Patent Publication Number: US-8996294-B2

Title: Inter-vehicle distance maintenance supporting system and method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application Serial No. 2007-327068, filed on Dec. 19, 2007, and Japanese Patent Application Serial No. 2008-205852, filed Aug. 8, 2008, each of which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention pertains to an inter-vehicle distance maintenance supporting system and an inter-vehicle distance maintenance supporting method. 
     BACKGROUND 
     The related art describes technology related to reaction force control for reducing discomfort of the driver. For example, Japanese Kokai Patent Application No. 2004-249891 describes an auxiliary device for driving vehicles. With this device, based on a confidence factor of false recognition and a confidence factor that an object is not present, a risk potential is computed, and, corresponding to the risk potential, a pattern of change in the reaction force generated in the vehicle equipment is corrected, so that the reaction force characteristics are taken to be good characteristics when the obstacle is not an object for reaction force control. 
     SUMMARY 
     Disclosed herein is an inter-vehicle distance maintenance supporting system for a host vehicle that provides an improved support running of a host vehicle. According to one embodiment of the present invention, an inter-vehicle distance maintenance supporting system for a host vehicle may include an obstacle detector configured to detect an obstacle ahead of the host vehicle, an inter-vehicle distance detector configured to detect an inter-vehicle distance between said host vehicle and said obstacle, a confidence factor computing device configured to compute a confidence factor for treating the obstacle as a preceding vehicle ahead of the host vehicle based on a state of the obstacle detected by said obstacle detector, a confidence factor correcting part configured to correct said confidence factor based on a relative-position relationship between said host vehicle and said obstacle, and a reaction force controller configured to apply a reaction force on an accelerator pedal based on said inter-vehicle distance detected by said inter-vehicle distance detector and said confidence factor corrected by said confidence factor correcting part. 
     According to another embodiment of the present invention, an inter-vehicle distance maintenance supporting system for a host vehicle may include an obstacle detector configured to detect an obstacle ahead of the host vehicle, an inter-vehicle distance detector configured to detect an inter-vehicle distance between said host vehicle and said obstacle, a confidence factor computing device configured to compute a confidence factor for treating the obstacle as a preceding vehicle ahead of the host vehicle based on a state of the obstacle detected by said obstacle detector, an accelerator pedal depression detector configured to detect depression of an accelerator pedal, a confidence factor correcting part configured to correct said confidence factor based on the depression of said accelerator pedal, and a reaction force controller configured to apply a reaction force on the accelerator pedal based on said inter-vehicle distance detected by said inter-vehicle distance detector and said confidence factor corrected by said confidence factor correcting part. 
     According to another embodiment of the present invention, an inter-vehicle distance maintenance supporting method for a host vehicle may include detecting an obstacle ahead of the host vehicle, detecting an inter-vehicle distance between said host vehicle and said obstacle, computing a confidence factor for treating the obstacle as a preceding vehicle of the host vehicle based on a detected obstacle state, correcting said confidence factor based on a relative-position relationship between said host vehicle and said obstacle, and applying a reaction force based on the inter-vehicle distance and the corrected confidence factor. 
     According to another embodiment of the present invention, an inter-vehicle distance maintenance supporting method for a host vehicle may include detecting an obstacle ahead of the host vehicle, detecting an inter-vehicle distance between said host vehicle and said obstacle, computing a confidence factor for treating the obstacle as a preceding vehicle of the host vehicle based on a detected obstacle state, detecting a depression of an accelerator pedal, correcting said confidence factor based on the depression of the accelerator pedal, and applying a reaction force based on the inter-vehicle distance and the corrected confidence factor. 
     According to another embodiment of the present invention, an inter-vehicle distance maintenance supporting system for a host vehicle may include an obstacle detecting means for detecting an obstacle ahead of the host vehicle, an inter-vehicle distance detecting means for detecting an inter-vehicle distance between said host vehicle and said obstacle, a confidence factor computing means for computing a confidence factor for treating the obstacle as a preceding vehicle ahead of the host vehicle based on a state of the obstacle detected by said obstacle detecting means, a confidence factor correcting means for correcting said confidence factor based on a relative-position relationship between said host vehicle and said obstacle, and a reaction force controlling means for applying a reaction force on an accelerator pedal based on said inter-vehicle distance detected by said inter-vehicle distance detecting means and said confidence factor corrected by said confidence factor correcting means. 
     According to another embodiment of the present invention, an inter-vehicle distance maintenance supporting system for a host vehicle may include an obstacle detecting means for detecting an obstacle ahead of the host vehicle, an inter-vehicle distance detecting means for detecting an inter-vehicle distance between said host vehicle and said obstacle, a confidence factor computing means for computing a confidence factor for treating the obstacle as a preceding vehicle ahead of the host vehicle based on a state of the obstacle detected by said obstacle detecting means, an accelerator pedal depression detecting means for detecting depression of an accelerator pedal, a confidence factor correcting means for correcting said confidence factor based on the depression of said accelerator pedal, and a reaction force controlling means for applying a reaction force on the accelerator pedal based on said inter-vehicle distance detected by said inter-vehicle distance detecting means and said confidence factor corrected by said confidence factor correcting means. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       These and other features, aspect, and advantages of the present invention will become apparent from the following description, appended claims, and the accompanying exemplary embodiments shown in the drawings, which are briefly described below. 
         FIG. 1  is a system diagram illustrating the inter-vehicle distance maintenance supporting system in an embodiment of the present invention. 
         FIG. 2  is a diagram illustrating the vehicle using the inter-vehicle distance maintenance supporting system shown in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating an arrangement of the controller. 
         FIG. 4  is a flow chart illustrating the procedure of the inter-vehicle distance maintenance supporting control block in the inter-vehicle distance maintenance supporting system in an embodiment of the invention. 
         FIG. 5  is a flow chart illustrating the procedure for confidence factor computing. 
         FIG. 6  is a diagram illustrating the method for computing the lateral offset value when the host vehicle travels on a curved road. 
         FIG. 7  is a flow chart illustrating the procedure for accelerator pedal depression detection. 
         FIG. 8  is a diagram illustrating a method for computing the lateral offset value when the host vehicle travels on a curved road. 
         FIG. 9  is a diagram illustrating the relationship between the inter-vehicle distance and the cutoff frequency correction value. 
         FIG. 10  is a diagram illustrating the lateral offset value and the confidence factor. 
         FIG. 11  is a flow chart illustrating the procedure for computing the first inter-vehicle distance threshold. 
         FIG. 12  is a diagram illustrating the relationship between the confidence factor and gain Kp. 
         FIG. 13  is a flow chart illustrating the procedure for driver operation judgment. 
         FIG. 14  is a flow chart illustrating the procedure for target pedal reaction force correction. 
         FIG. 15  is a diagram illustrating the relationship between the confidence factor and gain Kacc. 
         FIG. 16  is a flow chart illustrating the procedure for computing the second inter-vehicle distance threshold. 
         FIG. 17  is a flow chart further illustrating the procedure for computing the second inter-vehicle distance threshold. 
         FIG. 18  is a diagram illustrating the relationship between the preceding vehicle speed and the preceding-vehicle-speed-dependent reference distance. 
         FIG. 19  is a diagram illustrating the relationship between the slope of the road the host vehicle is traveling on and the slope-dependent correction time. 
         FIG. 20  is a flow chart illustrating the procedure for computing the deviation in inter-vehicle distance. 
         FIG. 21  is a flow chart illustrating the procedure for computing the final value of the target accelerator opening. 
         FIG. 22  is a diagram illustrating the relationship between the accelerator pedal depression amount and the target accelerator opening minimum value. 
         FIG. 23  is a diagram illustrating the relationship between the vehicle speed and the vehicle-speed-dependent gain. 
         FIG. 24  is a diagram illustrating the relationship between the slope of the road the host vehicle is traveling on and the slope-dependent corrected gain. 
         FIG. 25  is a diagram illustrating the relationship between the confidence factor and the minimum value of the torque down gain. 
         FIG. 26  is a flow chart illustrating the procedure for detecting depression of the accelerator pedal. 
         FIG. 27  is a diagram illustrating the relationship between the confidence factor and the accelerator opening speed threshold. 
         FIG. 28  is a flow chart illustrating the procedure for resetting the target accelerator opening. 
         FIG. 29  is a diagram illustrating the relationship between the inter-vehicle distance and the torque down gain increasing limiter. 
         FIG. 30  is a flow chart illustrating the procedure for confidence factor computing in Embodiment 2. 
         FIG. 31  is a diagram illustrating the relationship between the inter-vehicle distance and the prediction time. 
         FIG. 32  is a diagram illustrating the procedure for computing the position of the obstacle after the prediction time. 
         FIG. 33  is a flow chart illustrating the procedure for computing the confidence factor in Embodiment 3. 
         FIG. 34  is a flow chart illustrating the procedure for computing the confidence factor correction coefficient. 
         FIG. 35  is a diagram illustrating the relationship between the offset value and the confidence factor. 
         FIG. 36  is a flow chart illustrating the treatment procedure for accelerator pedal depression detection. 
     
