Patent Publication Number: US-2023146552-A1

Title: Merge assistance device and merge assistance method

Description:
BACKGROUND OF THE DISCLOSURE 
     1. Field of the Disclosure 
     The present disclosure relates to a merge assistance device and a merge assistance method. 
     2. Description of the Background Art 
     For putting autonomous driving into practical use, being developed is technology in which the own-vehicle position is determined by a locator and map information relevant to the determined own-vehicle position is utilized to achieve high-level vehicle control. 
     One example of high-level vehicle control is autonomous merging control in a scene of merging from a merging lane to a main lane. For achieving the autonomous merging control, accurate position information about a merging-possible area start point, e.g., a hatched-marking area, in particular, a hatched-marking area leading end, is needed. 
      Patent Document 1: Japanese Patent No. 4645429 
     Map information that a locator has includes position information about merge start points. The position information about merge start points is indirectly given as boundaries between section types such as a main lane, a merging lane, a split lane, and a ramp. A boundary at which the section type is switched from a ramp to a merging lane may be considered to be a merge start point. 
     A merge start point acquired from the map information that the locator has depends on the content of the map that the locator has, and does not necessarily coincide with a merging-possible area start point, e.g., a hatched-marking area leading end on a road. This is because the merge start point may be located frontward of the hatched-marking area leading end or may be located rearward thereof. 
     Patent Document 1 discloses an on-vehicle device which estimates a merging-possible area start point through image processing such as overhead image conversion using a hatched-marking area image acquired by an on-vehicle camera. However, in the case of estimating the merging-possible area start point through image processing on the hatched-marking area image, error occurs due to the conversion processing, thus causing a problem that estimation accuracy might be insufficient in autonomous merging control. 
     SUMMARY OF THE DISCLOSURE 
     The present disclosure has been made to solve the above problem and an object of the present disclosure is to provide a merge assistance device and a merge assistance method for estimating a merging-possible area start point with high accuracy by utilizing map information that a locator has. 
     A merge assistance device according to the present disclosure is for assisting merge from an own lane on which an own vehicle travels, to a target lane, and includes: an own-vehicle position acquisition unit for acquiring an own-vehicle position determined by a locator; a provisional merge point setting unit for setting a provisional merge point on the own lane on the basis of coordinate information of the own lane and the target lane included in map information stored in a map information database; a lane-to-lane distance calculation unit for calculating a lane-to-lane distance from each point on the own lane to each point on the target lane; an attention point setting unit which, on the basis of a provisional merge point lane-to-lane distance from the provisional merge point to the target lane calculated by the lane-to-lane distance calculation unit, sets one or more frontward attention points on the own lane on a side frontward of the provisional merge point and sets one or more rearward attention points on the own lane on a side rearward of the provisional merge point; a first approximation expression deriving unit for deriving a first approximation expression approximating coordinates of the one or more frontward attention points and the one or more rearward attention points; a second approximation expression deriving unit for deriving a second approximation expression approximating coordinates on the target lane including coordinates of a position at a shortest distance from the provisional merge point; and a merging-possible area start point estimation unit which, on the basis of an own-lane width and a target-lane width included in the map information, and the first approximation expression and the second approximation expression, estimates, on the target lane, a merging-possible area start point which is a start point of an area where the own vehicle becomes able to merge from the own lane to the target lane. 
     A merge assistance method according to the present disclosure is for assisting merge from an own lane on which an own vehicle travels, to a target lane, and includes: an own-vehicle position acquisition step of acquiring an own-vehicle position determined by a locator; a provisional merge point setting step of setting a provisional merge point on the own lane on the basis of coordinate information of the own lane and the target lane included in map information stored in a map information database; a lane-to-lane distance calculation step of calculating a lane-to-lane distance from each point on the own lane to each point on the target lane; an attention point setting step of, on the basis of a provisional merge point lane-to-lane distance from the provisional merge point to the target lane calculated in the lane-to-lane distance calculation step, setting one or more frontward attention points on the own lane on a side frontward of the provisional merge point and setting one or more rearward attention points on the own lane on a side rearward of the provisional merge point; a first approximation expression deriving step of deriving a first approximation expression approximating coordinates of the one or more frontward attention points and the one or more rearward attention points; a second approximation expression deriving step of deriving a second approximation expression approximating coordinates on the target lane including coordinates of a position at a shortest distance from the provisional merge point; and a merging-possible area start point estimation step of, on the basis of an own-lane width and a target-lane width included in the map information, and the first approximation expression and the second approximation expression, estimating, on the target lane, a merging-possible area start point which is a start point of an area where the own vehicle becomes able to merge from the own lane to the target lane. 
