Patent Publication Number: US-2023159035-A1

Title: Vehicle Behavior Estimation Method, Vehicle Control Method, and Vehicle Behavior Estimation Device

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle behavior estimation method, a vehicle control method, and a vehicle behavior estimation device. 
     BACKGROUND 
     Lane change estimation devices are known that estimate that an adjacent vehicle traveling in a second lane adjacent to a first lane where a host vehicle and a preceding vehicle of the host vehicle are traveling changes lanes into the first lane. For example, a lane change estimation device described in JP 2018-147040 A below calculates a first indicator value indicating the likelihood of an adjacent vehicle to change lanes as a larger value the smaller the relative speed between the adjacent vehicle and a preceding vehicle is, and determines the likelihood that the adjacent vehicle will change lanes on the basis of the calculated first indicator value. 
     SUMMARY 
     However, the lane change estimation device described in JP 2018-147040 A makes the determination on the basis of a current value of the relative speed. Therefore, when the relative speed between the adjacent vehicle and the preceding vehicle is small even if a driver of the adjacent vehicle does not intend to change lanes, it may be erroneously determined that a lane change is highly likely to be made. 
     It is an object of the present invention to improve accuracy in estimating the likelihood that an adjacent vehicle will change lanes into a first lane where a host vehicle is traveling. 
     According to an aspect of the present invention, there is provided a vehicle behavior estimation method including: detecting a speed of a first preceding vehicle traveling in front of a host vehicle in a first lane where the host vehicle is traveling; detecting a speed of an adjacent vehicle traveling in a second lane adjacent to the first lane; calculating a relative speed between the first preceding vehicle and the adjacent vehicle; predicting whether or not an absolute value of the relative speed will be at or below a speed threshold value within a predetermined time from a point time when a decrease in the absolute value of the relative speed starts to be detected; and estimating that the adjacent vehicle is likely to change lanes into the first lane when the absolute value of the relative speed is predicted to be at or below the speed threshold value within the predetermined time. 
     According to an aspect of the present invention, it is possible to improve accuracy in estimating the likelihood that an adjacent vehicle will change lanes into a first lane where a host vehicle is traveling. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a diagram illustrating an example of a schematic configuration of a host vehicle provided with a traveling assistance device according to an embodiment; 
         FIG.  2 A  is an illustrative diagram of a positional relationship between a host vehicle, a first preceding vehicle, and an adjacent vehicle; 
         FIG.  2 B  is an illustrative diagram of an example of a vehicle behavior estimation method according to a first embodiment; 
         FIG.  3    is a diagram illustrating an example of a functional configuration of a controller  17  of the first embodiment; 
         FIG.  4    is a flowchart of an example of the vehicle behavior estimation method of the first embodiment; 
         FIG.  5    is an illustrative diagram of an example of a vehicle behavior estimation method of a second embodiment; 
         FIG.  6    is a diagram illustrating an example of a functional configuration of the controller  17  of the second embodiment; 
         FIG.  7    is a flowchart of an example of the vehicle behavior estimation method of the second embodiment; 
         FIG.  8    is an illustrative diagram of a first modification of the second embodiment; 
         FIG.  9    is an illustrative diagram of a second modification of the second embodiment; 
         FIG.  10 A  is an illustrative diagram of an example of a vehicle behavior estimation method of a third embodiment; 
         FIG.  10 B  is an illustrative diagram of an example of the vehicle behavior estimation method of the third embodiment; 
         FIG.  11    is a diagram illustrating an example of a functional configuration of the controller  17  of the third embodiment; 
         FIG.  12    is a flowchart of an example of the vehicle behavior estimation method of the third embodiment; 
         FIG.  13 A  is an illustrative diagram of a positional relationship between the host vehicle, the first preceding vehicle, the adjacent vehicle, and a second preceding vehicle; 
         FIG.  13 B  is an illustrative diagram of an example of a vehicle behavior estimation method of a fourth embodiment; 
         FIG.  14    is a diagram illustrating an example of a functional configuration of the controller  17  of the fourth embodiment; and 
         FIG.  15    is a flowchart of an example of the vehicle behavior estimation method of the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the present invention are described with reference to the drawings. In the descriptions of the drawings, the same or similar portions are denoted by the same or similar reference signs, and duplicate descriptions are omitted. Each drawing is schematic, and may be different from reality. The following embodiments exemplify devices and methods for embodying the technological concept of the present invention, and the technological concept of the present invention is not specific to the devices and methods exemplified in the embodiments given below. The technological concept of the present invention may be modified in various ways within the technological scope described in the claims. 
     First Embodiment 
     (Configuration) 
     A host vehicle  1  includes a traveling assistance device  10  that assists driving of the host vehicle  1 . The traveling assistance device  10  detects a host vehicle position, which is a current position of the host vehicle  1 , and assists traveling of the host vehicle  1  on the basis of the detected host vehicle position. 
     For example, the traveling assistance device  10  automatically controls driving of the host vehicle  1  on the basis of the detected host vehicle position and a surrounding driving environment to assist the traveling of the host vehicle  1 . In other words, the traveling assistance device  10  is a type of vehicle control device that executes vehicle control of the host vehicle  1 . 
     The automatic control of the host vehicle  1  may include, for example, autonomous traveling control in which the host vehicle  1  is driven automatically without the involvement of an occupant (for example, a driver). The automatic control of the host vehicle  1  may also include, for example, automatically controlling at least one of acceleration or deceleration of the host vehicle  1 . 
     Additionally, for example, the traveling assistance device  10  may assist the driving of the host vehicle  1  by the driver according to the traveling environment around the host vehicle  1  to assist the traveling of the host vehicle  1 . On the basis of an estimated host vehicle position and the surrounding traveling environment, the traveling assistance device  10  may output a message for prompting the driver to adjust the inter-vehicle distance and speed, a message for telling the driver the prohibition of acceleration, or a notification sound or a display for prompting the driver to call attention to the surrounding traveling environment to assist driving by the driver. 
     The traveling assistance device  10  includes an object sensor  11 , a vehicle sensor  12 , a positioning device  13 , a map database  14 , a communication device  15 , a navigation system  16 , a controller  17 , an actuator  18 , and an output device  19 . In the drawings, the map database is denoted as “MAP DB”. 
     Note that the object sensor  11  and the controller  17  are an example of a “vehicle behavior estimation device” described in the claims. 
     The object sensor  11  includes a plurality of different types of sensors that detect objects around the host vehicle  1 . 
     For example, the object sensor  11  includes a camera mounted on the host vehicle  1 . The camera captures an image of a predetermined view angle range (shooting range) in front of the host vehicle  1 , and outputs a captured image to the controller  17 . 
     In addition, the object sensor  11  may include a distance measuring sensor such as a laser radar, a millimeter-wave radar, or a LIDAR (light detection and ranging or laser imaging detection and ranging) system. 
     The vehicle sensor  12  is mounted on the host vehicle  1 , and detects a variety of information (vehicle signals) obtainable from the host vehicle  1 . Examples of the vehicle sensor  12  include a vehicle speed sensor that detects a traveling speed (vehicle speed) of the host vehicle  1 , a vehicle wheel speed sensor that detects a rotational speed of each tire mounted on the host vehicle  1 , a triaxial acceleration sensor (G sensor) that detects acceleration (including deceleration) of the host vehicle  1  in three axial directions, a steering angle sensor that detects a steering angle (including a turning angle), a gyro sensor that detects an angular velocity generated in the host vehicle  1 , a yaw rate sensor that detects yaw rate, an accelerator sensor that detects an accelerator position of the host vehicle, and a brake sensor that detects the amount of braking operation by the occupant. 
     The positioning device  13  includes a global navigation satellite system (GNSS) receiver, and receives radio waves from a plurality of navigation satellites to measure a current position of the host vehicle  1 . The GNSS receiver may be, for example, a global positioning system (GPS) receiver or the like. For example, the positioning device  13  may be an inertial navigation system. 