    
    
     DETAILED DESCRIPTION 
     In the auxiliary device for driving vehicles in the related technology of the related art, if the preceding vehicle, as an object of reaction force control, temporarily moves to the adjacent lane, or if the host vehicle changes lanes so that the preceding vehicle is no longer detected, the accelerator pedal reaction force is quickly changed and transmitted to the driver. Consequently, it cannot be associated with a driver&#39;s prediction, so that discomfort is felt by the driver. This is undesirable. In the following, an explanation will be given regarding an example of an inter-vehicle distance maintenance supporting system that can prevent such a feeling of discomfort. 
     Embodiment 1 
     In the following, an explanation will be given regarding the inter-vehicle distance maintenance supporting system of Embodiment 1 of the present invention with reference to the figures.  FIG. 1  is a system diagram illustrating the inter-vehicle distance maintenance supporting system  1  of Embodiment 1 of the present invention.  FIG. 2  is a diagram illustrating the vehicle using inter-vehicle distance maintenance supporting system  1 . 
     First, the inter-vehicle distance maintenance supporting system  1  will be explained. Here, laser radar  10  is installed on the front grill or bumper of the vehicle. It emits IR light pulses in the horizontal direction to scan the region ahead of the vehicle. Laser radar  10  measures the reflected waves of IR light pulses reflected from plural reflective objects (usually the rear end of the preceding vehicle) ahead of the host vehicle, and, from the arrival time of the reflected waves, it detects the individual inter-vehicle distances of plural preceding vehicles and their direction. The detected inter-vehicle distance and the direction are output to controller  50 . In the present embodiment, the direction of the object ahead of the vehicle can be represented by the relative angle with respect to the host vehicle. The laser radar  10  scans about ±60° of the front region with respect to the normal direction of the front of the host vehicle, and objects ahead of the host vehicle present in said range can be detected. 
     In this case, vehicle speed sensor  20  detects the speed of the host vehicle by measuring the rotational velocity of the wheels and the rotational velocity from the transmission, and it outputs the detected host vehicle speed to controller  50 . Here, yaw rate sensor  30  detects the yaw rate of the vehicle, that is, the vehicle turning speed, and it outputs the detected yaw rate to controller  50 . 
     Controller  50  comprises a CPU as well as ROM, RAM and other CPU peripheral devices. It performs overall control of inter-vehicle distance maintenance supporting system  1 . Controller  50  uses the distance information input from laser radar  10  and the host vehicle speed input from vehicle speed sensor  20  to recognize the state of obstacles around the host vehicle, such as the relative distance and the relative speed between the host vehicle and each obstacle as the running state with respect to the obstacle. Based on the obstacle state, controller  50  computes the confidence factor for the obstacle ahead of the host vehicle, the first inter-vehicle distance threshold and the second inter-vehicle distance threshold. Then, it performs the following control based on the computed confidence factor, the first inter-vehicle distance threshold and the second inter-vehicle distance threshold. 
     Inter-vehicle distance maintenance supporting system  1  controls the reaction force generated when accelerator pedal  72  is depressed, so that the driver is notified of the surrounding environment, and the inter-vehicle distance maintenance supporting system can thus appropriately assist the driver, especially in maintaining an appropriate inter-vehicle distance with an obstacle ahead of the host vehicle. Also, by controlling the output amount of the engine torque with respect to the depression amount of accelerator pedal  72 , in the case of tracking mode, where said obstacle ahead of the host vehicle is tracked, it is possible to reduce operations performed by the driver in correcting accelerator pedal  72  and thus to reduce the physical load on the driver. At the same time, as the depression amount of accelerator pedal  72  is usually larger than that in the related art, by controlling the operation reaction force, it is easier to inform the driver of the operation reaction force generated at accelerator pedal  72 . In addition, when the output amount of the engine torque with respect to the accelerator pedal depression amount is reset to the normal relationship, by means of resetting corresponding to the accelerator pedal depression operation of the driver, it is possible to reduce discomfort caused by acceleration of the host vehicle even though the depression amount of accelerator pedal  72  is constant. 
     Inter-vehicle distance maintenance supporting system  1  also corrects the accelerator pedal operation reaction force and the engine torque output amount corresponding to the confidence factor that there is an obstacle ahead of the host vehicle. Here, the confidence factor of the obstacle ahead of the host vehicle is defined as the value indicating the confidence of the presence of an obstacle ahead of the host vehicle, that is, the confidence of the presence of an obstacle ahead of the host vehicle that becomes the object related to control of the operation reaction force and the engine torque. That is, it is defined as the value that represents the confidence of the ability to judge that an obstacle ahead of the host vehicle is indeed a preceding vehicle ahead of the host vehicle. By performing correction corresponding to the confidence factor of the obstacle ahead of the host vehicle, in the case when the host vehicle is passing the obstacle ahead of the host vehicle or a similar case when there is a deviation in the lateral position between the host vehicle and the obstacle ahead of the host vehicle, control can be released at an earlier time, so that the feeling of discomfort of the driver can be reduced. 
     More specifically, controller  50  computes the confidence factor for the obstacle ahead of the host vehicle from the relationship in lateral position (left/right direction) between the host vehicle and the obstacle ahead of the host vehicle. Then, based on the first inter-vehicle distance threshold for the obstacle ahead of the host vehicle, the target accelerator pedal reaction force is computed, and the computed target accelerator pedal reaction force is corrected corresponding to the confidence factor. The computed target correction value of the accelerator pedal reaction force is output to accelerator pedal reaction force controller  70 . 
     Then, controller  50  computes the target accelerator opening based on the second inter-vehicle distance threshold with respect to the obstacle ahead of the host vehicle and the accelerator pedal depression amount by the driver. Then, the computed target accelerator opening is corrected corresponding to the confidence factor, and the corrected target accelerator opening is output to engine controller  74 . Also, based on the accelerator pedal depression amount by the driver detected by accelerator pedal depression amount detecting part  73 , controller  50  judges whether the depression accelerator pedal  72  is depressed. When the target accelerator opening is reset to the accelerator pedal depression amount by the driver, controller  50  outputs the result of the target accelerator opening resetting treatment based on the determined accelerator pedal depression to engine controller  74 . 
     Corresponding to the reaction force control amount output from controller  50 , accelerator pedal reaction force controller  70  controls the torque generated by servo motor  71  assembled in the link mechanism of accelerator pedal  72 . Servo motor  71  controls the reaction force generated corresponding to the instruction value from accelerator pedal reaction force controller  70 , and it can control the depression force generated when the driver depresses accelerator pedal  72  at will. Also, accelerator pedal depression amount detecting part  73  is connected via a link mechanism to accelerator pedal  72 . Accelerator pedal depression amount detecting part  73  detects the depression amount (operation amount) of accelerator pedal  72  converted to the rotating angle of servo motor  71  via a link mechanism, and outputs it to controller  50 . 
     Also, with regard to the conventional accelerator pedal reaction force characteristics when the accelerator pedal reaction force is not controlled, for example, the accelerator pedal reaction force is set to be greater when the operation amount of accelerator pedal  72  is greater. The conventional accelerator pedal reaction force characteristics can be realized by means of, for example, the elastic force of a torsion spring (not shown in the figure) set to the rotating center of accelerator pedal  72 . 
     Engine controller  74  controls the generated engine torque to correspond to the target accelerator opening output from controller  50 . Engine controller  74  presets a relationship of the engine torque generation amount corresponding to the accelerator pedal depression amount. Here, engine controller  74  controls the engine torque by determining the engine torque generation amount based on the target accelerator opening output from controller  50  instead of the actual accelerator pedal depression amount due to depression by the driver, and adjusting the degree of opening of, for example, a throttle valve. That is, the target accelerator opening is the control instruction value of the engine torque. 
       FIG. 3  is a block diagram illustrating an arrangement of controller  50 . For example, controller  50  may comprise the following parts depending on the CPU software: obstacle recognition part  51 , confidence factor computing part  52 , first inter-vehicle distance threshold computing part  53 , accelerator pedal reaction force determining part  54 , driver operation judgment part  55 , accelerator pedal reaction force correcting part  56 , predicted slope value computing part  57 , second inter-vehicle distance threshold computing part  58 , target accelerator opening computing part  59 , accelerator pedal depression operation detecting part  60 , and target accelerator opening resetting part  61 . 
     Obstacle recognition part  51  computes the inter-vehicle distance and relative speed to an obstacle, such as the preceding vehicle, ahead of the host vehicle based on the signal input from laser radar  10 . In addition, it detects the state of the obstacle ahead of the host vehicle from the inter-vehicle distance, the relative speed, and the host vehicle speed input from vehicle speed sensor  20 . Confidence factor computing part  52  computes the confidence factor of the obstacle preset ahead of the host vehicle based on the yaw rate of the host vehicle input from yaw rate sensor  30 . 
     First inter-vehicle distance threshold computing part  53  computes the first inter-vehicle distance threshold with respect to the obstacle ahead of the host vehicle based on the obstacle state input from obstacle recognition part  51 . Accelerator pedal reaction force determining part  54  determines the accelerator pedal reaction force applied on accelerator pedal  72  based on the first inter-vehicle distance threshold computed by first inter-vehicle distance threshold computing part  53  and the inter-vehicle distance input from obstacle recognition part  51 . Driver operation judgment part  55  judges whether the driver is depressing accelerator pedal  72  based on the accelerator pedal depression amount input from accelerator pedal depression amount detecting part  73  and the confidence factor computed by confidence factor computing part  52 . Accelerator pedal reaction force correcting part  56  uses the judgment result of driver operation judgment part  55  and the confidence factor computed by confidence factor computing part  52  to correct the accelerator pedal reaction force computed by accelerator pedal reaction force determining part  54 , and outputs the corrected accelerator pedal reaction force to accelerator pedal reaction force controller  70 . 
     Second inter-vehicle distance threshold computing part  58  computes the second inter-vehicle distance threshold with respect to the obstacle ahead of the host vehicle based on the state of the obstacle input from obstacle recognition part  51 . On the basis of the second inter-vehicle distance threshold computed by second inter-vehicle distance threshold computing part  58 , the accelerator pedal depression amount input from accelerator pedal depression amount detecting part  73 , and the confidence factor computed by confidence factor computing part  52 , target accelerator opening computing part  59  computes the target accelerator opening (final value of the target accelerator opening) for use as the control instruction value of the engine torque to be finally realized. 
     From the accelerator pedal depression amount input from accelerator pedal depression amount detecting part  73  and the confidence factor computed by confidence factor computing part  52 , accelerator pedal depression operation detecting part  60  detects the accelerator pedal depression operation by the driver. On the basis of the detection result of accelerator pedal depression operation detecting part  60  and the confidence factor computed by confidence factor computing part  52 , target accelerator opening resetting part  61  resets the final value of the target accelerator opening computed by target accelerator opening computing part  59 , and it re-computes the target accelerator opening. 
     In the following, an explanation will be given in more detail regarding the operation of inter-vehicle distance maintenance supporting system  1  of Embodiment 1.  FIG. 4  is a flow chart illustrating the procedure of the inter-vehicle distance maintenance control operation in controller  50  as an embodiment. This operation is performed consecutively once every prescribed interval, for example, 50 msec. 
     First, in step S 100 , the running state is read. Here, the running state refers to information pertaining to the running state of the host vehicle including the state of the obstacle ahead of the host vehicle. Here, the inter-vehicle distance to the obstacle ahead of the host vehicle and the direction of the obstacle ahead of the host vehicle, such as a preceding vehicle, detected by laser radar  10  and the host vehicle speed detected by vehicle speed sensor  20  are read. 
     In step S 200 , based on the running state data read and recognized in step S 100 , the state of the obstacle ahead of the host vehicle is recognized. Here, based on the relative position of the obstacle and its movement direction/movement speed with respect to the host vehicle detected in the preceding process cycle and before that and stored in the memory of controller  50  and the current running state obtained in step S 100 , the current relative position and its movement direction/movement speed of the obstacle with respect to the host vehicle are recognized. Then, it recognizes where the obstacle with respect to running of the host vehicle is set and how it moves in relation. 
     In step S 300 , as the value representing the confidence that the obstacle ahead of the host vehicle and as the object for the operation reaction force control and engine torque control will remain present as the control object ahead of the host vehicle, the confidence factor of the obstacle is computed. The confidence factor may also be taken as the value that represents the probability of the obstacle ahead of the host vehicle being present in the road to be traveled by the host vehicle (predicted running path). 
     The predicted running path can be predicted based on the yaw rate detected by yaw rate sensor  30  and the host vehicle speed detected by vehicle speed sensor  20 . In this case, when the predicted running path is determined, filtering is performed for the yaw rate detected by yaw rate sensor  30  such that there is no variation in the predicted running path due to small variations in the yaw rate. This filter can be realized by, for example, a low-pass filter. 
     When heavy filtering is performed to remove noise and drift in the detected yaw rate, the response property slows. Consequently, for example, when the host vehicle changes lanes to pass the obstacle ahead of the host vehicle, the predicted running path determined based on the yaw rate cannot quickly respond to the motion of the host vehicle, especially turning of the steering wheel. As a result, it treats the obstacle ahead of the host vehicle to be passed as remaining in the predicted running path, so that the operation reaction force control and the engine torque control continues with the obstacle ahead of the host vehicle are taken as the object. As a result, the driver feels a braking-like discomfort when the host vehicle passes the obstacle ahead of the host vehicle. 
     According to Embodiment 1, when the driver depresses accelerator pedal  72  to pass the obstacle ahead of the host vehicle, the cutoff frequency in the filtering with respect to the yaw rate detected by yaw rate sensor  30  is corrected, and the mode changes to light filtering. As a result, a predicted running path that corresponds swiftly to the steering wheel operation by the driver is sought. 
     In the following, an explanation will be given regarding the treatment for computing the confidence factor in step S 300  with reference to the flow chart shown in  FIG. 5 .  FIG. 6  is a schematic diagram illustrating the relative positional relationship between the host vehicle and an obstacle when it appears ahead of the host vehicle while the host vehicle travels a curve in the road. As shown in  FIG. 6 , when deviation occurs in the lateral direction between the center of the host vehicle and the center of the obstacle, this lateral deviation is computed as offset value α and, from the computed lateral offset value α, the confidence factor is computed. 
     First, in step S 301 , whether accelerator pedal  72  is depressed down is detected. In the following, an explanation will be given regarding this process with reference to the flow chart shown in  FIG. 7 . In step S 3011 , by performing differential computation for accelerator pedal depression amount APO by the driver detected by accelerator pedal depression amount detecting part  73 , and the depression speed of accelerator pedal  72 , that is, accelerator opening speed dAPO, is computed. 
     In step S 3012 , it is determined whether the accelerator opening speed dAPO exceeds a prescribed accelerator opening speed threshold dAPO 1 . When dAPO≧dAPO 1 , it is determined that the driver is depressing accelerator pedal  72 , so that the process goes to step S 3013  to set accelerator depression operation flag Flg_APO to 1. On the other hand, if dAPO&lt;dAPO 1 , it is determined that the driver is not stepping down accelerator pedal  72 , that is, accelerator pedal  72  is maintained or reset, or accelerator pedal  72  is released, so that the process goes to step S 3014 , and accelerator depression operation flag Flg_APO is set at 0, that is, it is cleared. 
     In step S 302 , it is determined whether accelerator pedal  72  is depressed down based on the result of detection of the accelerator pedal depression operation in step S 301 . When accelerator pedal  72  is depressed down (Flg_APO=1), the process goes to step S 308 . On the other hand, when accelerator pedal  72  is not depressed down (Flg_APO=0), the process goes to step S 303 . 
     In step S 303 , filtering is performed with respect to yaw rate ω detected by yaw rate sensor  30 , and yaw rate filter value ω 1  is computed. Here, yaw rate filter value ω 1  can be computed using the following Formula 1 from cutoff frequency f 1 .
 