     In the merge assistance device and the merge assistance method according to the present disclosure, using the provisional merge point acquired from map information that the locator has, frontward attention points and rearward attention points are set on the own lane on the basis of the lane-to-lane distance from the provisional merge point to the target lane. Using the frontward attention points and the rearward attention points, the first approximation expression representing the own lane is derived. The second approximation expression is derived by approximating coordinates on the target lane including coordinates of the position at the shortest distance from the provisional merge point. The merging-possible area start point is estimated on the basis of the first approximation expression and the second approximation expression. This provides an effect of obtaining a merge assistance device and a merge assistance method that can estimate the merging-possible area start point with high accuracy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a function block diagram showing the configuration of a merge assistance device according to the first and second embodiments of the present disclosure; 
         FIG.  2    is a flowchart showing a merge assistance method according to the first embodiment; 
         FIG.  3    schematically shows a merge scene of an own vehicle merging from an own lane to a target lane in the merge assistance method according to the first embodiment; 
         FIG.  4    schematically shows a method for estimating a merging-possible area start point in the merge assistance method according to the first embodiment; 
         FIG.  5    is a flowchart showing a merge assistance method according to the second embodiment; 
         FIG.  6    schematically shows a merge scene of an own vehicle merging from an own lane to a target lane in the merge assistance device according to the second embodiment; 
         FIG.  7    is a function block diagram showing the configuration of a merge assistance device according to the third embodiment of the present disclosure; 
         FIG.  8    is a flowchart showing a merge assistance method according to the third embodiment; 
         FIG.  9    is a function block diagram showing the configuration of a merge assistance device according to the fourth embodiment of the present disclosure; 
         FIG.  10    is a flowchart showing a merge assistance method according to the fourth embodiment; 
         FIG.  11    is a function block diagram showing a hardware configuration for implementing the merge assistance device according to any of the first, third, and fourth embodiments; and 
         FIG.  12    is a function block diagram showing a hardware configuration for implementing the merge assistance device according to any of the first, third, and fourth embodiments. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED 
     Embodiments of the Disclosure 
     First Embodiment 
       FIG.  1    is a function block diagram showing the configuration of a merge assistance device  100  according to the first embodiment of the present disclosure. The merge assistance device  100  shown in  FIG.  1    includes an own-vehicle position acquisition unit  101 , a provisional merge point setting unit  102 , a lane-to-lane distance calculation unit  103 , an attention point setting unit  104 , an approximation expression deriving unit  105 , a merging-possible area start point estimation unit  106 , and a vehicle control unit  110 . In the following description, a case of mentioning “coordinates” may mean a “point” represented by the “coordinates”, and a case of mentioning a “point” may mean “coordinates” corresponding to the “point”. 
     From a locator  101   a  provided outside the merge assistance device  100 , the own-vehicle position acquisition unit  101  acquires an own-vehicle position, and map information relevant to the own-vehicle position and stored in a map information database  101   b  inside the locator  101   a . In  FIG.  1   , the map information database  101   b  is provided inside the locator  101   a . However, the map information database  101   b  may be provided outside the locator  101   a , or may be provided inside the merge assistance device  100 . 
     The own-vehicle position acquisition unit  101  outputs information such as the own-vehicle position and the map information acquired from the locator  101   a , to the provisional merge point setting unit  102 . 
     The locator  101   a  is for specifying the position of the own vehicle, and determines the own-vehicle position by using a global navigation satellite system (GNSS) receiver, for example. The own-vehicle position may be detected using a device such as an inertial navigation device. 
     The map information database  101   b  inside the locator  101   a  stores map information. The map information includes at least a section type, coordinate information of an own lane  1 , coordinate information of a target lane  2 , the positional relationship between the own lane  1  and the target lane  2 , an own-lane width, a target-lane width, and the like. The coordinate information of the own lane  1 , the coordinate information of the target lane  2 , and the like are provided at certain intervals to the map information. 