     The map database  14  may store high-definition map data (hereinafter simply referred to as “high-definition map”) suitable as map information for automatic driving. The high-definition map is map data with higher precision than map data for navigation (hereinafter simply referred to as “navigation map”). 
     Road information contained in the high-definition map includes information of information in units of lanes, which is more detailed than information in units of roads. Hereinafter, the information in units of lanes included in the high-definition map data may be denoted as “lane information”. 
     For example, the high-definition map includes, as the lane information, information on a lane node indicating a reference point on a lane reference line (for example, a center line in a lane) and information on a lane link indicating forms of lane sections between lane nodes. 
     The information on a lane node includes an identification number of the lane node, positional coordinates thereof, the number of connected lane links, and identification numbers of the connected lane links. The information on a lane link includes an identification number of the lane link, lane type, lane width, lane demarcation line type, lane shape, lane gradient, and lane demarcation line shape. 
     The high-definition map further includes information on ground objects, such as types and positional coordinates of ground objects such as stop lines, signs, buildings, utility poles, curbs, pedestrian crossings, and architectures that are present on or near lanes, and identification numbers of lane nodes and identification numbers of lane links corresponding to the positional coordinates of the ground objects. 
     The communication device  15  performs wireless communication with a communication device external to the host vehicle  1 . A communication method by the communication device  15  may be, for example, wireless communication through a public mobile phone network, vehicle-to-vehicle communication, road-to-vehicle communication, or satellite communication. 
     The navigation system  16  recognizes the current position of the host vehicle  1  by the positioning device  13 , and acquires map information in the current position from the map database  14 . The navigation system  16  sets a traveling route to a destination input by the occupant, and provides a route guidance to the occupant according to the traveling route. 
     The navigation system  16  also outputs information on the set traveling route to the controller  17 . When performing autonomous traveling control, the controller  17  automatically drives the host vehicle  1  so that the host vehicle  1  travels along the traveling route set by the navigation system  16 . 
     The controller  17  is an electronic control unit (ECU) that performs traveling assistance control of the host vehicle  1 . 
     The controller  17  automatically controls traveling of the host vehicle  1  on the basis of a surrounding traveling environment during the traveling assistance control of the host vehicle  1 . Alternatively, the controller  17  assists driving of the host vehicle  1  by the occupant according to the traveling environment around the host vehicle  1 . 
     Therefore, the controller  17  executes a vehicle behavior estimation method of an embodiment when executing the traveling assistance control of the host vehicle  1 . Details of the vehicle behavior estimation method are described later. 
     The controller  17  includes a processor  21  and peripheral components such as a storage device  22 . The processor  21  may be, for example, a central processing unit (CPU) or a micro-processing unit (MPU). 
     The storage device  22  may include a semiconductor storage device, a magnetic storage device, an optical storage device, or the like. The storage device  22  may include registers, a cache memory, and a memory used as a main storage device, such as a read only memory (ROM) and a random access memory (RAM). 
     Functions of the controller  17  described below are achieved by, for example, causing the processor  21  to execute a computer program stored in the storage device  22 . 
     Note that the controller  17  may be formed by dedicated hardware for executing information processing described below. 
     For example, the controller  17  may include a functional logic circuit that is set in a general-purpose semiconductor integrated circuit. For example, the controller  17  may include a programmable logic device (PLD) such as a field-programmable gate array (FPGA), or the like. 
     The actuator  18  operates a steering mechanism, an accelerator position, and a brake device of the host vehicle  1  to generate a vehicle behavior of the host vehicle  1  in response to control signals from the controller  17 . The actuator  18  includes a steering actuator, an accelerator position actuator, and a brake control actuator. The steering actuator controls a steering direction and a steering amount of the steering mechanism of the host vehicle  1 . 
     The accelerator position actuator controls the accelerator position of the host vehicle  1 . The brake control actuator controls the amount of braking by the brake device of the host vehicle  1 . 
     The output device  19  outputs information that the traveling assistance device  10  presents to the occupant for assisting driving (for example, a message for prompting to adjust the inter-vehicle distance and speed, a message for telling the prohibition of acceleration, or a notification sound or a display for prompting the driver to call attention to a surrounding traveling environment). The output device  19  may include, for example, a display device, lamp, or meter that outputs visual information, or a speaker that outputs audio information. 
     Next, an outline of a vehicle behavior estimation method according to the first embodiment by the controller  17  is described with reference to  FIGS.  2 A and  2 B . 
     As illustrated in  FIG.  2 A , assume that the host vehicle  1  is traveling in a first lane  2   a . In the first lane  2   a , a first preceding vehicle  3 , which is a preceding vehicle of the host vehicle  1 , is traveling in front of the host vehicle  1 . Additionally, in a second lane  2   b , which is a lane adjacent to the first lane  2   a , an adjacent vehicle  4  is traveling. A traveling direction of the first lane  2   a  is the same as a traveling direction of the second lane  2   b.    
     When the controller  17  detects the adjacent vehicle  4  traveling within the range of a predetermined distance from the host vehicle  1  in a front-rear direction, the controller  17  estimates whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     A solid line L 41  in  FIG.  2 B  indicates a change in vehicle speed of the adjacent vehicle  4  with elapsed time, and a broken line L 31  indicates a change in vehicle speed of the first preceding vehicle  3  with elapsed time. 
     When the adjacent vehicle  4  attempts to change lanes into the first lane  2   a  while there is a speed difference between the vehicle speed of the adjacent vehicle  4  and a vehicle speed of a vehicle traveling in the first lane  2   a , the adjacent vehicle  4  changes the vehicle speed of the adjacent vehicle  4 , and adjusts the vehicle speed of the adjacent vehicle  4  so as to reduce the speed difference between the vehicle speed of the vehicle traveling in the first lane  2   a  and the vehicle speed of the adjacent vehicle  4 . In an example of  FIG.  2 B , an absolute value vr1 of a relative speed between the first preceding vehicle  3  and the adjacent vehicle  4  starts to decrease from a time point t0. 
     Thus, the controller  17  determines whether or not the absolute value vr1 of the relative speed between the first preceding vehicle  3  and the adjacent vehicle  4  is decreasing. Then, when the relative speed continues to decrease from the time point t0 when the controller  17  starts to detect that the absolute value vr1 of the relative speed is decreasing, the controller  17  predicts whether or not the absolute value vr1 of the relative speed will be at or below a speed threshold value Tv within a predetermined time T0. Note that the speed threshold value Tv is a relative speed (for example, 4 km/h) that is small enough to determine that the first preceding vehicle  3  and the adjacent vehicle  4  are at substantially the same speed when the absolute value vr1 of the relative speed between the first preceding vehicle  3  and the adjacent vehicle  4  is at or below the speed threshold value Tv. The speed threshold value Tv is a value determined by obtaining through an experiment or the like in advance. 
     The controller  17  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when the controller  17  predicts that the absolute value vr1 of the relative speed will be at or below the speed threshold value Tv within the predetermined time T0. When the controller  17  does not predict that the absolute value vr1 of the relative speed will be at or below the speed threshold value Tv within the predetermined time T0, the controller  17  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     In the example of  FIG.  2 B , the absolute value vr1 is predicted to be at or below the speed threshold value Tv at a time point t1 within the predetermined time T0. Therefore, the controller  17  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Thus, the vehicle behavior estimation method of the first embodiment estimates the likelihood that the adjacent vehicle  4  will change lanes into the first lane  2   a  on the basis of whether the relative speed is decreasing or not and a prediction result on whether or not the absolute value of the relative speed will be at or below the speed threshold value Tv within the predetermined time from a time point where the relative speed starts to decrease. 
     This improves the accuracy of the estimation compared with estimating the likelihood of a lane change on the basis of a current value of the relative speed. 
     Next, functions of the controller  17  are described in detail with reference to  FIG.  3   . The controller  17  includes a host vehicle position estimation unit  30 , a lane structure acquisition unit  31 , an object detection unit  32 , an object tracking unit  33 , a target vehicle identification unit  34 , a surrounding vehicle state acquisition unit  35 , a vehicle behavior estimation unit  36 , a tracking target setting unit  37 , a traveling trajectory generation unit  38 , and a traveling control unit  39 . 