ω1=ω×(2 πf 1)/( S+ 2 πf 1)  (Formula 1)
 
     In Formula 1, S represents a Laplace operator. 
     In step  304 , host vehicle speed V detected by vehicle speed sensor  20  is read. In step S 305 , the turning radius (predicted turning radius) R of the predicted running path is computed from yaw rate filter value ω 1  computed in step S 303  and host vehicle speed V read in step S 304 . The predicted turning radius R can be computed using the following Formula 2.
 
 R=V/ω 1  (Formula 2)
 
     In step S 306 , the position of the obstacle ahead of the host vehicle is computed. As shown in  FIG. 8 , the position of the center of the curved road is taken as O, and the central angle between the host vehicle and the obstacle is taken as θR. Also, the position of the center of the host vehicle when the host vehicle reaches the current position of the obstacle is taken as E, and the distance between position E and obstacle center position B is taken as α. Also, R represents the turning radius of the curved road, and the predicted turning radius computed in step S 305  is adopted as is. 
     In the following, an explanation will be given regarding the geometric method for determining lateral offset value α using distances L 1 , L 2  and angles θ 1 , θ 2  to the left/right edges of the obstacle, as well as host vehicle speed V. The various vectors in  FIG. 8  are represented by the following Formulas 3-11. 
     
       
         
           
             
               
                 
                   
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                                     1 
                                   
                                   · 
                                   cos 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   1 
                                 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     9 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     BD 
                     → 
                   
                   = 
                   
                     
                       
                         AD 
                         → 
                       
                       - 
                       
                         
                           A 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           B 
                         
                         → 
                       
                     
                     = 
                     
                       ( 
                       
                         
                           
                             
                               
                                 
                                   
                                     - 
                                     
                                       L 
                                       2 
                                     
                                   
                                   · 
                                   sin 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   2 
                                 
                               
                               + 
                               
                                 
                                   L 
                                   · 
                                   sin 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                             
                           
                         
                         
                           
                             
                               
                                 
                                   
                                     L 
                                     2 
                                   
                                   · 
                                   cos 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   θ 
                                   2 
                                 
                               
                               - 
                               
                                 
                                   L 
                                   · 
                                   cos 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     10 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     OB 
                     → 
                   
                   = 
                   
                     
                       
                         OA 
                         → 
                       
                       + 
                       
                         AB 
                         → 
                       
                     
                     = 
                     
                       ( 
                       
                         
                           
                             
                               R 
                               - 
                               
                                 
                                   L 
                                   · 
                                   sin 
                                 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 θ 
                               
                             
                           
                         
                         
                           
                             
                               
                                 L 
                                 · 
                                 cos 
                               
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               θ 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     11 
                   
                   ) 
                 
               
             
           
         
       
     
     Also, obstacle width D 1  can be computed using the following formula (Formula 12).
 
∴ D   1   2   =L   1   2   +L   2   2 −2 L   1   ·L   2  cos(θ 1 −θ 2 )
 
∴ D   1 =√{square root over ( L   1   2   +L   2   2 −2 L   1   ·L   2  cos(θ 1 −θ 2 ))}  (Formula 12)
 
     Obstacle width D 1  can be used to compute distance L to the center of the obstacle using Formula (13). 
     
       
         
           
             
               
                 
                   
                     
                       L 
                       1 
                       2 
                     
                     + 
                     
                       L 
                       2 
                       2 
                     
                   
                   = 
                   
                     
                       
                         2 
                         · 
                         
                           ( 
                           
                             
                               
                                 ( 
                                 
                                   
                                     D 
                                     1 
                                   
                                   2 
                                 
                                 ) 
                               
                               2 
                             
                             + 
                             
                               L 
                               2 
                             
                           
                           ) 
                         
                       
                       ⁢ 
                       
                         
 
                       
                       ∴ 
                       L 
                     
                     = 
                     
                       
                         
                           
                             
                               L 
                               1 
                               2 
                             
                             + 
                             
                               L 
                               2 
                               2 
                             
                           
                           2 
                         
                         - 
                         
                           
                             ( 
                             
                               
                                 D 
                                 1 
                               
                               2 
                             
                             ) 
                           
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     13 
                   
                   ) 
                 
               
             
           
         
       
     
     With regard to angle θ between obstacle center position B and the central line of the host vehicle in the longitudinal direction, because vector CB=vector BD, it can be represented by Formula 14. 
     
       
         
           
             
               
                 
                   
                     
                       sin 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     = 
                     
                       
                         
                           
                             
                               L 
                               1 
                             
                             · 
                             sin 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             1 
                           
                         
                         + 
                         
                           
                             
                               L 
                               2 
                             
                             · 
                             sin 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             2 
                           
                         
                       
                       
                         2 
                         ⁢ 
                         L 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       cos 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     = 
                     
                       
                         
                           
                             
                               L 
                               1 
                             
                             · 
                             cos 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             1 
                           
                         
                         + 
                         
                           
                             
                               L 
                               2 
                             
                             · 
                             cos 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             θ 
                             2 
                           
                         
                       
                       
                         2 
                         ⁢ 
                         L 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     14 
                   
                   ) 
                 
               
             
           
         
       
     
     Also, because vector OB and vector OE are parallel, the central angle θR between the host vehicle and the obstacle can be represented by Formula 15 below. 
     
       
         
           
             
               
                 
                   
                     tan 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       R 
                     
                   
                   = 
                   
                     
                       
                         L 
                         · 
                         cos 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     
                       R 
                       - 
                       
                         
                           L 
                           · 
                           sin 
                         
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         θ 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     15 
                   
                   ) 
                 
               
             
           
         
       
     
     In step S 307 , lateral offset value α between the host vehicle and the obstacle is computed. If the angle in the front-left direction of the host vehicle is positive, the offset value α can be computed by the following Formulas 16, 17. 
     