     The locator  101   a  detects the present position of the own vehicle on the map on the basis of positioning data for the own-vehicle position acquired by the GNSS receiver and map information around the own-vehicle position stored in the map information database  101   b , for example, and outputs the detected present position to the own-vehicle position acquisition unit  101 . 
     The provisional merge point setting unit  102  sets a provisional merge point  4  on the own lane  1  on the basis of the coordinate information of the own lane  1  and the target lane  2  included in the map information stored in the map information database  101   b . As an example of the provisional merge point  4 , as shown in  FIG.  3   , coordinates of a section-type switchover point at which the section type is switched from ramp to merge may be calculated, and the calculated coordinates may be set as the provisional merge point  4 . 
     The lane-to-lane distance calculation unit  103  calculates a distance from each point on the own lane  1  to each point on the target lane  2 , i.e., a lane-to-lane distance. The lane-to-lane distance calculation unit  103  can also calculate a lane-to-lane distance from each attention point on the own lane  1  to each point on the target lane  2 . In particular, the lane-to-lane distance from the provisional merge point  4  to the target lane  2  is calculated as a provisional merge point lane-to-lane distance LM. A method for calculating the provisional merge point lane-to-lane distance LM will be described later. 
     The attention point setting unit  104  sets one or more frontward attention points  5  on the own lane  1  on the side frontward of the provisional merge point  4 , on the basis of the provisional merge point lane-to-lane distance LM outputted from the lane-to-lane distance calculation unit  103 . The attention point setting unit  104  sets one or more rearward attention points  6  on the own lane  1  on the side rearward of the provisional merge point  4 .  FIG.  3    shows examples of the frontward attention points  5  and the rearward attention points  6 . A method for setting the frontward attention points  5  and the rearward attention points  6  will be described later. 
     The approximation expression deriving unit  105  includes a first approximation expression deriving unit  105   a  and a second approximation expression deriving unit  105   b . The first approximation expression deriving unit  105   a  and the second approximation expression deriving unit  105   b  may be integrated as one unit. 
     The approximation expression deriving unit  105  derives an approximation expression approximating coordinates on the own lane  1  or the target lane  2 . The first approximation expression deriving unit  105   a  in the approximation expression deriving unit  105  derives a first approximation expression approximating coordinates of one or more frontward attention points and one or more rearward attention points. That is, the first approximation expression represents an approximation of the own lane  1 . On the other hand, the second approximation expression deriving unit  105   b  in the approximation expression deriving unit  105  derives a second approximation expression approximating coordinates on the target lane  2  including coordinates of the position at the shortest distance from the provisional merge point  4 . That is, the second approximation expression represents an approximation of the target lane  2  near the provisional merge point  4  on the own lane  1 . The details of deriving for the approximation expressions and the like will be described later. 
     The merging-possible area start point estimation unit  106  estimates, on the target lane  2 , a merging-possible area start point which is a start point of an area where the own vehicle becomes able to merge from the own lane  1  which is the merging lane to the target lane  2 , on the basis of the own-lane width, the target-lane width, the first approximation expression, and the second approximation expression. 
     The vehicle control unit  110  controls traveling of the own vehicle toward the merging-possible area start point on the target lane  2  estimated by the merging-possible area start point estimation unit  106 . 
     The components of the merge assistance device  100  according to the first embodiment and the locator  101   a  are as described above. 
      Next, operation of the merge assistance device  100  according to the first embodiment, i.e., a merge assistance method, will be described with reference to  FIG.  2   ,  FIG.  3   , and  FIG.  4   . 
       FIG.  2    is a flowchart showing the merge assistance method according to the first embodiment,  FIG.  3    schematically shows a merge scene in the merge assistance method according to the first embodiment, and  FIG.  4    schematically shows a method for estimating the merging-possible area start point in the merge assistance method according to the first embodiment. 
     Hereinafter, with reference to the flowchart in  FIG.  2   , the merge assistance method according to the first embodiment will be described. First, in step S 101 , the own-vehicle position is acquired by the locator  101   a  determining the own-vehicle position. The information of the own-vehicle position detected by the locator  101   a  is outputted to the own-vehicle position acquisition unit  101 . 