     The host vehicle position estimation unit  30  estimates a host vehicle position on the map stored in the map database  14  where the host vehicle  1  is currently traveling, a posture of the host vehicle  1  (for example, the direction of traveling of the host vehicle  1 ), and a speed of the host vehicle  1 . 
     The host vehicle position estimation unit  30  estimates the host vehicle position of the host vehicle  1  on the basis of, for example, a positioning result of the positioning device  13 , odometry by the vehicle sensor  12 , and a result of detection of a target around the host vehicle  1  by the object sensor  11 . 
     The lane structure acquisition unit  31  acquires information on a lane structure, which is the structure of lanes, intersections, and surrounding ground objects (for example, curbs) in front of the host vehicle  1  on a road where the host vehicle  1  is traveling, from the map database  14  on the basis of the host vehicle position of the host vehicle  1  estimated by the host vehicle position estimation unit  30 . 
     he object detection unit  32  detects objects around the host vehicle  1  by the object sensor  11 , and acquires the relative position, speed, and size of each of the surrounding objects with respect to the host vehicle  1 . The object detection unit  32  determines whether the detected object is a vehicle or not. For example, when the position of the detected object is on the road and the size of the object is within a predetermined range assumed as a vehicle size, the object detection unit  32  determines that the detected object is a vehicle. 
     The object detection unit  32  detects the positions, postures (for example, the directions of traveling), and speeds of detected other vehicles around the host vehicle  1 . 
     The object tracking unit  33  tracks the objects detected by the object detection unit  32 . Specifically, the object tracking unit  33  integrates detection results at a current time of each object output from the object detection unit  32  with past-time detection results to verify (associate) the identity of the each object between different times, and tracks the relative position, speed, and posture of the each object around the host vehicle  1  with respect to the host vehicle  1  on the basis of the association. 
     The target vehicle identification unit  34  identifies the first preceding vehicle  3  traveling in front of the host vehicle  1  in the first lane  2   a  and the adjacent vehicle  4  traveling in the second lane  2   b  on the basis of the lane structure around the host vehicle  1  acquired by the lane structure acquisition unit  31 , the object detection results by the object detection unit  32 , and the tracking results by the object tracking unit  33 . 
     The target vehicle identification unit  34  may identify, as a lane change determination target, the adjacent vehicle  4  traveling within the range of a predetermined distance previously determined in the front-rear direction from the host vehicle  1 . For example, the adjacent vehicle  4  traveling in the second lane  2   b  within a range from a front-rear direction position of the host vehicle  1  to a front-rear direction position of the first preceding vehicle  3  may be identified as a lane change determination target. 
     Alternatively, the adjacent vehicle  4  traveling in the second lane  2   b  within a range from the front-rear direction position of the host vehicle  1  to a position rearward by a predetermined distance may be identified as a lane change determination target. 
     It should be noted that although the present example uses the relative positional relationships and relative distances between the host vehicle  1  and the adjacent vehicle  4  and between the first preceding vehicle  3  and the adjacent vehicle  4  to identify the adjacent vehicle  4  as the lane change determination target, the present invention is not limited thereto. The adjacent vehicle  4  as the lane change determination target may be identified using other indicators that can define a length of spacing between the vehicles, such as inter-vehicle times between the host vehicle  1  and the adjacent vehicle  4  and between the first preceding vehicle  3  and the adjacent vehicle  4 . In other words, among vehicles traveling in the second lane  2   b , the adjacent vehicle  4  that is present in such a position that it may change lanes and interrupt between the host vehicle  1  and the first preceding vehicle  3  is identified as the adjacent vehicle  4  that is a lane change determination target. 
     When the adjacent vehicle  4  as the lane change determination target is identified, the controller  17  estimates whether or not the adjacent vehicle  4  will change lanes by continuously monitoring the likelihood of the adjacent vehicle  4  as the determination target to change lanes. Therefore, the controller  17  repeatedly estimates the likelihood of the adjacent vehicle  4  to change lanes at different times, and accumulates the estimation results to calculate a “certainty degree” (i.e., the level of likelihood) of a lane change as a variable. 
     The target vehicle identification unit  34  determines whether or not the adjacent vehicle  4  is identified for the first time as a lane change determination target, and initializes the certainty degree for the adjacent vehicle  4  to “0” when the adjacent vehicle  4  is identified for the first time as a lane change determination target. 
     The surrounding vehicle state acquisition unit  35  generates a speed profile of the adjacent vehicle  4  and a speed profile of the first preceding vehicle  3 . For example, the surrounding vehicle state acquisition unit  35  may generate, as a speed profile, a history of vehicle speeds output from the object tracking unit  33  at each time from a past time point by a predetermined time to the current time. 
     The vehicle behavior estimation unit  36  estimates whether or not the adjacent vehicle  4  will change lanes into the first lane  2   a  (for example, a section between the host vehicle  1  and the first preceding vehicle  3  in the first lane  2   a ) on the basis of the speed profile of the adjacent vehicle  4  and the speed profile of the first preceding vehicle  3 . 
     The vehicle behavior estimation unit  36  includes a vehicle speed prediction unit  36   a  and an intention estimation unit  36   b.    
     The vehicle speed prediction unit  36   a  determines whether or not the following condition A for estimating the likelihood of the adjacent vehicle  4  to change lanes into the first lane  2   a  is satisfied on the basis of the speed profile of the adjacent vehicle  4  and the speed profile of the first preceding vehicle  3  generated by the surrounding vehicle state acquisition unit  35 . 
     (Condition A): It is predicted that the adjacent vehicle  4  is trying to match the vehicle speed thereof to the vehicle speed of the first preceding vehicle  3 , and it is predicted that a vehicle speed difference between the vehicle speed of the adjacent vehicle  4  and the vehicle speed of the first preceding vehicle  3  will be at or below the speed threshold value Tv by the time the predetermined time T0 elapses. 
     The predetermined time T0 may be obtained, for example, experimentally, and may be set to, for example, from 3.0 to 5.0 [sec]. 
     The vehicle speed prediction unit  36   a  calculates an acceleration α from the speed profile of the adjacent vehicle  4 , and adds a product of an elapsed time Δt from the current time and the acceleration α to a current vehicle speed Vs(t) to calculate a future predicted speed Vsp of the adjacent vehicle  4  (Vsp=Vs(t)+α×Δt). Note that here, the acceleration α is an acceleration in which acceleration in the traveling direction is positive, and acceleration in a direction opposite to the traveling direction is negative. 
     Similarly, a future predicted speed Vlp of the first preceding vehicle  3  is calculated. 
     The vehicle speed prediction unit  36   a  predicts whether or not the adjacent vehicle  4  is trying to match the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3  on the basis of the absolute value vr1 of the relative speed, which is a speed difference between the predicted speed Vsp and the predicted speed Vlp. 
     For example, when it is detected that the absolute value vr1 is decreasing, the vehicle speed prediction unit  36   a  predicts that the adjacent vehicle  4  is trying to match the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 . When it is not detected that the absolute value vr1 is decreasing, the vehicle speed prediction unit  36   a  predicts that the adjacent vehicle  4  is not trying to match the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 . Note that when the absolute value vr1 starts to decrease and then continues to decrease for a previously determined predetermined time (for example, 1.0 sec) that is shorter than the predetermined time T0, it may be detected that the absolute value vr1 is starting to decrease. This prevents the adjacent vehicle  4  from being predicted to be trying to match the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3  when the adjacent vehicle  4  is temporarily decelerating without intending to match the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 . 
     Furthermore, the vehicle speed prediction unit  36   a  determines whether or not the absolute value vr1 will be at or below the speed threshold value Tv within the predetermined time T0 from the start of the decrease in the absolute value vr1. When the absolute value vr1 is at or below the speed threshold value Tv within the predetermined time T0 from the start of the decrease in the absolute value vr1, the vehicle speed prediction unit  36   a  determines that the condition A is satisfied. 