       
         
           
             
               
                 
                   
                     
                       When 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     &gt; 
                     0 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     α 
                     = 
                     
                       
                          
                         
                           EB 
                           → 
                         
                          
                       
                       = 
                       
                         
                            
                           
                             
                               OB 
                               → 
                             
                             - 
                             
                               OE 
                               → 
                             
                           
                            
                         
                         = 
                         
                            
                           
                             ( 
                             
                               
                                 
                                   
                                     R 
                                     - 
                                     
                                       
                                         L 
                                         · 
                                         sin 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                     - 
                                     
                                       
                                         R 
                                         · 
                                         cos 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         θ 
                                         R 
                                       
                                     
                                   
                                 
                               
                               
                                 
                                   
                                     
                                       
                                         L 
                                         · 
                                         cos 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       θ 
                                     
                                     - 
                                     
                                       
                                         R 
                                         · 
                                         sin 
                                       
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         θ 
                                         R 
                                       
                                     
                                   
                                 
                               
                             
                             ) 
                           
                            
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     16 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     
                       When 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       θ 
                     
                     &lt; 
                     0 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     α 
                     = 
                     
                       
                         - 
                         
                            
                           
                             EB 
                             → 
                           
                            
                         
                       
                       = 
                       
                         
                           - 
                           
                              
                             
                               
                                 OB 
                                 → 
                               
                               - 
                               
                                 OE 
                                 → 
                               
                             
                              
                           
                         
                         = 
                         
                           - 
                           
                              
                             
                               ( 
                               
                                 
                                   
                                     
                                       R 
                                       - 
                                       
                                         
                                           L 
                                           · 
                                           sin 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                       - 
                                       
                                         
                                           R 
                                           · 
                                           cos 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         
                                           θ 
                                           R 
                                         
                                       
                                     
                                   
                                 
                                 
                                   
                                     
                                       
                                         
                                           L 
                                           · 
                                           cos 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         θ 
                                       
                                       - 
                                       
                                         
                                           R 
                                           · 
                                           sin 
                                         
                                         ⁢ 
                                         
                                             
                                         
                                         ⁢ 
                                         
                                           θ 
                                           R 
                                         
                                       
                                     
                                   
                                 
                               
                               ) 
                             
                              
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     17 
                   
                   ) 
                 
               
             
           
         
       
     
     On the other hand, in step S 302 , if it is determined that accelerator pedal  72  is depressed, the process goes to step S 308 , and the cutoff frequency is corrected to change the filtering with respect to yaw rate ω detected by yaw rate sensor  30  to light filtering. Here, cutoff frequency correction value f′ is set based on inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle. By correcting the cutoff frequency in this way, it is possible to correct the confidence factor, which is to be explained later. 
       FIG. 9  is a diagram illustrating the relationship between inter-vehicle distance L and cutoff frequency correction value f′. When inter-vehicle distance L is greater than prescribed inter-vehicle distance L 1 , cutoff frequency correction value f′ is fixed at minimum value f 1  to remove the noise and drift of the detected value of the yaw rate. Here, minimum value f 1  refers to the cutoff frequency adopted in the filtering performed when accelerator pedal  72  is depressed down. When inter-vehicle distance L becomes less than prescribed inter-vehicle distance L 1 , cutoff frequency correction value f′ is gradually increased. When inter-vehicle distance L becomes greater than prescribed inter-vehicle distance L 2 , cutoff frequency correction value f′ is fixed at maximum value f 2 . As a result, if the driver tries to pass the obstacle ahead of the host vehicle so that the driver depresses accelerator pedal  72  to approach the obstacle ahead of the host vehicle, it is possible to detect the yaw rate with a high response. 
     In step S 309 , cutoff frequency correction value f′ computed in step S 308  is used to compute yaw rate correction value ω 2  after filtering (yaw rate filter correction value). The yaw rate correction value ω 2  is computed using the following Formula 18.
 
ω2=ω×(2 πf ′)/( S+ 2 πf ′)  (Formula 18)
 
     In step S 310 , host vehicle speed V detected by vehicle speed sensor  20  is read. In step S 311 , predicted turning radius R is computed. Here, predicted turning radius R can be computed using the following Formula 19 from yaw rate correction value ω 2  computed in step S 309  and host vehicle speed V.
 
 R=V/ω 2  (Formula 19)
 
     In step S 312 , Formulas 3-15 above are used to detect the position of the obstacle ahead of the host vehicle. In step S 313 , Formulas 16 and 17 above are used to compute lateral offset value α between the host vehicle and the obstacle. 
     In step S 314 , lateral offset value α computed in step S 307  or S 313  is used to compute confidence factor Prob of the obstacle.  FIG. 10  is a diagram illustrating the relationship between the lateral offset value α and confidence factor Prob. 
     As shown in  FIG. 10 , when lateral offset value α=0, that is, when host vehicle center position A and center position B of the obstacle ahead of the host vehicle are in agreement when the host vehicle reaches the position of the obstacle ahead, keeping the obstacle ahead of the host vehicle as the object for control is ensured, so that confidence factor Prob=1. That is, a higher confidence that the obstacle is present ahead of the host vehicle, means that a larger value is set for confidence factor Prob. A larger lateral offset value α, means higher that there is a possibility that the obstacle ahead of the host vehicle will not remain as an obstacle ahead of the host vehicle. In this case, confidence factor Prob that the current obstacle ahead of the host vehicle will remain to be an object for control is gradually reduced. When α&gt;(D0/2+D1/2) or α&lt;(D0/2−D1/2), the superposing amount between the host vehicle and the obstacle ahead of the host vehicle in the lateral direction disappears, and confidence factor Prob equals 0. 
     In this way, after confidence factor Prob of the obstacle is computed in step S 300 , the process goes to step S 400 . In step S 400 , the first inter-vehicle distance threshold with respect to the obstacle ahead of the host vehicle for use in the accelerator pedal reaction force control is computed. In the following, an explanation will be given regarding the operation carried out here with reference to the flow chart shown in  FIG. 11 . 
     In step S 401 , first, inter-vehicle distance threshold (steady-state value) Lh 1 * is computed. The inter-vehicle distance threshold (steady-state value) Lh 1 * corresponds to the inter-vehicle distance threshold when it is assumed that the vehicle speed of the obstacle, such as a preceding vehicle, is constant in the formula for computing the first inter-vehicle distance threshold for the obstacle ahead of the host vehicle. In this embodiment, inter-vehicle distance threshold (steady-state value) Lh 1  is set to correspond to host vehicle speed VSP and relative speed Vr with the obstacle recognized in steps S 100  and S 200  (Lh 1 *=f(VSP, Vr)). 
     In step S 402 , preceding vehicle speed Va is computed using Formula 20 based on host vehicle speed VSP and relative speed Vr.
 
Va+VFSP+Vr  (Formula 20)
 
     In step S 403 , the following Formula 21 is used to compute acceleration/deceleration αa of the preceding vehicle.
 
 Aa=d ( Va )/ dt   (Formula 21)
 
     In step S 404 , it is determined whether parameter Tr 1  for inter-vehicle distance threshold (transient value) for computing inter-vehicle distance threshold (transient value) Lr 1 * is computed/refreshed. As the condition for computing/refreshing the parameter Tr 1  for the inter-vehicle distance threshold (transient value), it is determined whether alarm flag Fw computed in step S 500  to be explained later is set. If the alarm flag is not set (Fw=OFF), the process goes to step S 405 . On the other hand, if the alarm flag is set (Fw=ON), the process goes to step S 408  without refreshing parameter Tr 1  for the inter-vehicle distance threshold (transient value). 
     In step S 405 , it is determined whether the preceding vehicle is decelerating. In this embodiment, it is determined whether acceleration/deceleration αa of the preceding vehicle computed in step S 403  exceeds a prescribed level. If the acceleration/deceleration αa of the preceding vehicle is less than prescribed level α 0  (αa≦α 0 ), it is determined that the preceding vehicle is decelerating, so that preceding vehicle deceleration judgment flag Fdec_a=ON. Then, the process goes to step S 406 . On the other hand, when acceleration/deceleration αa of the preceding vehicle exceeds the prescribed level α 0  (αa&gt;α 0 ), the preceding vehicle deceleration judgment flag Fdec_a=OFF, and the process goes to step S 407 . Here, prescribed level α 0  is a threshold for judging whether the preceding vehicle is decelerating, and it is preset to an appropriate value. Here, the acceleration/deceleration αa of the preceding vehicle and deceleration judgment threshold α 0  are taken to have positive values in acceleration, and negative values in deceleration. 
     In step S 406 , when it is determined that the preceding vehicle is decelerating, the following formula (Formula 22) is used to compute and refresh parameter Tr 1  for the inter-vehicle distance threshold (transient value). 
     
       
         
           
             
               
                 
                   
                     Tr 
                     1 
                   
                   = 
                   
                     
                       ( 
                       
                         L 
                         - 
                         
                           L 
                           
                             h 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             1 
                           
                           * 
                         
                       
                       ) 
                     
                     Vr 
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     22 
                   
                   ) 
                 
               
             
           
         
       
     
     As can be seen from Formula 22, the parameter Tr 1  for the inter-vehicle distance threshold (transient value) represents the portion (L−Lh 1 *) corresponding to the tolerable distance of real inter-vehicle distance L with respect to inter-vehicle distance threshold (steady-state value) Lh 1 * when the preceding vehicle starts decelerating at a relative speed coefficient. 
     In step S 407 , when it is determined that the preceding vehicle is not decelerating, parameter Tr 1  for the inter-vehicle distance threshold (transient value) is cleared (Tr 1 =0). 
     In step S 408 , the following formula (Formula 23) is used to compute inter-vehicle distance threshold (transient value) Lr 1 *.
 
 Lr 1*= Tr 1× Vr   (Formula 23)
 
     Here, inter-vehicle distance threshold (transient value) Lr 1 * corresponds to the inter-vehicle distance threshold when it is assumed that the obstacle ahead of the host vehicle, such as a preceding vehicle, is decelerating in the formula for computing the first inter-vehicle distance threshold. 
     In step S 409 , first inter-vehicle distance threshold L 1 * is computed using inter-vehicle distance threshold (steady-state value) Lh 1 * computed in step S 401  and the inter-vehicle distance threshold computed in step S 408 . In this embodiment, the following formula (Formula 24) is used to compute first inter-vehicle distance threshold L 1 * as the sum of the inter-vehicle distance threshold (steady-state value) Lh 1 * and the inter-vehicle distance threshold (transient value) Lr 1 *.
 
 L 1* =Lh 1* +Lr 1*  (Formula 24)
 
     After computing of first inter-vehicle distance threshold L 1 * in step S 400 , the process goes to step S 500 . In step S 500 , the alarm flag Fw is computed. More specifically, the alarm flag Fw is determined using actual inter-vehicle distance L between the host vehicle and the preceding vehicle read in step S 100  and first inter-vehicle distance threshold, L 1 * computed in step S 400 . When the actual inter-vehicle distance L is less than first inter-vehicle distance threshold L 1 * (L 1 *&gt;L), the alarm flag Fw=ON. On the other hand, if actual inter-vehicle distance L is greater than the first inter-vehicle distance threshold L 1 * (L* 1 ≦L), alarm flag Fw=OFF. 
     Then, in step S 600 , based on the first inter-vehicle distance threshold L 1 *, target accelerator pedal reaction force FA* for applying on accelerator pedal  72  is determined. In order to compute target accelerator pedal reaction force FA*, first, difference (deviation in inter-vehicle distance) ΔL 1  between first inter-vehicle distance threshold L 1 * and actual inter-vehicle distance L is computed using the following formula (Formula 25).
 