     In step S 102 , from the map information stored in the map information database  101   b  inside the locator  101   a , coordinates of a section-type switchover point at which the section type is switched from ramp to merge in map information around the own-vehicle position are calculated and set as the provisional merge point  4 . This setting method for the provisional merge point  4  is merely an example. Another setting method may be used as long as the provisional merge point  4  is set on the own lane  1  on the basis of coordinate information of the own lane  1  on which the own vehicle is traveling and the target lane  2 , included in the map information stored in the map information database  101   b . 
     In step S 103 , on the basis of the provisional merge point  4  set in step S 102 , the coordinate information of the target lane  2  included in the map information, and the positional relationship between the own lane  1  and the target lane  2 , the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  is calculated. For example, as shown in  FIG.  3   , in a case where the target lane  2  is almost straight, the length of a perpendicular from the provisional merge point  4  to the target lane  2  on a plan view of the map may be calculated as the provisional merge point lane-to-lane distance LM. Alternatively, the shortest distance from the provisional merge point  4  on the own lane  1  to the target lane  2  may be calculated as the provisional merge point lane-to-lane distance LM. 
     In step S 104 , on the basis of the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2 , a number N of frontward attention points  5  and a number M of rearward attention points  6  with respect to the provisional merge point  4  are determined. That is, on the basis of the provisional merge point lane-to-lane distance LM outputted from the lane-to-lane distance calculation unit  103 , the attention point setting unit  104  sets N frontward attention points  5  at intervals on the own lane  1  on the side frontward of the provisional merge point  4 . In addition, the attention point setting unit  104  sets M rearward attention points  6  at intervals on the own lane  1  on the side rearward of the provisional merge point  4 . As the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  becomes longer, the numbers of the attention points proportionally increase. It is desirable that the intervals of the attention points are constant, but the intervals may not necessarily be constant. 
     The number N of the frontward attention points  5  may be one or more, i.e., N ≥ 1. The number M of the rearward attention points  6  may be one or more, i.e., M ≥ 1. For example, in the case of  FIG.  3   , if the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  is great, a hatched-marking area leading end (not shown) is considered to be present frontward of the provisional merge point  4 , and thus a large number of frontward attention points  5  are set. That is, the number N of the frontward attention points  5  is set to 4, and the number M of the rearward attention points  6  is set to 1. 
     In step S 105 , the first approximation expression representing the own lane  1  near the provisional merge point  4  is derived using the coordinate information of the frontward attention point  5  and the rearward attention point  6  included in the coordinate information of the own lane  1 . Deriving for the first approximation expression is performed by the first approximation expression deriving unit  105   a . 
     In step S 106 , the second approximation expression for the target lane  2  is derived by approximating coordinates on the target lane  2  including the coordinates of the position at the shortest distance from the provisional merge point  4 . Deriving for the second approximation expression is performed by the second approximation expression deriving unit  105   b . 
     In step S 107 , among points at which a lane-to-lane distance LS which is a distance between a point on the second approximation expression representing the target lane  2  and a point on the first approximation expression representing the own lane  1  is equal to or smaller than a lane-to-lane threshold distance, a point closest to the own-vehicle position, on the target lane  2 , is estimated as a merging-possible area start point. The lane-to-lane threshold distance is set to an appropriate value in accordance with the own-lane width and the target-lane width. 
     In the subsequent process from step S 105 , as shown in  FIG.  4   , the target lane  2  may be approximated as a straight line (second approximation expression), and with the target lane  2  defined as x axis and an axis orthogonal to the x axis defined as y axis, the approximation expression (first approximation expression) for the own lane  1  may be represented as a function f, i.e., y = f(x). In  FIG.  4   , the y-axis-direction interval between the first approximation expression and the second approximation expression corresponds to the lane-to-lane distance LS. Among points on the target lane  2  at which the lane-to-lane distance LS is equal to or smaller than the lane-to-lane threshold distance, a point closest to the own-vehicle position is estimated as the merging-possible area start point. 
      The vehicle control unit  110  outputs a control quantity for the own vehicle on the basis of the merging-possible area start point on the target lane  2  estimated by the merging-possible area start point estimation unit  106 . For example, in a case of using vehicle control technology such as model prediction control, vehicle control is performed with a constraint condition set so as not to perform lane change before the merging-possible area start point. 
