Patent Publication Number: US-7902693-B2

Title: Driver assisting system, method for assisting driver, and vehicle incorporating same

Description:
RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/990,603 (U.S. Pat. No. 7,560,826), filed Nov. 18, 2004, which claims the benefit of Japanese Patent Application No. 2003-391124 filed on Nov. 20, 2003 in the Japanese Patent Office, the disclosure of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a driver assisting system, a method for assisting a driver, and a vehicle incorporating the driver assisting system. 
     DESCRIPTION OF RELATED ART 
     JP10-166889A discloses a driver assisting system, which, when a distance to a preceding vehicle drops to a predetermined value, sets an increased magnitude of reaction force of an accelerator pedal. JP10-166890A discloses a similar driver assisting system. JP2000-54860A discloses a driver assisting system, which, when an automatic control is being carried out, sets an increased magnitude of reaction force of an accelerator pedal. US 2003/0163240 A1, published Aug. 28, 2003, discloses a driving assist system which adjusts reaction force of an accelerator pedal upon detection of a discontinuous change in environment around a vehicle. 
     US 2003/0060936 A1, published Mar. 27, 2003, discloses a driving assist system for assisting effort by an operator (a driver) to operate a vehicle while traveling. This system comprises a data acquisition system acquiring data including information on vehicle state and information on environment in a field around the vehicle, a controller, and at least one actuator. The controller determines a future environment in the field around the vehicle using the acquired data, makes an operator response plan in response to the determined future environment, which plan prompts the operator to operate the vehicle in a desired manner for the determined future environment, and to generates a command. The actuator is coupled to an operator controlled input device to mechanically affect operation of the input device in a manner that prompts the operator in response to the command to operate the vehicle in the desired manner. 
     There is a need for a system and vehicle to effectively forward not only a risk derived from information on environment in field around a vehicle but also a potential risk, which might become actualized, to the driver. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a driver assisting system for assisting a driver, within a driver&#39;s seat, operating a driver controlled input device of a vehicle, comprising: 
     a running environment detecting section configured to detect an obstacle appearing in a running environment around the vehicle and obtain information of a vehicle speed standard for a road on which the vehicle is traveling; 
     a vehicle speed sensor configured to detect a vehicle speed of the vehicle; 
     the vehicle speed standard being a vehicle speed value calculated based on a recommended vehicle speed, which the vehicle is recommended to travel at, taking into account the speed limit for the road, an error of the vehicle speed sensor and fuel economy, 
     a risk calculating section configured to calculate a first type of risk perceived by the driver and associated with a possible collision with the detected obstacle and a second type of risk unrelated to the detected obstacle and derived from determining an amount by which the detected vehicle speed exceeds the vehicle speed standard; 
     a tactile stimulus controlling section configured to translate the calculated first type of risk and second type of risk into different first and second forms of tactile stimulus, respectively; and 
     a tactile stimulus forwarding device configured to forward the calculated first and second type of risk to the driver by applying the first and second forms of tactile stimulus to the driver, via a single device, wherein the single device is the driver&#39;s seat or the driver controlled input device. 
     The foregoing and other features, aspects and advantages of the present invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating hardware of a first exemplary implementation of a driver assisting system according to the present invention; 
         FIG. 2  is a perspective view a vehicle in the form of an automobile incorporating the driver assisting system; 
         FIG. 3  is a driver controlled device, in the form of an accelerator pedal, of the vehicle; 
         FIG. 4  is a flow chart illustrating operation of the first exemplary implementation; 
         FIGS. 5(   a ) to  5 ( b ) are time charts illustrating operation of the first exemplary implementation; 
         FIG. 6  is a flow chart illustrating operation of a second exemplary implementation of a driver assisting system according to the present invention; 
         FIGS. 7(   a ) to  7 ( d ) are time charts illustrating operation of the second exemplary implementation; 
         FIG. 8  illustrates an exemplary relationship between a vehicle speed standard as corrected and a time elapsed from a moment immediately after vehicle speed has exceeded the vehicle speed standard; 
         FIG. 9  illustrates an exemplary varying of a click starting period with different values of an excess by which the vehicle speed exceeds the vehicle speed standard; 
         FIG. 10  illustrates an exemplary varying of a click interval with different values of the excess by which the vehicle speed exceeds the vehicle speed standard; 
         FIG. 11  illustrates an exemplary varying of a first correction coefficient with different values of the excess; 
         FIG. 12  illustrates an exemplary varying of a second correction coefficient with different values of risk perceived (RP); 