Δ L 1= L 1*−1  (Formula 25)
 
     Then, from first inter-vehicle distance threshold L 1 * and inter-vehicle distance deviation ΔL 1 , Formula 26 is used to compute target accelerator pedal reaction force FA*.
 
 FA*=Kp×ΔL 1  (Formula 26)
 
     In Formula 26, Kp represents the gain for computing target accelerator pedal reaction force FA* from inter-vehicle distance deviation ΔL 1 , and it is set based on confidence factor Prob of the obstacle computed in step S 300 .  FIG. 12  is a diagram illustrating the relationship between confidence factor Prob and gain Kp. As shown in  FIG. 12 , a smaller confidence factor Prob, means lower a gain Kp. Here, target accelerator pedal reaction force FA* is computed such that it is larger when actual inter-vehicle distance L decreases with respect to first inter-vehicle distance threshold L 1 *, and it is smaller when gain Kp computed based on confidence factor Prob is less. When an obstacle is present right ahead of the host vehicle, confidence factor Prob=1. On the other hand, when the host vehicle and the obstacle superpose each other by about half, confidence factor Prob=0.8. Also, for example, when the right end of the host vehicle and the left end of the obstacle agree, confidence factor Prob=0.6. 
     In this way, target accelerator pedal reaction force FA* is computed in step S 600 . Then, the process goes to step S 700 . In step S 700 , it is determined whether the operator has further depressed accelerator pedal  72 . In the following, an explanation will be given regarding the operation carried out in this case with reference to the flow chart shown in  FIG. 13 . 
     In step S 701 , as the condition for refreshing accelerator opening retention value Acch, it is determined whether alarm flag Fw computed in step S 500  is set. When alarm flag Fw is not set (Fw=OFF), the process goes to step S 702 . On the other hand, when alarm flag Fw is set (Fw=ON), the process goes to step S 703 . 
     In step S 702 , accelerator pedal depression amount APO of accelerator pedal  72  by the driver depresses the pedal and detected by accelerator pedal depression amount detecting part  73  is set as accelerator opening retention value Acch. Then, accelerator depression increment ΔAcc of accelerator pedal  72  is cleared (ΔAcc=0). Here, accelerator depression increment ΔAcc indicates whether accelerator pedal  72  is further depressed from the accelerator opening retention value Acch, that is, whether the accelerator pedal is further depressed down. 
     In step S 703 , it is determined whether accelerator pedal depression amount APO detected by accelerator pedal depression amount detecting part  73  is less than accelerator opening retention value Acch. When accelerator pedal depression amount APO is less than accelerator opening retention value Acch, the process goes to step S 704 . On the other hand, if accelerator pedal depression amount APO is greater than accelerator opening retention value Acch, the process goes to step S 705 . 
     In step S 704 , while accelerator pedal depression amount APO detected by accelerator pedal depression amount detecting part  73  is set as accelerator opening retention value Acch, accelerator depression increment ΔAcc is cleared (ΔAcc=0). On the other hand, in step S 705 , accelerator depression increment ΔAcc is computed using the following formula (Formula 27) from accelerator pedal depression amount APO and accelerator opening retention value Acch.
 
ΔAcc=APO−Acch  (Formula 27)
 
     In this way, after determination of driver action in step S 700 , that is, after determination of whether the driver has depressed down accelerator pedal  72 , the process goes to step S 800 . In step S 800 , based on depression by the driver determined in step S 700 , target accelerator pedal reaction force FA* computed in step S 600  is corrected. In the following, an explanation will be given regarding the operation carried out here with reference to the flow chart shown in  FIG. 14 . 
     First, in step S 801 , based on accelerator depression increment ΔAcc computed in step S 700 , target pedal reaction force correction coefficient K_fa for correcting target accelerator pedal reaction force FA* is computed according to the following formula (Formula 28).
 
 K   —   fa= 100−(ΔAcc× K acc)   (Formula 28)
 
     Here, Kacc is the gain for computing target pedal reaction force correction coefficient K_fa from accelerator depression increment ΔAcc, and it is set based on confidence factor Prob of the obstacle computed in step S 300 .  FIG. 15  is a diagram illustrating the relationship between confidence factor Prob and gain Kacc. As shown in  FIG. 15 , when confidence factor Prob approaches one, gain Kacc is set to the minimum value, such as the smaller confidence factor Prob results in a higher gain Kacc. Here, the maximum value of target pedal reaction force correction coefficient K_fa is 100, and the minimum is 0. 
     In step S 802 , target accelerator pedal reaction force correction value FA*corr is computed by means of the following formula (Formula 29) from target pedal reaction force correction coefficient K_fa computed in step S 801  and target accelerator pedal reaction force FA* computed in step S 600 .
 
 FA *corr= K   —   fa×FA*/ 100  (Formula 29)
 
     Consequently, a smaller confidence factor Prob results in a larger gain Kacc, and larger correction amount of target accelerator pedal reaction force FA* with respect to accelerator depression increment ΔAcc. That is, in this case, target accelerator pedal reaction force correction value FA*corr decreases and accelerator pedal  72  can be depressed down more easily. Also, a larger accelerator depression increment ΔAcc results in a smaller target pedal reaction force correction coefficient K_fa, and smaller the target accelerator pedal reaction force correction value FA*corr. 
     In this way, after target accelerator pedal reaction force correction value FA*corr is computed in step S 800 , the process goes to step S 900 . In step S 900 , the second inter-vehicle distance threshold for the obstacle for the engine torque control is computed. In the following, an explanation will be given in more detail regarding the computing of the second inter-vehicle distance threshold with reference to the flow chart shown in  FIG. 16 . 
     In step S 910 , the slope of the road the host vehicle is traveling is determined. First, if the torque amplification rate of the engine torque converter is Rt, the automatic transmission gear ratio is Rat, and the differential gear ratio is Rdef, the relationship between driving shaft torque Tw and engine torque Te can be represented by the following formula (Formula 3).
 
 Tw=Rt×Rat×Rdef×Te   (Formula 30)
 
     Also, if the brake cylinder area is Ab, the rotor effective radius is Rb, and the pad frictional coefficient is μb, the relationship between brake hydraulic pressure instruction value Pbr and brake torque Tbr is represented by the following formula (Formula 31).
 
 Tbr= 8 ×Ab×Rb×μb×Pbr   (Formula 31)
 
     In addition, aerodynamic resistance Fa and rotary resistance Fr acting on the host vehicle can be computed using the following formulas 32 and 33, respectively.
 
 Fa=μa×Sv×VSP   2   (Formula 32)
 
 Fr=μr×Mv×g   (Formula 33)
 
     Here, μa represents the aerodynamic resistivity, Sv represents the front projection area, μr represents the rotary resistivity, Mv represents weight of the vehicle, g represents the acceleration of gravity, and VSP represents the host vehicle speed. 
     From driving shaft torque Tw generated by the engine torque and the brake hydraulic pressure, aerodynamic resistance Fa and rotary resistance Fr computed using the above Formulas 30-33, the acceleration of the host vehicle is determined and compared with the actual acceleration, so that slope SLP of the road the host vehicle is traveling can be determined using the following formula (Formula 34). 
     
       
         
           
             
               
                 
                   SLP 
                   = 
                   
                     
                       
                         
                           T 
                           w 
                         
                         - 
                         
                           T 
                           br 
                         
                         - 
                         
                           
                             R 
                             w 
                           
                           ⁡ 
                           
                             ( 
                             
                               
                                 F 
                                 a 
                               
                               + 
                               
                                 F 
                                 r 
                               
                             
                             ) 
                           
                         
                       
                       
                         
                           M 
                           v 
                         
                         ⁢ 
                         
                           R 
                           w 
                         
                       
                     
                     - 
                     
                       s 
                       · 
                       VSP 
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     34 
                   
                   ) 
                 
               
             
           
         
       
     
     Here, s represents a Laplace operator, and Rw represents the coefficient used in computing the slope. 
     In step S 920 , second inter-vehicle distance threshold L 2 * with respect to the obstacle ahead of the host vehicle is computed. In the following, an explanation will be given in more detail regarding computing of the second inter-vehicle distance threshold performed in step S 920  with reference to the flow chart shown in  FIG. 17 . 
     First of all, in step S 921 , based on preceding vehicle speed Va, preceding-vehicle-speed-dependent reference distance Lh 2 * is computed.  FIG. 18  is a diagram illustrating the relationship between preceding vehicle speed Va and preceding-vehicle-speed-dependent reference distance Lh 2 *. As shown in  FIG. 18 , the preceding-vehicle-speed-dependent reference distance Lh 2 * is set such that it is increased slowly from minimum value L 2 min so that a higher preceding vehicle speed Va means a farther distance at which the output amount of the engine torque with respect to accelerator pedal depression amount APO is controlled. 
     In step S 922 , slope-dependent correction time T_slp is computed based on slope SLP of the road the host vehicle is traveling.  FIG. 19  is a diagram illustrating the relationship between slope SLP and slope-dependent correction time T_slp. As shown in  FIG. 19 , when slope SLP is positive, that is, when the vehicle travels up a slope, slope-dependent correction time T_slp is set to a negative value. On the other hand, when the slope SLP is negative, that is, when the vehicle travels down a slope, slope-dependent correction time T_slp is set to a positive value, such that a larger absolute value of the slope SLP means a larger absolute value of slope-dependent correction time T_slp. Also, when the absolute value of slope SLP exceeds a prescribed level, the absolute value of slope-dependent correction time T_slp is fixed at a prescribed value. 
     In step S 923 , relative-speed-dependent correction distance Lr 2 * is computed. From the preset reference time T 1  and slope-dependent correction time T_slp computed in step S 922 , relative-speed-dependent correction distance Lr 2 * is computed using the following formula (Formula 35).
 
 Lr 2*=( T 1 +T   —   slp )×(− Vr )  (Formula 35)
 
     In step S 924 , second inter-vehicle distance threshold L 2 * is computed. From preceding-vehicle-speed-dependent reference distance Lh 2 * computed in step S 921  and relative-speed-dependent correction distance Lr 2 * computed in step S 923 , second inter-vehicle distance threshold L 2 * is computed using the following formula (Formula 36).
 