     Effects of First Embodiment 
     As described above, in the merge assistance device and the merge assistance method according to the first embodiment, using the provisional merge point  4  which is a merge start point acquired from map information in the locator  101   a , the frontward attention point  5  and the rearward attention point  6  are set on the own lane  1  on the basis of the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2 . Using the attention points, the first approximation expression representing the own lane  1  near the provisional merge point  4  is derived, and further, the second approximation expression is derived by approximating the target lane  2  including the coordinates of the position at the shortest distance from the provisional merge point  4 . A point on the target lane  2  at which the lane-to-lane distance LS between the own lane  1  and the target lane  2  is equal to or smaller than the lane-to-lane threshold distance is estimated as the merging-possible area start point. This provides an effect of obtaining a merge assistance device and a merge assistance method that enable highly accurate estimation for the merging-possible area start point when the own vehicle merges from the own lane  1  to the target lane  2 . 
     Second Embodiment 
     A merge assistance device used in a merge assistance method according to the second embodiment of the present disclosure is the same as that in the first embodiment, and therefore description of the merge assistance device is omitted. With reference to  FIG.  5    and  FIG.  6   , the merge assistance method according to the second embodiment will be described. 
       FIG.  5    is a flowchart showing the merge assistance method according to the second embodiment, and  FIG.  6    schematically shows a merge scene of the own vehicle merging from the own lane  1  to the target lane  2  in the merge assistance device according to the second embodiment. Hereinafter, with reference to the flowchart in  FIG.  5   , the merge assistance method according to the second embodiment will be described. 
     Processing in steps S 201  to S 203  is the same as the processing in steps S 101  to S 103  in the flowchart in  FIG.  2   , and therefore description thereof is omitted. 
     In step S 204 , whether or not the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  is equal to or greater than a provisional merge point threshold distance, is determined. 
     If the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  is equal to or greater than the provisional merge point threshold distance, i.e., in the case of Yes in step S 204 , in step S 205 , a lane-to-lane distance L1 to the target lane  2  is calculated for coordinates on the own lane  1  on the side frontward of the provisional merge point  4 , on the basis of the provisional merge point  4  set in step S 202 , the coordinate information of the target lane  2  included in the map information, and the positional relationship between the own lane  1  and the target lane  2 . 
     In performing step S 205 , it is assumed that the lane-to-lane distance L1 monotonically decreases as the coordinates on the own lane  1  advances frontward from the provisional merge point  4 . 
     In step S 206 , on the own lane  1  on the side frontward of the provisional merge point  4 , a point N 1  closest to the provisional merge point  4  among points on the own lane  1  at which the lane-to-lane distance L1 from coordinates on the own lane  1  to the target lane  2  is smaller than the lane-to-lane threshold distance, is set as a frontward specific attention point. For example, in the merge scene shown in  FIG.  6   , the point N 1  which is the frontward specific attention point is the second frontward attention point  5  counted frontward from the provisional merge point  4 . Further, on the own lane  1  on the side rearward of the point N 1 , a point M1 closest to the point N 1  which is the frontward specific attention point among points on the own lane  1  at which the lane-to-lane distance L1 from the own lane  1  to the target lane  2  is equal to or greater than the lane-to-lane threshold distance, is set as a rearward specific attention point. 
     In step S 207 , using the coordinate information of the frontward specific attention point (N 1 ) and the rearward specific attention point (M1) set in step S 206 , the first approximation expression which is an approximation expression for the own lane  1  near the provisional merge point  4  is derived. 
     In step S 208 , a second approximation expression is derived by approximating coordinates on the target lane  2  including coordinates of the position at the shortest distance from the provisional merge point  4 . 
     In step S 209 , among points at which the distance between a point on the second approximation expression representing the target lane  2  and a point on the first approximation expression representing the own lane  1 , i.e., the lane-to-lane distance LS, is equal to or smaller than the lane-to-lane threshold distance, a point closest to the own-vehicle position, on the target lane  2 , is estimated as a merging-possible area start point. 