     On the contrary, when the absolute value vr1 is not at or below the speed threshold value Tv within the predetermined time T0 from the start of the decrease in the absolute value vr1, the vehicle speed prediction unit  36   a  determines that the condition A is not satisfied. 
     The intention estimation unit  36   b  estimates whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  on the basis of the determination result of the vehicle speed prediction unit  36   a . The intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when the condition A is satisfied, and does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when the condition A is not satisfied. 
     The intention estimation unit  36   b  repeatedly estimates the likelihood of the adjacent vehicle  4  to change lanes on the basis of whether the condition A is satisfied or not, which has been repeatedly determined at different times by the vehicle speed prediction unit  36   a.    
     The intention estimation unit  36   b  accumulates estimation results obtained by the repeated estimations, and calculates a certainty degree. For example, each time the intention estimation unit  36   b  estimates that there is a likelihood of a lane change, the intention estimation unit  36   b  increases the certainty degree by a predetermined step amount (for example, the certainty degree is counted up). On the other hand, when the intention estimation unit  36   b  does not estimate that there is a likelihood of a lane change, the intention estimation unit  36   b  does not change the certainty degree or increases the certainty degree by a step amount smaller than the step amount when it is estimated that there is the likelihood of a lane change. 
     When the certainty degree exceeds a threshold value, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a.    
     When the certainty degree does not exceed the threshold value (for example, when the adjacent vehicle  4  accelerates and moves more forward than the first preceding vehicle  3  before the certainty degree exceeds the threshold value, or when the adjacent vehicle  4  decelerates and moves more rearward than the host vehicle  1  before the certainty degree exceeds the threshold value, the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  will change lanes into the first lane  2   a.    
     When an inter-vehicle distance control such as adaptive cruise control (ACC) is executed in the host vehicle  1 , the tracking target setting unit  37  sets a vehicle being a tracking target for the inter-vehicle distance control. Note that an inter-vehicle distance in the inter-vehicle distance control is an inter-vehicle distance in a direction along the traveling direction of the host vehicle. 
     When the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a , the tracking target setting unit  37  sets the adjacent vehicle  4  as a tracking target for the inter-vehicle distance control. 
     When it is not estimated that the adjacent vehicle  4  will change lanes into the first lane  2   a , the tracking target setting unit  37  sets the first preceding vehicle  3  as a tracking target as it is. 
     When the adjacent vehicle  4  is set as the tracking target for the inter-vehicle distance control, the traveling trajectory generation unit  38  generates a target traveling trajectory and a target speed profile of the host vehicle  1  on the basis of the relative speed and relative positional relationship between the adjacent vehicle  4  and the host vehicle  1 . Note that the target speed profile refers to a target speed according to an elapsed time. 
     At this time, the traveling trajectory generation unit  38  generates the target speed profile so as to maintain the inter-vehicle distance between the host vehicle  1  and the adjacent vehicle  4 . Here, the absolute value vr1 of the relative speed between the first preceding vehicle  3  and the adjacent vehicle  4  is at or below the speed threshold value Tv. Accordingly, generating the target speed profile so as to maintain the inter-vehicle distance between the host vehicle  1  and the adjacent vehicle  4  allows the target speed profile to be generated so as to maintain the inter-vehicle distance between the host vehicle  1  and the first preceding vehicle  3 . 
     When the first preceding vehicle  3  is set as the tracking target for the inter-vehicle distance control, the traveling trajectory generation unit  38  generates the target traveling trajectory and target speed profile of the host vehicle  1  on the basis of the relative speed and relative positional relationship between the host vehicle  1  and the first preceding vehicle  3 . 
     Additionally, when the inter-vehicle distance control is not executed in the host vehicle  1 , the traveling trajectory generation unit  38  generates a driving action plan for the host vehicle  1  on the basis of a route space map representing the presence or absence of routes and objects around the host vehicle  1  and a risk map quantifying the danger of a driving field. 
     The traveling trajectory generation unit  38  generates a target traveling trajectory for driving the host vehicle  1  on the basis of the planned driving action, motion characteristics of the host vehicle  1 , the route space map, and the risk map. 
     When the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a , the traveling trajectory generation unit  38  generates any of a target speed profile that increases or maintains the inter-vehicle distance between the host vehicle  1  and the first preceding vehicle  3 , a target speed profile that reduces or maintains the vehicle speed of the host vehicle  1 , or a target speed profile at which the host vehicle  1  does not accelerate. 
     This can facilitate the adjacent vehicle  4  to change lanes into the first lane  2   a  (for example, the section between the host vehicle  1  and the first preceding vehicle  3  in the first lane  2   a ). 
     The traveling control unit  39  drives the actuator  18  so that the host vehicle  1  travels on the target traveling trajectory at a speed according to the target speed profile generated by the traveling trajectory generation unit  38 . 
     As a result, when the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a , the traveling control unit  39  executes at least one of a vehicle control that increases or maintains the inter-vehicle distance between the host vehicle  1  and the first preceding vehicle  3 , a vehicle control that reduces or maintains the vehicle speed of the host vehicle  1 , or a vehicle control that prohibits acceleration of the host vehicle  1 . 
     In addition, when the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a , the output device  19  may execute driving assistance by outputting a message such as a sound and/or a display that tells the driver. 
     For example, the output device  19  may output any one of a message that prompts to increase or maintain the inter-vehicle distance between the host vehicle  1  and the first preceding vehicle  3 , a message that prompts to reduce or maintain the vehicle speed of the host vehicle  1 , a message that prohibits acceleration of the host vehicle  1 , or a message that prompts to call attention to the surrounding traveling environment. 
     (Operation) 
     Next, an example of the vehicle behavior estimation method by the traveling assistance device  10  according to the first embodiment is described with reference to  FIG.  4   . 
     At step S 1 , the host vehicle position estimation unit  30  estimates the host vehicle position of the host vehicle  1 . The lane structure acquisition unit  31  acquires information on a lane and an intersection in front of the host vehicle  1  and a lane structure therearound. The object detection unit  32  detects objects around the host vehicle  1  by the object sensor  11 , and the object tracking unit  33  tracks each of the objects detected by the object detection unit  32 . 
     At step S 2 , the target vehicle identification unit  34  determines whether or not the adjacent vehicle  4  to be determined as to whether or not to change lanes is present around the host vehicle  1 . When the adjacent vehicle  4  is present (step S 2 : Y), processing proceeds to step S 3 . When the adjacent vehicle  4  is not present (step S 2 : N), processing ends. 
     At step S 3 , the target vehicle identification unit  34  determines whether or not the adjacent vehicle  4  is detected for the first time as a lane change determination target. When the adjacent vehicle  4  is detected for the first time (step S 3 : Y), processing proceeds to step S 4 . If the adjacent vehicle  4  has already been detected as the lane change determination target in a previous processing loop (step S 3 : N), processing proceeds to step S 5 . 
     At step S 4 , the target vehicle identification unit  34  initializes the “certainty degree”, which is the variable that indicates the level of likelihood of the adjacent vehicle  4  to change lanes, to “0”. Then, processing proceeds to step S 5 . 
     At step S 5 , the surrounding vehicle state acquisition unit  35  generates the speed profile of the adjacent vehicle  4  and the speed profile of the first preceding vehicle  3 . If the speed profiles have already been generated in the previous processing loop, new speed profiles are generated again. Here, assuming that the adjacent vehicle  4  and the first preceding vehicle  3  maintain the current accelerations, as illustrated in  FIG.  2 B , the speed profile of the adjacent vehicle  4  and the speed profile of the first preceding vehicle  3  can be generated from the current vehicle speeds and accelerations of the adjacent vehicle  4  and the first preceding vehicle  3 . 
     At step S 6 , the vehicle speed prediction unit  36   a  determines whether the above condition A is satisfied or not. When the condition A is satisfied (step S 6 : Y), processing proceeds to step S 7 . When the condition A is not satisfied (step S 6 : N), processing proceeds to step S 8 . 