 L 2* =Lh 2* +Lr 2*  (Formula 36)
 
     After second inter-vehicle distance threshold L 2 * is computed in step S 920 , in step S 930 , inter-vehicle distance deviation ΔL 2  is computed from actual inter-vehicle distance L and second inter-vehicle distance threshold L 2 *. In the following, an explanation will be given regarding the operation carried out here with reference to the flow chart shown in  FIG. 20 . 
     In step S 931 , it is determined whether the actual inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle detected by laser radar  10  is less than second inter-vehicle distance threshold L 2 * computed in step S 920 . If L≦L 2 *, the process goes to step S 932 , and inter-vehicle distance deviation ΔL 2  is computed according to the following formula (Formula 37).
 
Δ L 2 =L 2* −L   (Formula 37)
 
     When it is determined that L&gt;L 2 * in step S 931 , the process goes to step S 933 , and inter-vehicle distance deviation ΔL 2  is set to 0, that is, it is cleared. 
     In this way, after computing the second inter-vehicle distance threshold in step S 900 , the process goes to step S 1000 . In step S 1000 , from second inter-vehicle distance threshold L 2 * computed in step S 900  as well as inter-vehicle distance deviation ΔL 2 , target accelerator pedal opening final value APO 0 * for controlling the output amount of the engine torque with respect to accelerator pedal depression amount APO by the driver is computed. In the following, an explanation will be given in more detail regarding the treatment for computing the final value of the target accelerator opening carried out in step S 1000  with reference to the flow chart shown in  FIG. 21 . 
     First, in step S 1010 , target accelerator opening minimum value APO_min with respect to accelerator pedal depression amount APO is computed.  FIG. 22  is a diagram illustrating the relationship between accelerator pedal depression amount APO and target accelerator opening minimum value APO_min. As indicated by the solid line in  FIG. 22 , the target accelerator opening minimum value APO_min is set such that it is determined uniquely with respect to accelerator pedal depression amount APO; a larger accelerator pedal depression amount APO means a larger target accelerator opening minimum value APO_min. 
     In step S 1020 , torque down gain Ka 0  is computed using the following formula (Formula 38) from vehicle-speed-dependent gain K v  and inter-vehicle distance deviation ΔL 2  computed in step S 930  and inter-vehicle distance deviation ΔL 2 .
 
 Ka 0=100− ΔL 2 ×Kv   (Formula 38)
 
     Here, vehicle-speed-dependent gain Kv is the amount of change of torque down gain Ka 0  with respect to inter-vehicle distance deviation ΔL 2 , and it is computed from the plot shown in  FIG. 23 . As shown in  FIG. 23 , as host vehicle speed VSP increases, the vehicle-speed-dependent gain Kv gradually decreases, so that the amount of change of torque down gain Ka 0  with respect to inter-vehicle distance deviation ΔL 2  is decreased. When host vehicle speed VSP exceeds a prescribed level, vehicle-speed-dependent gain Kv is fixed at a prescribed value. 
     In step S 1030 , torque down gain Ka 0  computed in step S 1020  is corrected corresponding to slope SLP of the road the host vehicle is traveling. First, from the plot shown in  FIG. 24 , slope-dependent corrected gain Ka_slp is computed. When the slope SLP is positive, that is, when the vehicle travels up a slope, slope-dependent corrected gain Ka_slp is set to a positive value. On the contrary, when slope SLP is negative, that is, when the vehicle travels down a slope, slope-dependent corrected gain Ka_slp is set to a negative value. A larger absolute value of slope SLP means a larger absolute value of slope-dependent corrected gain Ka_slp. Also, when the absolute value of slope SLP exceeds a prescribed level, the absolute value of slope-dependent corrected gain Ka_slp is fixed at a prescribed value. 
     By means of slope-dependent corrected gain Ka_slp computed based on slope SLP of the road the host vehicle is traveling, torque down gain Ka 0  computed in step S 1020  is corrected, and torque down gain Ka 1  is re-computed. The torque down gain Ka 1  is computed using the following formula (Formula 39).
 
Ka1−Ka0+K_slp  (Formula 39)
 
     Here, torque down gain Ka 1  has a maximum value of 100 and minimum value of 0. 
     In step S 1040 , based on confidence factor Prob computed in step S 300 , torque down gain Ka 1  computed in step S 1030  is corrected. First, from the plot shown in  FIG. 25 , torque down gain minimum value Ka_min is computed corresponding to confidence factor Prob. As shown in  FIG. 25 , a smaller confidence factor Prob of the obstacle means a larger torque down gain minimum value Ka_min. By restricting torque down gain Ka 1  computed in step S 1030  using torque down gain minimum value Ka_min computed based on confidence factor Prob, final torque down gain Ka is computed. More specifically, by means of select a high torque down gain Ka 1  and torque down gain minimum value Ka_min as shown in the following (Formula 40), torque down gain Ka is computed.
 
 Ka =max( Ka 1,  Ka _min)  (Formula 40)
 
     In step S 1050 , target accelerator pedal opening final value APO 0 * is computed. As shown in the following formula (Formula 41), target accelerator pedal opening final value APO 0 * is computed by interior-dividing target accelerator opening minimum value APO_min computed in step S 1010  and accelerator pedal depression amount APO of the driver in torque down gain Ka computed in step S 1040 . 
     
       
         
           
             
               
                 
                   
                     APO 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       0 
                       * 
                     
                   
                   = 
                   
                     
                       APO 
                       · 
                       
                         Ka 
                         100 
                       
                     
                     + 
                     
                       APO_min 
                       · 
                       
                         
                           100 
                           - 
                           Ka 
                         
                         100 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     41 
                   
                   ) 
                 
               
             
           
         
       
     
     In this way, after computing target accelerator pedal opening final value APO 0 * in step S 1000 , the process goes to step S 1100 . In step S 1100 , the operation for detecting the accelerator pedal depression operation is carried out. In the following, an explanation will be given regarding the operation carried out in step S 1100  with reference to the flow chart shown in  FIG. 26 . 
     In step S 1101 , by differential computation for accelerator pedal step-down amount APO by the driver detected by accelerator pedal step-down amount detecting part  73 , the depression speed of accelerator pedal  72 , that is, accelerator pedal opening speed dAPO, is computed. 
     In step S 1102 , it is determined whether an obstacle exists ahead of the host vehicle. When an obstacle ahead of the host vehicle is detected by laser radar  10 , the process goes to step S 1103 . In step S 1103 , accelerator opening speed threshold dAPO 1  computed based on confidence factor Prob is set in accelerator opening speed threshold dAPO 0  as the threshold for judging the depression operation of accelerator pedal  72 .  FIG. 27  is a diagram illustrating the relationship between confidence factor Prob of the obstacle and accelerator opening speed threshold dAPO 1 . As shown in  FIG. 27 , the larger the confidence factor Prob, the larger accelerator opening speed threshold dAPO 1 . When confidence factor Prob is smaller, accelerator opening speed threshold dAPO 1  is set smaller. Consequently, the smaller the confidence factor Prob of the obstacle, the earlier the depression by the driver on the accelerator pedal can be detected. 
     When it is determined that no obstacle exists ahead of the host vehicle in step S 1102 , the process goes to step  1104 , and preset value dAPO 2  is set as accelerator opening speed threshold dAPO 0 . Here, value dAPO 2  when there no obstacle exists ahead of the host vehicle corresponds to the minimum value of accelerator opening speed threshold dAPO 1  in the plot of confidence factor Prob and accelerator opening speed threshold dAPO 1  shown in  FIG. 27 . 
     In step S 1105 , it is determined whether accelerator opening speed dAPO computed in step S 1101  exceeds accelerator opening speed threshold dAPO 0  set in step S 1103  or S 1104 . If dAPO≧dAPO 0 , it is determined that accelerator pedal  72  is depressed down, and the process goes to step S 1106 , and accelerator step-down operation flag Flg_APO is set to 1. On the other hand, when dAPO&lt;dAPO 0 , it is determined that the driver is not stepping down accelerator pedal  72 , that is, accelerator pedal  72  is maintained or reset, or accelerator pedal  72  is released. Then, the process goes to step S 1107 , and accelerator step-down operation flag Flg_APO is set to 0, that is, it is cleared. 
     In this way, after detection of the depression operation of accelerator pedal  72  in step S 1100 , the process goes to step S 1200 . In step S 1200 , the target accelerator opening is reset. In the following, an explanation will be given regarding the operation carried out in step S 1200  with reference to the flow chart shown in  FIG. 28 . 
     In step S 1201 , it is determined whether an obstacle exists ahead of the host vehicle. When an obstacle ahead of the host vehicle is detected by laser radar  10 , the process goes to step S 1202 , and it is determined whether torque down gain Ka computed in step S 1040  is smaller than the previous-cycle value of the torque down gain output value Ka_out_z. If Ka≦Ka_out_z, the process goes to step S 1203 , and the change rate limiter for torque down gain Ka is set. Here, limiter Ka_up for increasing torque down gain Ka and limiter Ka_dn for decreasing it are set, respectively. Here, limiter Ka_up for increasing the torque down gain is set to zero, and limiter Ka_dn for decreasing the torque down gain are set to preset value Ka_dn 1 . 
     When it is determined that Ka&gt;Ka_out_z in step S 1202 , the process goes to step S 1204 , and it is determined whether accelerator step-down operation flag Flg_APO set in step S 1100  is 1. If the accelerator step-down operation flag Flg_APO=1, that is, accelerator pedal  72  is depressed down, the process goes to step S 1205 . In step S 1205 , as limiter Ka_up for increasing the torque down gain, value Ka_up 1  is set based on inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle, while limiter Ka_dn for decreasing the torque down gain is set to zero.  FIG. 29  is a diagram illustrating the relationship between inter-vehicle distance L and limiter Ka_up 1  for increasing the torque down gain. As shown in  FIG. 29 , with the minimum value of Ka_up_min and the maximum value of Ka_up 2 , the limiter Ka_up 1  for increasing the torque down gain is set such that it is increased slowly as inter-vehicle distance L increases. 
     When it is determined in step S 1204  that accelerator step-down operation flag Flg_APO=0, that is, accelerator pedal  72  is not depressed down, the process goes to step S 1206 . In step S 1206 , both limiter Ka_up for increasing the torque down gain and limiter Ka_dn for decreasing the torque down gain are set to zero. 
     When it is determined in step S 1201  that no obstacle exists ahead of the host vehicle, the process goes to step S 1207 , and it is determined whether accelerator step-down operation flag Flg_APO is 1. If Flg_APO=1, the process goes to step S 1208 , and, as limiter Ka_up for increasing the torque down gain, Ka_up 2  corresponding to the maximum value on the plot shown in  FIG. 29  is set. In addition, limiter Ka_dn for decreasing the torque down gain is set to zero. When it is determined in step S 1207  that Flg_APO=0, the process goes to step S 1209 , and both limiter Ka_up for increasing the torque down gain and limiter Ka_dn for decreasing the torque down gain are set to zero. 
     In step S 1210 , the change rate limiter process is performed using limiter Ka_up for increasing the torque down gain and limiter Ka_dn for decreasing the torque down gain for torque down gain Ka computed in step S 1040  to compute torque down gain output value Ka_out. 
     In step S 1211 , based on torque down gain output value Ka_out computed in step S 1210 , target accelerator opening APO* for use as the instruction value to engine controller  74  is computed.
         When Ka=Ka_out       

     With the following formula (Formula 45), target accelerator pedal opening final value APO 0 * computed in step S 1000  is set to target accelerator opening APO*.
 