     On the other hand, in step S 204 , if the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  is smaller than the provisional merge point threshold distance, i.e., in the case of No in step S 204 , in step S 220 , a lane-to-lane distance L2 from the own lane  1  to the target lane  2  is calculated for coordinates on the own lane  1  on the side rearward of the provisional merge point  4 , on the basis of the provisional merge point  4  set in step S 202 , the coordinate information of the target lane  2  included in the map information, and the positional relationship between the own lane  1  and the target lane  2 . 
     In performing step S 220 , it is assumed that the lane-to-lane distance L2 monotonously increases as the coordinates on the own lane  1  become away rearward from the provisional merge point  4 . Here, targets for which the lane-to-lane distance L2 is calculated are coordinates on the own lane  1  at which the lane-to-lane distance L2 is equal to or greater than the provisional merge point threshold distance. 
     In step S 221 , on the own lane  1  on the side frontward of the provisional merge point  4 , a point N 2  closest to the provisional merge point  4  among points at which the lane-to-lane distance L2 from the own lane  1  to the target lane  2  is smaller than the provisional merge point threshold distance, is set as a frontward specific attention point. In addition, on the side rearward of the point N 2 , a point M 2  closest to the point N 2  among points at which the lane-to-lane distance L2 between the own lane  1  and the target lane  2  is equal to or greater than the provisional merge point threshold distance, is set as a rearward specific attention point. 
     In processing after step S 221 , the processing in steps S 207  to S 209  described above is performed. 
     In the subsequent processing from step S 207 , as shown in  FIG.  4   , the target lane  2  may be approximated as a straight line, and with the target lane  2  defined as x axis and an axis orthogonal to the x axis defined as y axis, the approximation expression for the own lane  1  may be represented as a function f, i.e., y = f(x). 
     The vehicle control unit  110  outputs a control quantity for the own vehicle on the basis of the merging-possible area start point estimated by the merging-possible area start point estimation unit  106 . For example, in a case of using vehicle control technology such as model prediction control, vehicle control is performed with a constraint condition set so as not to perform lane change before the merging-possible area start point. 
     The merge assistance method according to the second embodiment is as described above. 
     Effects of Second Embodiment 
     As described above, in the merge assistance method according to the second embodiment, using a merge start point acquired from the map information in the locator, i.e., the provisional merge point  4 , on the basis of the provisional merge point lane-to-lane distance LM from the provisional merge point  4  to the target lane  2  and the provisional merge point threshold distance, the frontward specific attention point and the rearward specific attention point are set on the own lane  1 . Using the coordinates of the frontward specific attention point and the rearward specific attention point, the first approximation expression representing the own lane  1  near the provisional merge point  4  is derived, and the second approximation expression approximating the target lane  2  including the coordinates of the position at the shortest distance from the provisional merge point  4  is derived. Among points on the target lane  2  at which the lane-to-lane distance between the own lane  1  and the target lane  2  is equal to or smaller than the lane-to-lane threshold distance, a point closest to the own-vehicle position is estimated as the merging-possible area start point. This provides an effect of obtaining a merge assistance method that enables highly accurate estimation for the merging-possible area start point when the own vehicle merges from the own lane  1  to the target lane  2 . 
     Third Embodiment 
       FIG.  7    is a function block diagram showing the configuration of a merge assistance device  200  according to the third embodiment of the present disclosure. The merge assistance device  200  shown in  FIG.  7    further includes a merging-possible area start point correction unit  107  in addition to the configuration of the merge assistance device  100  according to the first embodiment. Description of the same components as those in the merge assistance device  100  according to the first embodiment is omitted. 
     The merging-possible area start point correction unit  107  corrects a distance (hereinafter, referred to as start point distance LB) from the provisional merge point  4  to the merging-possible area start point estimated by the merging-possible area start point estimation unit  106 , so that the start point distance LB becomes equal to a first start point threshold distance or a second start point threshold distance determined on the basis of a section ID of a road outputted from the locator  101   a , in accordance with a condition. Then, the merging-possible area start point correction unit  107  outputs the merging-possible area start point after the correction (hereinafter, referred to as corrected merging-possible area start point), to the vehicle control unit  110 . 
     The first start point threshold distance and the second start point threshold distance may be set to values different among maps specified by section IDs of roads. 
     With reference to a flowchart in  FIG.  8   , a merge assistance method according to the third embodiment will be described. The flowchart in  FIG.  8    shows operations after the step in which the merging-possible area start point is estimated by the merge assistance method according to the first or second embodiment. 