     At step S 7 , the intention estimation unit  36   b  increases the certainty degree by a predetermined step amount. 
     At step S 8 , the intention estimation unit  36   b  determines whether or not the certainty degree is larger than a threshold value. When the certainty degree is larger than the threshold value (step S 8 : Y), processing proceeds to step S 9 . 
     At step S 9 , the intention estimation unit  36   b  estimates that the adjacent vehicle  4  will change lanes into the first lane  2   a . Then, processing ends. 
     On the other hand, at step S 8 , when it is not determined that the certainty degree is larger than the threshold value (step S 8 : N), processing returns to step S 1 . 
     At step S 1 , the host vehicle position of the host vehicle  1  is estimated again, information on the lane structure is acquired, objects around the host vehicle  1  are detected, and each object is tracked. 
     At step S 2 , when the adjacent vehicle  4  is present (step S 2 : Y), steps S 3  to S 8  are repeated. 
     On the other hand, when the adjacent vehicle  4  is not present (step S 2 : N), for example, when the adjacent vehicle  4  accelerates and moves more forward than the first preceding vehicle  3  or when the adjacent vehicle  4  decelerates and moves more rearward than the host vehicle  1 , the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  will change lanes into the first lane  2   a , and processing ends. 
     Effects of First Embodiment 
     (1) The object detection unit  32  detects the speed of the first preceding vehicle  3  traveling in front of the host vehicle  1  in the first lane  2   a  where the host vehicle  1  is traveling, and detects the speed of the adjacent vehicle  4  traveling in the second lane  2   b  adjacent to the first lane  2   a . The vehicle speed prediction unit  36   a  calculates the relative speed between the first preceding vehicle  3  and the adjacent vehicle  4 , and predicts whether or not the absolute value of the relative speed will be at or below the speed threshold value within the predetermined time from the time point where the decrease in the absolute value of the relative speed starts to be detected. When it is predicted that the absolute value of the relative speed will be at or below the speed threshold value within the predetermined time, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Thus, the intention of the adjacent vehicle  4  to change lanes is estimated on the basis of the change over time in the absolute value of the relative speed. This can prevent an erroneous determination in situations where an erroneous determination occurs in the conventional technology, for example, such as a case where a relative speed with a preceding vehicle happens to be small. 
     Additionally, when the adjacent vehicle  4  changes lanes between the host vehicle  1  and the first preceding vehicle  3 , the adjacent vehicle  4  matches to the vehicle speed of the first preceding vehicle  3  in many cases. The first embodiment estimates the likelihood of the lane change by focusing on the change over time of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3 . Therefore, the intention of the adjacent vehicle  4  to change lanes can be detected early. 
     (2) The intention estimation unit  36   b  may calculate the certainty degree by accumulating the estimation results obtained by repeatedly estimating the likelihood of the adjacent vehicle  4  to change lanes between the host vehicle  1  and the first preceding vehicle  3 . The intention estimation unit  36   b  may estimate that the adjacent vehicle  4  will change lanes into the first lane  2   a  when the certainty degree exceeds the threshold value. 
     Thus, the accuracy of the estimation can be further improved by estimating on the basis of the results obtained by repeatedly estimating at a plurality of different times. 
     Second Embodiment 
     (Configuration) 
     Next, a second embodiment is described. The vehicle behavior estimation unit  36  of the second embodiment determines whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  on the basis of a condition regarding a positional relationship between the adjacent vehicle  4  and the first preceding vehicle  3  when changing lanes, in addition to the condition A of the first embodiment regarding the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3 . 
     Reference is made to  FIG.  5   . A solid line L 42  indicates a relative position in the front-rear direction of the adjacent vehicle  4  with respect to the host vehicle  1  (i.e., a relative distance in the front-rear direction from the host vehicle  1  to the adjacent vehicle  4 ), and a broken line L 32  indicates a relative position in the front-rear direction of the first preceding vehicle  3  with respect to the host vehicle  1  (i.e., a relative distance in the front-rear direction from the host vehicle  1  to the first preceding vehicle  3 ). Note that  FIG.  5    illustrates a state in which the host vehicle  1  is following the first preceding vehicle  3  and traveling at the same speed as that of the first preceding vehicle  3 . 
     When the adjacent vehicle  4  changes lanes into the first lane  2   a  while matching the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 , the adjacent vehicle  4  is considered to change lanes into a range behind the first preceding vehicle  3 . 
     Accordingly, the vehicle behavior estimation unit  36  determines whether or not the front-rear direction position of the adjacent vehicle  4  is located behind the front-rear direction position of the first preceding vehicle  3  at a time point t1 where the absolute value vr1 of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3  of the first embodiment is at or below the speed threshold value Tv. 
     When the above condition A is satisfied and the front-rear direction position of the adjacent vehicle  4  is located behind the front-rear direction position of the first preceding vehicle  3  at the time point t1, the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a . For example, the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into a range behind the first preceding vehicle  3 . 
     On the other hand, when the above condition A is not satisfied or the front-rear direction position of the adjacent vehicle  4  is not located behind the front-rear direction position of the first preceding vehicle  3  at the time point t1, the vehicle behavior estimation unit  36  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Functions of the controller  17  of the second embodiment are described in detail with reference to  FIG.  6   . The controller  17  of the second embodiment has the same configuration as that of the first embodiment. Among components of the controller  17  of the second embodiment, the same components as those of the first embodiment are denoted by the same reference signs. 
     The vehicle behavior estimation unit  36  of the second embodiment includes a position prediction unit  36   c.    
     The position prediction unit  36   c  predicts a front-rear direction position of the adjacent vehicle  4  and a front-rear direction position of the first preceding vehicle  3  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv on the basis of the predicted speed Vsp of the adjacent vehicle  4  and the predicted speed Vlp of the first preceding vehicle  3  calculated by the vehicle speed prediction unit  36   a.    
     For example, a predicted position Psp of the front-rear direction of the adjacent vehicle  4  can be predicted as a sum of a product of the elapsed time Δt from the current time and the predicted speed Vlp plus a current front-rear direction position Ps(t) of the adjacent vehicle  4  (Psp=Ps(t)+Vsp×Δt). 
     Similarly, a predicted position Plp of the front-rear direction of the first preceding vehicle  3  can also be predicted. 
     The position prediction unit  36   c  determines whether the following condition B for estimating the likelihood of the adjacent vehicle  4  to change lanes into the first lane  2   a  is satisfied or not. 
     (Condition B): It is predicted that the front-rear direction position Psp of the adjacent vehicle  4  is located behind the front-rear direction position Plp of the first preceding vehicle  3  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv. 
     The intention estimation unit  36   b  estimates whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  on the basis of the determination result of the vehicle speed prediction unit  36   a  and the determination result of the position prediction unit  36   c . The intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when both the conditions A and B are satisfied. 
     On the other hand, when at least one of the condition A or B is not satisfied, the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     The other function of the intention estimation unit  36   b  is the same as that of the first embodiment. 
     (Operation) 
     Next, an example of the vehicle behavior estimation method by the traveling assistance device  10  according to the second embodiment is described with reference to  FIG.  7   . 
     Processing of steps S 1   l  to S 15  is the same as that of steps S 1  to S 5  of the first embodiment described with reference to  FIG.  4   . 
     At step S 16 , the vehicle speed prediction unit  36   a  determines whether the above condition A is satisfied or not. Additionally, the position prediction unit  36   c  determines whether the above condition B is satisfied or not. The intention estimation unit  36   b  determines whether both the conditions A and B are satisfied or not. When both the conditions A and B are satisfied (step S 16 : Y), processing proceeds to step S 17 . When at least one of the condition A or B is not satisfied (step S 16 : N), processing proceeds to step S 18 . 
     Processing of steps S 17  to S 19  is the same as that of steps S 7  to S 9  of the first embodiment described with reference to  FIG.  4   . 
     First Modification 
     The position prediction unit  36   c  may estimate whether or not the adjacent vehicle  4  will change lanes into a range in front of the host vehicle  1  and behind the first preceding vehicle  3 . 
