus  11 . That is, the collision avoidance assistance program executable by the processor  11   a  and various information related thereto may be stored in the memory  11   b  of the autonomous driving apparatus  11 . That is, as one application of the autonomous driving apparatus  11 , the function of the collision avoidance assistance may be realized. 
     Further, the collision avoidance assistance apparatus according to the present disclosure can be applied not only to the autonomous driving vehicle such as the vehicle  2  of the embodiment described above, also to the manual driving vehicle driven by a driver.