APO*=APO0*  (Formula 42)
         When Ka ≠Ka_out       

     By means of the following formula (Formula 43), target accelerator opening APO* is computed. 
     
       
         
           
             
               
                 
                   
                     APO 
                     * 
                   
                   = 
                   
                     
                       APO 
                       · 
                       
                         Ka_out 
                         100 
                       
                     
                     + 
                     
                       APO_min 
                       · 
                       
                         
                           100 
                           - 
                           Ka_out 
                         
                         100 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Formula 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     43 
                   
                   ) 
                 
               
             
           
         
       
     
     In step S 1300 , target accelerator pedal opening APO* computed in step S 1200  is output to engine controller  74 , and, at the same time, target accelerator pedal reaction force correction value FA*corr computed in step S 800  is output to accelerator pedal reaction force controller  70 . Engine controller  74  controls the engine torque generation amount according to target accelerator opening APO* to perform engine torque control. Accelerator pedal reaction force controller  70  controls the accelerator pedal depression reaction force generated on accelerator pedal  72  corresponding to the target accelerator pedal reaction force correction value FA*corr. At this point, the current cycle ends. 
     In the Embodiment 1, the following operation effects can be displayed. 
     (1) When the inter-vehicle distance between the host vehicle and the obstacle ahead of the host vehicle becomes less than a prescribed distance (first inter-vehicle distance threshold L 1 *), inter-vehicle distance maintenance supporting system  1  applies a reaction force on accelerator pedal  72 . Here, controller  50  computes confidence factor Prob indicating the continued presence of the obstacle ahead of the host vehicle based on the state of the obstacle. In addition, when it is detected that accelerator pedal  72  is depressed down, confidence factor Prob is corrected. As a result, when the driver tries to pass the obstacle ahead of the host vehicle by stepping down accelerator pedal  72 , the obstacle ahead of the host vehicle can quickly be cancelled as an object for control of the reaction force, so that it is possible to prevent the problem in the related art of the obstacle ahead of the host vehicle being kept as an object control of the reaction force continues giving discomfort to the driver. 
     (2) When it is detected that accelerator pedal  72  is depressed down, controller  50  corrects confidence factor Prob based on inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle. When the host vehicle tries to pass the obstacle ahead of the host vehicle, by correcting confidence factor Prob corresponding to the relative-position relationship in this state, it is possible to reliably correct confidence factor Prob. 
     (3) More specifically, the shorter the inter-vehicle distance L, the smaller the confidence factor Prob. Because it is believed that the passing time is shorter when the host vehicle is closer to the obstacle ahead of the host vehicle, by reducing confidence factor Prob, it is possible to quickly cancel the obstacle ahead of the host vehicle as an object for control. 
     (4) Controller  50  filters the detected value of yaw rate sensor  30 , and uses the filtered yaw rate in computing the predicted running path of the host vehicle. Then, the relative position of the obstacle ahead of the host vehicle with respect to the predicted running path, or, more specifically, offset value α, is used in computing confidence factor Prob. As a result, it is possible to compute confidence factor Prob based on the predicted running path that reflects the driver turning the steering wheel. 
     (5) When the confidence factor Prob is corrected, the filtering of the yaw rate is changed. When heavy filtering is performed, although it is possible to remove the noise and drift, the response nevertheless becomes slower. Here, when filtering is changed to light filtering, it is possible to obtain a yaw rate with quick response, and it is possible to compute swiftly the predicted running path that reflects the driver turning the steering wheel. 
     (6) Controller  50  increases the cutoff frequency for use in filtering when inter-vehicle distance L becomes shorter. In the state of passing the obstacle ahead of the host vehicle, the driver does not notice the noise and drift in the detected value of the yaw rate generated due to the driver turning the steering wheel. Here, by changing to light filtering with swift response ability, it is possible to obtain a detected value that can rapidly reflect the driver turning the steering wheel. 
     Embodiment 2 
     In the following, an explanation will be given regarding the inter-vehicle distance maintenance supporting system of Embodiment 2 of the present invention. The basic configuration of the inter-vehicle distance maintenance supporting system in Embodiment 2 is the same as that of Embodiment 1 above. In the following, an explanation will be given mainly regarding the points of difference from Embodiment 1 above. 
     In Embodiment 2, the future position of the obstacle ahead of the host vehicle with respect to the host vehicle is predicted, and the predicted position of the obstacle ahead of the host vehicle is used to compute confidence factor Prob. Here, prediction time t indicating the time in seconds to be predicted for the future position is set using inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle. 
     In the following, an explanation will be given regarding computing the confidence factor Prob in Embodiment 2 with reference to the flow chart shown in  FIG. 30 . This is executed in step S 300  of the flow chart shown in  FIG. 4 . Steps S 301 -S 307  are the same as that shown in the flow chart of  FIG. 5 , so an explanation is omitted. 
     In step S 321 , Formula 1 above is used to compute yaw rate filter value ω 1 . In step S 322 , host vehicle speed V is read, and, in step S 323 , predicted turning radius R is computed from Formula 2 above. 
     In step S 324 , the position of the obstacle ahead of the host vehicle after prediction time t is computed.  FIG. 31  shows the relationship between inter-vehicle distance L and prediction time t. When inter-vehicle distance L is greater than prescribed inter-vehicle distance L 1 , prediction time t=0, and the current position of the obstacle is computed. On the other hand, when inter-vehicle distance L is less than prescribed inter-vehicle distance L 1 , prediction time t is gradually increased. When it becomes less than prescribed inter-vehicle distance L 2 , prediction time t is fixed at the maximum value t 1 . 
     The closer the host vehicle is to the obstacle ahead of the host vehicle, the earlier the time for passing the obstacle ahead of the host vehicle can be predicted. Consequently, by predicting the position of the obstacle ahead of the host vehicle at a certain time later, it is possible to swiftly cancel the obstacle ahead of the host vehicle as an object for control. 
     In the following, an explanation will be given in reference to  FIG. 32  regarding the method for computing the position of the obstacle ahead of the host vehicle at prediction time t later. In  FIG. 32 , D 1  represents the inter-vehicle distance between the host vehicle and the obstacle ahead of the host vehicle at the present time, and X 1  represents the lateral position of the obstacle ahead of the host vehicle with respect to the host vehicle. Here, inter-vehicle distance D 1  and lateral position X 1  correspond to distance L and offset value α in  FIG. 8 , respectively. If the longitudinal relative velocity and the lateral relative velocity between the host vehicle and the obstacle ahead of the host vehicle are vVy and vVx, respectively, the position of the obstacle ahead of the host vehicle at prediction time t later has the longitudinal position (inter-vehicle distance) of (D 1 +t×vVy) and the lateral position (offset value) of (X 1 +t×vVx). 
     In step S 325 , the position of the obstacle ahead of the host vehicle after the prediction time computed in step S 324  is used to compute offset value α, and, in step S 326 , confidence factor Prob is computed. 
     Consequently, in Embodiment 2 explained above, in addition to the effects of Embodiment 1 above, the following effects can be realized. 
     (1) Controller  50  computes the relative position of the obstacle at prescribed time t later with respect to the predicted running path, and the relative position of the obstacle at computed prescribed time t is used to compute confidence factor Prob. In the state when accelerator pedal  72  is depressed down to pass the obstacle ahead of the host vehicle, by computing confidence factor Prob using the position of the obstacle at the prescribed time (prediction time) t, it is possible to swiftly cancel the obstacle ahead of the host vehicle as an object for control. 
     (2) The less the inter-vehicle distance L is between the host vehicle and the obstacle ahead of the host vehicle, the longer the prediction time t is set. Consequently, when the obstacle ahead of the host vehicle is to be passed, it is possible to swiftly cancel the obstacle ahead of the host vehicle as an object for control. 
     Also, instead of inter-vehicle distance L, accelerator pedal step-down amount APO of accelerator pedal  72  may be used, and a greater the accelerator pedal step-down amount APO, a greater prediction time t is set. 
     Embodiment 3 
     In the following, an explanation will be given regarding the inter-vehicle distance maintenance supporting system of Embodiment 3 of the present invention. The basic configuration of the inter-vehicle distance maintenance supporting system in Embodiment 3 is the same as that in Embodiment 1. In the following, an explanation will be given mainly on the points of difference from Embodiment 1. 
     In Embodiment 1, by changing the response of filtering with respect to yaw rate ω detected by yaw rate sensor  30 , confidence factor Prob is decreased in an earlier stage when passing the obstacle ahead of the host vehicle. In Embodiment 3, confidence factor Prob computed based on offset value α is directly corrected. 
     In the following, an explanation will be given regarding computing confidence factor Prob in Embodiment 3 with reference to the flow chart shown in  FIG. 33 . This treatment is executed in step S 300  in the flow chart shown in  FIG. 4 . Steps S 301 -S 307  are the same as in the flow chart shown in  FIG. 5 , and will not be explained in detail again. 
     In step S 331 , the above formula (Formula 1) is used to compute yaw rate filter value ω 1 . In step S 332 , host vehicle speed V is read. In step S 333 , predicted turning radius R is computed from the above formula (Formula 2). In step S 334 , the Formulas 3-15 are used to compute the position of the obstacle ahead of the host vehicle. In step S 335 , Formulas 16 and 17 are used to compute offset value α. 
     In step S 336 , the confidence factor correction coefficient for correcting the confidence factor is computed. More specifically, the slope of the confidence factor computing formula (confidence factor correction coefficient) is changed to correspond to inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle, and the formula for computing confidence factor Prob is changed. In the following, an explanation will be given regarding this case with reference to the flow chart shown in  FIG. 34 . 
     In step S 3361 , preset constant Da and confidence factor change amount ΔProb are used to set the first confidence factor computing formula represented by the following formula (Formula 44).
 