     For example, the position prediction unit  36   c  may predict a front-rear direction position Pop of the host vehicle  1  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv, and may determine whether the following condition B1 instead of the above condition B is satisfied or not. 
     (Condition B1): It is predicted that at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv, the front-rear direction position Psp of the adjacent vehicle  4  is located in a range in front of the front-rear direction position Pop of the host vehicle  1  and behind the front-rear direction position Plp of the first preceding vehicle  3 . 
     When both the conditions A and B1 are satisfied, the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the range in front of the host vehicle  1  and behind the first preceding vehicle  3  in the first lane  2   a . When at least one of the condition A or B1 is not satisfied, the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     In the example of  FIG.  5   , it is predicted that the relative position in the front-rear direction of the adjacent vehicle  4  indicated by the solid line L 42  is located in a range between the relative position in the front-rear direction of the first preceding vehicle  3  indicated by the broken line L 32  and the position of the host vehicle  1 , which is an origin, at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv (condition B1). It is also predicted that the absolute value vr1 will be at or below the speed threshold value Tv at the time point t1 within the predetermined time T0 (condition A). 
     Therefore, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Note that the “range in front of the front-rear direction position Pop of the host vehicle and behind the front-rear direction position Plp of the first preceding vehicle  3 ” in the above condition B1 may be a range R from a position forward by a first predetermined distance L1 from the front-rear direction position Psp of the host vehicle  1  at the time point t1 to a position rearward by a second predetermined distance L2 from the front-rear direction position Plp of the first preceding vehicle  3  at the time point t1, as illustrated in  FIG.  8   . 
     In this case, the second predetermined distance L2 may be set shorter than the first predetermined distance L1. 
     Second Modification 
     In addition, when the adjacent vehicle  4  changes lanes into the range in front of the host vehicle  1  and behind the first preceding vehicle  3  in the first lane  2   a , it is easier for the adjacent vehicle  4  to maintain an inter-vehicle distance when the adjacent vehicle  4  enters the first lane  2   a  at a position closer to the first preceding vehicle  3  located forward in the front-rear direction than to the host vehicle  1  located rearward in the front-rear direction. 
     In other words, the position of the host vehicle  1  located rearward in the front-rear direction is confirmed either by looking rearward diagonally or by a rearview mirror, and therefore is more difficult to confirm than the position of the first preceding vehicle  3  located forward in the front-rear direction. Thus, making the front inter-vehicle distance shorter than the rear inter-vehicle distance allows for a margin in the rear inter-vehicle distance while maintaining the front inter-vehicle distance. 
     Accordingly, as illustrated in  FIG.  9   , when a distance D2 between the front-rear direction position Psp of the adjacent vehicle  4  and the front-rear direction position Plp of the first preceding vehicle  3  is shorter than a distance D1 between the front-rear direction position Psp of the adjacent vehicle  4  and the front-rear direction position Pop of the host vehicle  1  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv, it can be estimated that the adjacent vehicle  4  is likely to be adjusting the speed of the adjacent vehicle  4  in order to change lanes into the first lane  2   a  at a position closer to the first preceding vehicle  3  than to the host vehicle  1 . 
     Accordingly, for example, the position prediction unit  36   c  may determine whether the following condition B2 instead of the above condition B is satisfied or not. 
     (Condition B2): It is predicted that at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv, the front-rear direction position Psp of the adjacent vehicle  4  is located in the range in front of the front-rear direction position Pop of the host vehicle  1  and behind the front-rear direction position Plp of the first preceding vehicle  3 , and it is also predicted that the front-rear direction position Psp of the adjacent vehicle  4  is closer to the front-rear direction position Plp of the first preceding vehicle  3  than to the front-rear direction position Pop of the host vehicle  1 . 
     When both the conditions A and B2 are satisfied, the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a . On the other hand, when at least one of the condition A or B2 is not satisfied, the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Note that the first and second modifications above can also be applied to a third and a fourth embodiments described below. 
     Effects of Second Embodiment 
     (1) The object detection unit  32  may detect the positions of the adjacent vehicle  4  and the first preceding vehicle  3 . When it is predicted that the absolute value vr1 of the relative speed will be at or below the speed threshold value Tv within the predetermined time from the time point where the decrease in the absolute value vr1 of the relative speed starts to be detected, the position prediction unit  36   c  may predict the front-rear direction position of the adjacent vehicle  4  and the front-rear direction position of the first preceding vehicle  3  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv. The position prediction unit  36   c  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when the front-rear direction position of the adjacent vehicle  4  is located behind the front-rear direction position of the first preceding vehicle  3  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv. 
     When the adjacent vehicle  4  changes lanes into the first lane  2   a  while matching the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 , the adjacent vehicle  4  is considered to change lanes into the range behind the first preceding vehicle  3 . 
     By estimating whether or not the front-rear direction position of the adjacent vehicle  4  is adjusted to a position more rearward than the front-rear direction position of the first preceding vehicle  3 , the likelihood of the adjacent vehicle  4  to change lanes into the first lane  2   a  can be estimated accurately. 
     (2) The position prediction unit  36   c  may predict the front-rear direction position of the host vehicle  1  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv. The position prediction unit  36   c  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  when the front-rear direction position of the adjacent vehicle  4  is located in the range in front of the front-rear direction position of the host vehicle  1  and behind the front-rear direction position of the first preceding vehicle  3  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv. 
     When the adjacent vehicle  4  changes lanes into the first lane  2   a  while matching the vehicle speed of the adjacent vehicle  4  to the vehicle speed of the first preceding vehicle  3 , the adjacent vehicle  4  is considered to change lanes into the range behind the first preceding vehicle  3  and in front of the host vehicle  1 . 
     By estimating whether or not the front-rear direction position of the adjacent vehicle  4  is adjusted to a position more rearward than the front-rear direction position of the first preceding vehicle  3  and more forward than the front-rear direction position of the host vehicle  1 , the likelihood of the adjacent vehicle  4  to change lanes into the first lane  2   a  can be estimated accurately. 
     (3) The range in front of the front-rear direction position of the host vehicle  1  and behind the front-rear direction position of the first preceding vehicle  3  may be the range from the position more forward by the first predetermined distance L1 than the front-rear direction position of the host vehicle  1  to the position more rearward by the second predetermined distance L2 than the front-rear direction position of the first preceding vehicle  3 . 
     This allows for accurate estimation of the likelihood that the adjacent vehicle  4  will change lanes into the first lane  2   a  while maintaining a spacing between the adjacent vehicle  4  and the host vehicle  1  and a spacing between the adjacent vehicle  4  and the first preceding vehicle  3 . 
     (4) The second predetermined distance L2 may be set shorter than the first predetermined distance L1. 
     When the adjacent vehicle  4  changes lanes into the range in front of the host vehicle  1  and behind the first preceding vehicle  3  in the first lane  2   a , it is easier for the adjacent vehicle  4  to maintain the inter-vehicle distance when the adjacent vehicle  4  enters the first lane  2   a  at a position closer to the first preceding vehicle  3  located forward in the front-rear direction than to the host vehicle  1  located rearward in the front-rear direction. Therefore, the adjacent vehicle  4  is considered to change lanes to a position closer to the first preceding vehicle  3  than to the host vehicle  1 . 
     Setting the second predetermined distance L2 shorter than the first predetermined distance L1 allows for accurate estimation of the likelihood that the adjacent vehicle  4  will change lanes into the range in front of the host vehicle  1  and behind the first preceding vehicle  3 . 
     (5) For the same reason, when the front-rear direction position of the adjacent vehicle  4  at the time point t1 where the absolute value vr1 of the relative speed is at or below the speed threshold value Tv is closer to the front-rear direction position of the first preceding vehicle  3  than to the front-rear direction position of the host vehicle  1 , the position prediction unit  36   c  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     This allows for accurate estimation of the likelihood that the adjacent vehicle  4  will change lanes into the range in front of the host vehicle  1  and behind the first preceding vehicle  3 . 