     In step S 301 , whether the estimated merging-possible area start point on the target lane  2  is present frontward or rearward of the provisional merge point  4 , is determined. If the merging-possible area start point is present frontward of the provisional merge point  4 , i.e., in the case of Yes in step S 301 , in step S 302 , whether or not the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to or smaller than the first start point threshold distance, is determined. The first start point threshold distance may be variable in accordance with the section ID of the road outputted from the locator  101   a , or may be set separately. 
     If the start point distance LB from the provisional merge point  4  to the merging-possible area start point is greater than the first start point threshold distance, i.e., in the case of No in step S 302 , in step S 303 , the merging-possible area start point is corrected to a point on the target lane  2  on the side frontward of the provisional merge point  4 , at which the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to the first start point threshold distance, as the corrected merging-possible area start point. On the other hand, if the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to or smaller than the first start point threshold distance, i.e., in the case of Yes in step S 302 , the merging-possible area start point estimated by the merging-possible area start point estimation unit  106  is used. 
     If the merging-possible area start point is not present frontward of the provisional merge point  4 , i.e., in the case of No in step S 301 , in step S 304 , whether or not the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to or smaller than the second start point threshold distance, is determined. The second start point threshold distance may be variable in accordance with the section ID of the road outputted from the locator  101   a , or may be set separately. 
     If the start point distance LB from the provisional merge point  4  to the merging-possible area start point is greater than the second start point threshold distance, i.e., in the case of No in step S 304 , in step S 305 , the merging-possible area start point is corrected to a point on the target lane  2  on the side rearward of the provisional merge point  4 , at which the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to the second start point threshold distance, as the corrected merging-possible area start point. On the other hand, if the start point distance LB from the provisional merge point  4  to the merging-possible area start point is equal to or smaller than the second start point threshold distance, i.e., in the case of Yes in step S 304 , the merging-possible area start point estimated by the merging-possible area start point estimation unit  106  is used. 
     Effects of Third Embodiment 
     As described above, in the merge assistance device  200  and the merge assistance method according to the third embodiment, if the start point distance LB from the provisional merge point  4  acquired from the map information in the locator  101   a  to the merging-possible area start point estimated in the first or second embodiment is greater than the first start point threshold distance or the second start point threshold distance, correction is performed so that a point away from the provisional merge point  4  by the first start point threshold distance or the second start point threshold distance becomes the corrected merging-possible area start point. This provides an effect of obtaining a merge assistance device and a merge assistance method in which, for example, even in a case where inaccurate information is included in the map information outputted from the locator  101   a  and therefore the merging-possible area start point estimated by the merging-possible area start point estimation unit  106  has an abnormal value, the corrected merging-possible area start point can be used, thus enhancing the possibility that autonomous merging control can be performed. 
     Fourth Embodiment 
       FIG.  9    is a function block diagram showing the configuration of a merge assistance device  300  according to the fourth embodiment of the present disclosure. The merge assistance device  300  shown in  FIG.  9    further includes a hatched-marking detection unit  108  in addition to the configuration of the merge assistance device  200  according to the third embodiment. Description of the same components as those in the merge assistance device according to the first or third embodiment is omitted. 
     The hatched-marking detection unit  108  outputs hatched-marking area information obtained on the basis of image information acquired by an on-vehicle front camera (not shown), to the merging-possible area start point correction unit  107 . The hatched-marking area information includes information about a hatched-marking area leading end. 
     With reference to a flowchart shown in  FIG.  10   , operations of the merging-possible area start point correction unit  107  and the hatched-marking detection unit  108  will be described. The flowchart in  FIG.  10    shows operations after the step in which the merging-possible area start point is estimated or corrected by the merge assistance method according to the third embodiment. 
     In step S 401 , on the basis of the hatched-marking area information detected by the hatched-marking detection unit  108 , the merging-possible area start point correction unit  107  determines whether the merging-possible area start point is located frontward or rearward of the hatched-marking area leading end. 
      If the merging-possible area start point is located frontward of the hatched-marking area leading end, i.e., in the case of Yes in step S 401 , in step S 402 , whether or not a hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is equal to or smaller than a hatched-marking threshold distance, is determined. 