Prob A= 1−ΔProb×α/ Da   (Formula 44)
 
     In step S 3362 , preset constant Db and confidence factor change amount ΔProb are used to set the second confidence factor computing formula represented by the following formula (Formula 45).
 
Prob B= 1−ΔProb×α/ Db   (Formula 45)
 
       FIG. 35  is a diagram illustrating the relationship between the confidence factors ProbA, ProbB and offset value α. As shown in  FIG. 35 , when the absolute value of offset value α is increased, the confidence factors ProbA, ProbB are gradually reduced from 1. Here, the slope of confidence factor ProbB is set steeper than that of confidence factor ProbA. 
     In step S 3363 , it is determined whether inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle is greater than prescribed inter-vehicle distance L 1 . If L&gt;L 1 , the process goes to step S 3364 , and the first confidence factor computing formula represented by Formula 44 is used to compute confidence factor ProbB (Prob=ProbA). On the other hand, when the result of judgment in step S 3363  is NO, the process goes to step S 3365 , and it is determined whether inter-vehicle distance L is less than prescribed inter-vehicle distance L 2  (&lt;L 1 ). If L&lt;L 2 , the process goes to step S 3366 , and the second confidence factor computing formula represented by Formula 45 is used to compute confidence factor Prob for determination (Prob=ProbB). 
     If the judgment result in step S 3365  is NO, the process goes to step S 3367 , and Formula 46 is used to set the confidence factor computing formula.
 
Prob=Prob B ×( L−L 2)/( L 1 −L 2)+Prob B ×( L 1 −L )/( L 1 −L 2)  (Formula 46)
 
     Here, Formula 46 is for interior-dividing confidence factor ProbA and confidence factor ProbB in inter-vehicle distance L, and it corresponds to the intermediate region between ProbA and ProbB shown in  FIG. 35 . The prescribed inter-vehicle distances L 1 , L 2  are preset to appropriate values. Also, the first inter-vehicle distance threshold L 1 * and second inter-vehicle distance threshold L 2 * may also be used as the prescribed inter-vehicle distances L 1 , L 2 , respectively. 
     Then, in step S 337 , confidence factor Prob is computed using the computing formula determined in step S 336 . Also, when it is determined in step S 302  that accelerator pedal  72  is not depressed, the first confidence factor computing formula as Formula 44 is used to compute confidence factor Prob. 
     In Embodiment 3 explained above, in addition to the effects of the Embodiment 1, the following operation effects can be realized. 
     (1) Controller  50  changes the coefficient for use in computing confidence factor Prob from the relative position of the obstacle corresponding to inter-vehicle distance L. More specifically, as shown in Formula 44 and Formula 45, the confidence factor computing formula is set, and, by changing the coefficient used in the formulas corresponding to inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle, confidence factor Prob is corrected. As a result, in the state when accelerator pedal  72  is depressed down to pass, the obstacle ahead of the host vehicle can be swiftly canceled as an object for control. 
     (2) As shown in  FIG. 35 , the shorter the inter-vehicle distance L, the steeper the slope of confidence factor Prob with respect to offset value α, and the larger the absolute value of the coefficient of the confidence factor computing formula. As a result, in the state when the host vehicle approaches to pass the obstacle ahead of the host vehicle, the obstacle can be swiftly canceled as an object for control. 
     Embodiment 4 
     In the following, an explanation will be given regarding Embodiment 4 of the inter-vehicle distance maintenance supporting system of the present invention. The basic configuration of Embodiment 4 is the same as that of Embodiment 1. Consequently, in the following, an explanation will be given mainly regarding the points of difference from Embodiment 1. 
     In Embodiment 1 above, when accelerator pedal  72  is depressed, the filtering for yaw rate ω is changed to a light filtering. However, in some cases, the operator may lift accelerator pedal  72  before the host vehicle has fully passed the obstacle ahead of the host vehicle, that is, before the current obstacle ahead of the host vehicle is fully canceled as an object for control. Consequently, in Embodiment 4, even after accelerator pedal  72  is no longer depressed, the light filtering is continued to compute confidence factor Prob for a prescribed time, so that the obstacle ahead of the host vehicle is reliably canceled as an object for control. 
     In the following, an explanation will be given regarding detecting depression of the accelerator pedal with reference to the flow chart shown in  FIG. 36 . This is executed in step S 301  of the flow chart of the confidence factor computing shown in  FIG. 5 . 
     In step S 3021 , accelerator opening speed dAPO is computed. In step S 3022 , it is determined whether accelerator opening speed dAPO exceeds accelerator opening speed threshold dAPO 1  that has been preset. If dAPO≧dAPO 1 , the process goes to step S 3023 , and accelerator step-down operation flag Flg_APO is set to 1. In addition, delay counter Cnt_APO is set to 0. 
     In step S 3022 , if it is found that dAPO&lt;dAPO 1 , it is determined that the driver is not depressing accelerator pedal  72 , that is, accelerator pedal  72  is held constant or reset, or accelerator pedal  72  is released. It then goes to step S 3024 , and delay counter Cnt_APO is counted up. In step S 3025 , it is determined whether delay counter Cnt_APO exceeds a preset time T_APO (say, 1 sec). If delay counter Cnt_APO is greater than prescribed time T_APO, the process goes to step S 3026 , and accelerator step-down operation flag Flg_APO is set to 0, that is, it is cleared. 
     On the other hand, when delay counter Cnt_APO is less than prescribed time T_APO, the process goes to step S 3027 , and it is determined whether accelerator pedal step-down amount APO is zero. If accelerator pedal step-down amount APO is zero, it is determined that the driver has lifted his foot from accelerator pedal  72 , and it then goes to step S 3026 . Then, accelerator step-down operation flag Flg_APO is set to 0, that is, it is cleared. 
     In step S 3027 , if it is determined that accelerator pedal step-down amount APO is non-zero, the process goes to step S 3028 , and it is determined whether accelerator opening speed dAPO is less than preset accelerator opening speed threshold dAPO 2 . If accelerator opening speed dAPO is less than preset accelerator opening speed threshold dAPO 2 , it is determined that the driver is resetting accelerator pedal  72  at a speed higher than the prescribed operation speed, so that the process goes to step S 3026 , and accelerator step-down operation flag Flg_APO is set to zero, that is, it is cleared. Also, accelerator opening speed dAPO has a positive value when accelerator pedal  72  is depressed, and it has a negative value when accelerator pedal  72  is reset. 
     In step S 3028 , when it is determined that accelerator opening speed dAPO is greater than preset accelerator opening speed threshold dAPO 2 , it is determined that the driver is resetting accelerator pedal  72  at a speed lower than a prescribed speed, or the driver is keeping the depression amount of accelerator pedal  72  nearly constant. It is finished as is. 
     In this way, even when accelerator pedal  72  is no longer depressed, or, more specifically, even if the depression is not greater than accelerator opening speed threshold dAPO 1 , it is still determined that accelerator pedal  72  is depressed down during a prescribed time T_APO. Consequently, confidence factor Prob is computed using yaw rate correction value ω 2  with a high response until the obstacle ahead of the host vehicle is canceled as an object for control for sure. 
     In Embodiment 4, detecting of the depression operation of accelerator pedal  72  may be performed along with Embodiment 2 or 3. 
     In the Embodiment 4, in addition to the effects of Embodiments 1-3, the following operation effects can be displayed. 
     Until a prescribed time after detection that accelerator pedal  72  is no longer depressed, it is determined that accelerator pedal  72  is being depressed, and confidence factor Prob is corrected. As a result, even when the driver lifts his foot from accelerator pedal  72  before the host vehicle fully passes the obstacle ahead of the host vehicle, correction of confidence factor Prob is still continued. Consequently, it is possible to prevent the discomfort felt by the driver caused by control of the operation reaction force with the obstacle ahead of the host vehicle to be passed as the object and the engine torque. 
     In Embodiments 1-4 the device that perform control of the accelerator pedal reaction force and control of the engine torque has been explained based on inter-vehicle distance L between the host vehicle and the obstacle ahead of the host vehicle. However, the present invention is not limited to this case. One may also adopt a scheme in which correction of the confidence factor is performed in a device that controls only the accelerator pedal reaction force corresponding to inter-vehicle distance L. Also, in the example explained above, correction is performed on target accelerator pedal reaction force FA* and target accelerator opening APO* corresponding to confidence factor Prob. However, the configuration may also be such that only one of them is corrected. 
       FIG. 9  is a diagram illustrating the relationship between inter-vehicle distance L and cutoff frequency correction value f′.  FIG. 31  shows the relationship between inter-vehicle distance L and prediction time t. It may be set such that the shorter the inter-vehicle distance L, the larger the cutoff frequency correction value f′ or the longer the prediction time t, and it is not restricted to the characteristics shown in  FIG. 9  or  31 . 
     In the above embodiments presented as examples, laser radar  10  and yaw rate sensor  30  function as obstacle detecting means; laser radar  10  functions as the inter-vehicle distance detecting means; accelerator pedal reaction force controller  70  functions as the accelerator pedal reaction force control means; and confidence factor computing part  52  functions as the confidence factor computing means and the confidence factor correcting means. Of course, confidence factor computing part  52  can function as the confidence factor computing means and the confidence factor correcting means even when accelerator pedal  72  is not depressed down. Yaw rate sensor  30  functions as the yaw rate detecting means; accelerator pedal step-down operation detecting part  60  functions as the accelerator pedal depression detecting means, and accelerator pedal reaction force correcting part  56  can function as the accelerator pedal reaction force correcting means. However, the present invention is not limited to the aforementioned scheme. As the obstacle detecting means, instead of laser radar  10 , one may make use of a millimeter wave radar as another scheme, and the state of the obstacle can also be detected by inter-vehicle communication or the like. Merely an example is presented above. When the present invention is explained, there is no specific restriction on the corresponding relationship between the items of description of the embodiment and the description items in the Claims. 
     Given the disclosure of the present invention, one versed in the art would appreciate that there may be other embodiments and modifications within the scope of the invention. Accordingly, all modifications attainable by one versed in the art from the present disclosure within the scope of the present invention are to be included as further embodiments of the present invention. The scope of the present invention is to be defined as set forth in the following claims.