     Third Embodiment 
     (Configuration) 
     Next, a third embodiment is described. The vehicle behavior estimation unit  36  of the third embodiment determines whether or not the adjacent vehicle  4  is trying to change lanes between the host vehicle  1  and the first preceding vehicle  3  while avoiding an inadvertent approach to the host vehicle  1  or the first preceding vehicle  3 , in addition to the above conditions A and B. Note that the inadvertent approach means an approach to the extent that the occupant feels uncomfortable, and is hereinafter simply described as approach. 
     Reference is made to  FIG.  10 A . The vehicle behavior estimation unit  36  of the third embodiment detects a front-rear direction position of the adjacent vehicle  4  at a time point where the adjacent vehicle  4  starts a speed adjustment for changing lanes. The time point t0 when the decrease in the absolute value vr1 of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3  starts to be detected is used as the time point where the speed adjustment is started. 
     In an example of  FIG.  10 A , at the time point t0, the front-rear direction position of the adjacent vehicle  4  is within a range of a predetermined distance L3 from the front-rear direction position of the host vehicle  1 , and is close to the front-rear direction position of the host vehicle  1 . Accordingly, because there is a risk of an approach, the lane cannot be changed. 
     However, when the above conditions A and B1 (or B2) are satisfied, it is estimated that the adjacent vehicle  4  is adjusting the speed thereof so as to be able to change lanes without approaching the host vehicle  1  by the subsequent time point t1. 
     In an example of  FIG.  10 B , at the time point t0, the front-rear direction position of the adjacent vehicle  4  is within a range of a predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 , and is close to the front-rear direction position of the first preceding vehicle  3 . Accordingly, because there is a risk of an approach, the lane cannot be changed. 
     However, when the above conditions A and B (or B1 or B2) are satisfied, it is estimated that the adjacent vehicle  4  is adjusting the speed thereof so as to be able to change lanes without approaching the first preceding vehicle  3  by the subsequent time point t1. 
     Therefore, the vehicle behavior estimation unit  36  of the third embodiment determines whether or not the front-rear direction position of the adjacent vehicle  4  at the time point t0 when the decrease in the absolute value vr1 of the relative speed starts to be detected is within the range of the predetermined distance L3 from the front-rear direction position of the host vehicle  1  or within the range of the predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 . The predetermined distance L3 may be the same as or different from the first predetermined distance L1 of the first modification of the second embodiment. The predetermined distance L4 may be the same as or different from the second distance L2 of the first modification of the second embodiment. 
     When the above conditions A and B are satisfied and the front-rear direction position of the adjacent vehicle  4  at the time point t0 is within the range of the predetermined distance L3 from the front-rear direction position of the host vehicle  1  or within the range of the predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 , the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a . For example, the vehicle behavior estimation unit  36  estimates that the adjacent vehicle  4  will change lanes into a range behind the first preceding vehicle  3  and in front of the host vehicle  1 . 
     On the contrary, when at least one of the above condition A or B is not satisfied or the front-rear direction position of the adjacent vehicle  4  at the time point t0 is neither within the range of the predetermined distance L3 from the front-rear direction position of the host vehicle  1  nor within the range of the predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 , the vehicle behavior estimation unit  36  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     Functions of the controller  17  of the third embodiment are described in detail with reference to  FIG.  11   . The controller  17  of the third embodiment has the same configuration as that of the second embodiment. Among components of the controller  17  of the third embodiment, the same components as those of the second embodiment are denoted by the same reference signs. 
     The vehicle behavior estimation unit  36  of the third embodiment includes an approach determination unit  36   d.    
     The approach determination unit  36   d  acquires the front-rear direction positions of the adjacent vehicle  4  and the first preceding vehicle  3  at the time point t0 when the decrease in the absolute value vr1 of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3  starts to be detected from the object detection unit  32 . 
     The approach determination unit  36   d  determines whether the following condition C for estimating the likelihood of the adjacent vehicle  4  to change lanes into the first lane  2   a  is satisfied or not. 
     (Condition C): The front-rear direction position of the adjacent vehicle  4  at the time point t0 is within the range of the predetermined distance L3 from the front-rear direction position of the host vehicle  1  or within the range of the predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 . 
     The intention estimation unit  36   b  estimates whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  on the basis of the determination result of the vehicle speed prediction unit  36   a , the determination result of the position prediction unit  36   c , and the determination result of the approach determination unit  36   d.    
     When all of the conditions A, B, and C are satisfied, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     On the other hand, when at least one of the condition A, B, or C is not satisfied, the intention estimation unit  36   b  does not estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     The other function of the intention estimation unit  36   b  is the same as that of the first embodiment. 
     (Operation) 
     Next, an example of the vehicle behavior estimation method by the traveling assistance device  10  according to the third embodiment is described with reference to  FIG.  12   . 
     Processing of steps S 21  to S 25  is the same as that of steps S 1  to S 5  of the first embodiment described with reference to  FIG.  4   . 
     At step S 26 , the vehicle speed prediction unit  36   a  determines whether the above condition A is satisfied or not. Additionally, the position prediction unit  36   c  determines whether the above condition B is satisfied or not. The approach determination unit  36   d  determines whether the above condition C is satisfied or not. 
     The intention estimation unit  36   b  determines whether all of the conditions A, B, and C are satisfied or not. When all of the conditions A, B, and C are satisfied (step S 26 : Y), processing proceeds to step S 27 . When at least one of the condition A, B, or C is not satisfied (step S 26 : N), processing proceeds to step S 28 . 
     Processing of steps S 27  to S 29  is the same as that of steps S 7  to S 9  of the first embodiment described with reference to  FIG.  4   . 
     Effects of Third Embodiment 
     When the front-rear direction position of the adjacent vehicle  4  at the time point t0 when the decrease in the absolute value vr1 of the relative speed starts to be detected is within the range of the predetermined distance L3 from the front-rear direction position of the host vehicle  1  or within the range of the predetermined distance L4 from the front-rear direction position of the first preceding vehicle  3 , the approach determination unit  36   d  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a.    
     This allows for accurate estimation of the likelihood that the adjacent vehicle  4  will change lanes between the host vehicle  1  and the first preceding vehicle  3  while avoiding an approach to the host vehicle  1  or the first preceding vehicle  3 . 
     Fourth Embodiment 
     (Configuration) 
     Next, a fourth embodiment is described. The vehicle behavior estimation unit  36  of the fourth embodiment determines whether or not the adjacent vehicle  4  is more likely to change lanes into the first lane  2   a  on the basis of information on a vehicle preceding the adjacent vehicle  4 . 
     Now, as illustrated in  FIG.  13 A , assume that there is a second preceding vehicle  5  traveling in front of the adjacent vehicle  4  in the second lane  2   b.    
     Reference is made to  FIG.  13 B . The adjacent vehicle  4  starts to decelerate at the time point t0, whereas the second preceding vehicle  5  is traveling at a constant speed even after the time point t0. Therefore, a speed difference between the adjacent vehicle  4  and the second preceding vehicle  5  increases, as a result of which an inter-vehicle distance between the adjacent vehicle  4  and the second preceding vehicle  5  also increases. 
     Additionally, the absolute value vr1 of the relative speed between the vehicle speed of the adjacent vehicle  4  and the vehicle speed of the first preceding vehicle  3  decreases. 
     In such a case, it can be estimated that the adjacent vehicle  4  is highly likely to stop following the second preceding vehicle  5  and change lanes behind the first preceding vehicle  3 . 
     Therefore, in addition to estimating whether or not the adjacent vehicle  4  is likely to change lanes into the first lane  2   a  on the basis of the conditions A, B, and C, the vehicle behavior estimation unit  36  of the fourth embodiment estimates that the adjacent vehicle  4  is more likely to change lanes into the first lane  2   a  when the second preceding vehicle  5  is determined to be faster than the adjacent vehicle  4 , compared to when the second preceding vehicle  5  is not determined to be faster than the adjacent vehicle  4 . 