     If the hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is equal to or smaller than the hatched-marking threshold distance, i.e., in the case of Yes in step S 402 , the corrected merging-possible area start point corrected by the merging-possible area start point correction unit  107  is used. In a case where the merging-possible area start point is not corrected by the merging-possible area start point correction unit  107 , the merging-possible area start point estimated by the merging-possible area start point estimation unit  106  is used. 
     If the merging-possible area start point is not located frontward of the hatched-marking area leading end, i.e., in the case of No in step S 401 , or if the hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is greater than the hatched-marking threshold distance, i.e., in the case of No in step S 402 , in step S 403 , the merging-possible area start point is corrected to a point at which the hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is equal to the hatched-marking threshold distance, as the corrected merging-possible area start point. 
     Effects of Fourth Embodiment 
     As described above, in the merge assistance device  300  and the merge assistance method according to the fourth embodiment, using the hatched-marking area information acquired from image information obtained by an image camera or the like, processing is further performed for the merging-possible area start point or the corrected merging-possible area start point estimated or corrected in the merge assistance method according to the third embodiment, so that, if the hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is greater than the hatched-marking threshold distance, the merging-possible area start point is corrected to a point on the frontward side at which the hatched-marking distance LZ from the hatched-marking area leading end to the merging-possible area start point is equal to the hatched-marking threshold distance, as the corrected merging-possible area start point. This provides an effect of obtaining a merge assistance device and a merge assistance method in which, for example, even in a case where inaccurate information is included in the map information outputted from the locator  101   a  and therefore the merging-possible area start point estimated by the merging-possible area start point estimation unit  106  or the corrected merging-possible area start point corrected by the merging-possible area start point correction unit  107  has an abnormal value, the further corrected merging-possible area start point can be used, thus enhancing the possibility that autonomous merging control can be performed. 
     In the above description, the configuration in which the functions of components of the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments are implemented by one of hardware and software, etc., has been described. However, without limitation thereto, some of the components of the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments may be implemented by dedicated hardware, and the other components may be implemented by software, etc. 
     For example, as shown in function block diagrams in  FIG.  11    and  FIG.  12   , for some components, the functions thereof may be implemented by a processing circuit  50  as dedicated hardware, and for the other components, the processing circuit  50  as a processor  51  may read and execute a program  54 , stored in a memory  52 , for causing a computer or the like to execute a software part of the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments, thereby implementing the functions of the other components. 
     Further, as shown in  FIG.  12   , setting data to be used in the function units and the like of the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments may be installed as a part of software to the memory  52  from a storage medium  53  storing the program  54  for causing a computer or the like to execute a software part of the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments. 
     As described above, the merge assistance devices  100 ,  200 ,  300  according to the first, third, and fourth embodiments can implement the above-described functions by hardware, software, etc., or a combination thereof. 
     Although the disclosure is described above in terms of various exemplary embodiments and implementations, it should be understood that the various features, aspects, and functionality described in one or more of the individual embodiments are not limited in their applicability to the particular embodiment with which they are described, but instead can be applied, alone or in various combinations to one or more of the embodiments of the disclosure. 
     It is therefore understood that numerous modifications which have not been exemplified can be devised without departing from the scope of the present disclosure. For example, at least one of the constituent components may be modified, added, or eliminated. At least one of the constituent components mentioned in at least one of the preferred embodiments may be selected and combined with the constituent components mentioned in another preferred embodiment. 
     DESCRIPTION OF THE REFERENCE CHARACTERS 
     
         
           1  own lane 
           2  target lane 
           4  provisional merge point 
           5  frontward attention point 
           6  rearward attention point 
           50  processing circuit 
           51  processor 
           52  memory 
           53  storage medium 
           54  program 
           100 ,  200 ,  300  merge assistance device 
           101  own-vehicle position acquisition unit 
           101   a  locator 
           101   b  map information database 
           102  provisional merge point setting unit 
           103  lane-to-lane distance calculation unit 
           104  attention point setting unit 
           105  approximation expression deriving unit 
           105   a  first approximation expression deriving unit 
           105   b  second approximation expression deriving unit 
           106  merging-possible area start point estimation unit 
           107  merging-possible area start point correction unit 
           108  hatched-marking detection unit 
           110  vehicle control unit