     Functions of the controller  17  of the fourth embodiment are described in detail with reference to  FIG.  14   . The controller  17  of the fourth embodiment has the same configuration as that of the third embodiment. Among components of the controller  17  of the fourth embodiment, the same components as those of the third embodiment are denoted by the same reference signs. 
     The vehicle behavior estimation unit  36  of the fourth embodiment includes an adjacent preceding vehicle comparison unit  36   e.    
     The target vehicle identification unit  34  identifies the second preceding vehicle  5  on the basis of a lane structure around the host vehicle  1  acquired by the lane structure acquisition unit  31 , a result of object detection by the object detection unit  32 , and a result of tracking by the object tracking unit  33 . Additionally, the surrounding vehicle state acquisition unit  35  generates a speed profile of the second preceding vehicle  5 . 
     When all of the above conditions A, B, and C are satisfied, the adjacent preceding vehicle comparison unit  36   e  calculates a predicted speed of the adjacent vehicle  4  and a predicted speed of the second preceding vehicle  5  at a time after the time point t0 when the decrease in the absolute value vr1 of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3  starts to be detected. 
     The adjacent preceding vehicle comparison unit  36   e  determines whether the following condition D for estimating whether or not the adjacent vehicle  4  is highly likely to change lanes into the first lane  2   a  is satisfied or not. 
     (Condition D): It is predicted that the predicted speed of the second preceding vehicle  5  is higher than the predicted speed of the adjacent vehicle  4  at the time after the time point t0. 
     When all of the conditions A, B, and C are satisfied, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  is likely to change lanes into the first lane  2   a . As a result, the intention estimation unit  36   b  increases the certainty degree by a predetermined step amount (for example, the certainty degree is counted up). 
     Additionally, when the condition D is satisfied, the intention estimation unit  36   b  estimates that the adjacent vehicle  4  is more likely to change lanes into the first lane  2   a . In this case, the intention estimation unit  36   b  further adds an additional point to the certainty degree. 
     Therefore, when the condition D is satisfied, the certainty degree is higher than when the condition D is not satisfied. In other words, the intention estimation unit  36   b  estimates that the lane change is more likely to occur. 
     (Operation) 
     Next, an example of the vehicle behavior estimation method by the traveling assistance device  10  according to the fourth embodiment is described with reference to  FIG.  15   . 
     Processing of steps S 31  to S 37  is the same as that of steps S 21  to S 27  of the third embodiment described with reference to  FIG.  12   . After step S 37 , processing proceeds to step S 38 . 
     At step S 38 , the adjacent preceding vehicle comparison unit  36   e  determines whether the second preceding vehicle  5  is present or not. When the second preceding vehicle  5  is present (step S 38 : Y), processing proceeds to step S 39 . When the second preceding vehicle  5  is not present (step S 38 : N), processing proceeds to step S 41 . 
     At step S 39 , the adjacent preceding vehicle comparison unit  36   e  determines whether the condition D is satisfied or not. When the condition D is satisfied (step S 39 : Y), processing proceeds to step S 40 . When the condition D is not satisfied (step S 39 : N), processing proceeds to step S 41 . 
     At step S 40 , the intention estimation unit  36   b  further adds an additional point to the certainty degree. Then, processing proceeds to step S 41 . 
     Processing of steps S 41  and S 42  is the same as that of steps S 28  and S 29  of the third embodiment described with reference to  FIG.  12   . 
     Modifications 
     Under the condition D, instead of determining whether or not it is predicted that the predicted speed of the second preceding vehicle  5  is higher than the predicted speed of the adjacent vehicle  4 , it may be determined whether or not the speed difference between the predicted speed of the adjacent vehicle  4  and the predicted speed of the second preceding vehicle  5  tends to increase. 
     Additionally, under the condition D, instead of determining whether or not it is predicted that the predicted speed of the second preceding vehicle  5  is higher than the predicted speed of the adjacent vehicle  4 , it may be determined whether or not the speed difference between the predicted speed of the adjacent vehicle  4  and the predicted speed of the second preceding vehicle  5  is at or above a threshold value. 
     Note that, at the time after the time point t0, the absolute value vr1 of the relative speed between the adjacent vehicle  4  and the first preceding vehicle  3  decreases, and the speed difference between the adjacent vehicle  4  and the first preceding vehicle  3  becomes small. 
     Accordingly, under the condition D, it may be determined whether or not it is predicted that the predicted speed of the second preceding vehicle  5  is higher than the predicted speed of the first preceding vehicle  3  instead of the predicted speed of the adjacent vehicle  4 . 
     Additionally, even when the second preceding vehicle  5  is traveling far in front of the adjacent vehicle  4 , and the adjacent vehicle  4  adjusts the vehicle speed thereof to match the first preceding vehicle  3  even though there is a sufficient space in front of the adjacent vehicle  4 , it can be estimated that the adjacent vehicle  4  is highly likely to change lanes behind the first preceding vehicle  3  in the first lane  2   a.    
     Therefore, under the condition D, it may be determined whether or not it is predicted that an inter-vehicle distance between the adjacent vehicle  4  and the second preceding vehicle  5  is at or above a distance threshold value instead of determining whether or not it is predicted that the predicted speed of the second preceding vehicle  5  is higher than the predicted speed of the adjacent vehicle  4  at the time after the time point t0. 
     Effects of Fourth Embodiment 
     (1) The object detection unit  32  detects the speed of the second preceding vehicle  5  traveling in front of the adjacent vehicle  4  in the second lane  2   b . When the second preceding vehicle  5  is determined to be faster than the adjacent vehicle  4 , the adjacent preceding vehicle comparison unit  36   e  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is more likely to change lanes into the first lane  2   a  than when the second preceding vehicle  5  is not determined to be faster than the adjacent vehicle  4 . 
     As a result, when the vehicle speed of the second preceding vehicle  5  is high and the distance from the adjacent vehicle  4  is widening, it can be estimated that the adjacent vehicle  4  is adjusting the vehicle speed thereof to match the first preceding vehicle  3 , and the adjacent vehicle  4  is highly likely to change lanes behind the first preceding vehicle  3 . 
     (2) The object detection unit  32  detects the position of the second preceding vehicle  5  traveling in front of the adjacent vehicle  4  in the second lane  2   b . When the inter-vehicle distance between the adjacent vehicle  4  and the second preceding vehicle  5  is at or above the distance threshold value, the adjacent preceding vehicle comparison unit  36   e  and the intention estimation unit  36   b  may estimate that the adjacent vehicle  4  is more likely to change lanes into the first lane  2   a  than when the inter-vehicle distance is below the distance threshold value. 
     As a result, when the adjacent vehicle  4  adjusts the vehicle speed thereof to match the first preceding vehicle  3  even though there is a sufficient space in front of the adjacent vehicle  4 , it can be estimated that the adjacent vehicle  4  is highly likely to change lanes behind the first preceding vehicle  3 . 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
     REFERENCE SIGNS LIST 
     
         
           1 : Host vehicle 
           2   a : First lane 
           2   b : Second lane 
           3 : First preceding vehicle 
           4 : Adjacent vehicle 
           5 : Second preceding vehicle 
           10 : Traveling assistance device 
           11 : Object sensor 
           12 : Vehicle sensor 
           13 : Positioning device 
           14 : Map database 
           15 : Communication device 
           16 : Navigation system 
           17 : Controller 
           18 : Actuator 
           19 : Output device 
           21 : Processor 
           22 : Storage device 
           30 : Host vehicle position estimation unit 
           31 : Lane structure acquisition unit 
           32 : Object detection unit 
           33 : Object tracking unit 
           34 : Target vehicle identification unit 
           35 : Surrounding vehicle state acquisition unit 
           36 : Vehicle behavior estimation unit 
           36   a : Vehicle speed prediction unit 
           36   b : Intention estimation unit 
           36   c : Position prediction unit 
           36   d : Approach determination unit 
           36   e : Adjacent preceding vehicle comparison unit 
           37 : Tracking target setting unit 
           38 : Traveling trajectory generation unit 
           39 : Traveling control unit