         FIG. 13  illustrates an exemplary varying of a third correction coefficient with different values of an accelerator pedal position; 
         FIG. 14A  illustrates an exemplary varying of a reset period with different values of the time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 14B  illustrates an exemplary varying of a reset period with different values of the content of a click frequency counter; 
         FIG. 15A  illustrates an exemplary varying of a click interval with different values of the time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 15B  illustrates an exemplary varying of a click interval with different values of the content of a click frequency counter; 
         FIG. 16A  illustrates an exemplary varying of a first correction coefficient with different values of the time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 16B  illustrates an exemplary varying of the first correction coefficient with different values of the content of a click frequency counter; 
         FIG. 17  is a flow chart illustrating operation of a third exemplary implementation of the present invention; 
         FIGS. 18(   a ) to  18 ( d ) are time charts illustrating operation of the third exemplary implementation; 
         FIG. 19  illustrates two examples of logic; 
         FIG. 20  illustrates varying of number of times a click reaction force is repeated with different excess levels; 
         FIG. 21  is a flow chart illustrating operation of a fourth exemplary implementation of the present invention; 
         FIG. 22  illustrates an exemplary relationship between a vehicle speed standard as corrected and time elapsed after a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 23  illustrates an exemplary varying of a correction coefficient with different values of an accelerator pedal position; 
         FIG. 24A  illustrates an exemplary varying of a reset period with different values of time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 24B  illustrates an exemplary varying of a reset period with different values of time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 25A  illustrates an exemplary varying of a click interval with different values of time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 25B  illustrates an exemplary varying of a click interval with different values of the content of a click frequency counter; 
         FIG. 26A  illustrates an exemplary varying of a correction coefficient with different values of time elapsed from a moment immediately after the vehicle speed has exceeded the vehicle speed standard; 
         FIG. 26B  illustrates an exemplary varying of a correction coefficient with different values of the content of a click frequency counter; 
         FIG. 27  illustrates an exemplary varying of a correction coefficient with different values of an accelerator pedal position; 
         FIG. 28  is a block diagram illustrating hardware of a fifth exemplary implementation of a driver assisting system according to the present invention; 
         FIG. 29  is a perspective view of a driver&#39;s seat; 
         FIG. 30  is a cross section taken through the line  30 - 30  in  FIG. 29  in an uninflated state; 
         FIG. 31  is the same cross section in an inflated state; 
         FIG. 32  is a flow chart illustrating operation of the fifth exemplary implementation; 
         FIGS. 33(   a ) to  33 ( c ) are time charts illustrating operation of the fifth exemplary implementation; 
         FIG. 34  is a perspective view of a driver controlled device in the form of a steering system with a steering wheel; 
         FIG. 35  is a side view of the steering system: 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     First Exemplary Implementation of the Invention 
     Referring to the accompanying drawings, various exemplary implementations of a driver assisting system according to the present invention are described. For better understanding of some of the exemplary implementations, reference should be made to the previously-mentioned US 2003/0060936 A1, published Mar. 27, 2003, which has been hereby incorporated by reference in its entirety. 
       FIG. 1  is a block diagram showing elements of a first exemplary implementation of a driver assisting system  1  according to the present invention.  FIG. 2  is a perspective view of an automobile installed with the driver assisting system  1 . 
     The driver assisting system  1  includes a laser radar  10 . As shown in  FIG. 2 , the laser radar  10  is mounted to the vehicle at a front bumper or a front grille thereof. It scans horizontally and laterally about 6 degrees to each side of an axis parallel to the vehicle longitudinal centerline, propagates infrared pulses forwardly and receives the reflected radiation by an obstacle, such as, a rear bumper of a preceding vehicle. The laser radar  10  can provide distances to vehicles in front of the vehicle and angular locations of the preceding vehicles. The laser radar  10  provides an output to a controller  60 . 
     The driver assisting system  1  also includes a front camera  20 . The front camera  20  of a CCD or CMOS type is mounted to the vehicle in the vicinity of the internal rear view mirror. It acquires image data of a region in front of the vehicle. The region extends from a camera axis laterally to each side by about 30 degrees. The front camera  20  provides an output to the controller  60 . 
     The driver assisting system  1  also includes a vehicle speed sensor  30 . The vehicle speed sensor  30  may determine vehicle speed by processing outputs from wheel speed sensors. The vehicle speed sensor  20  may include an engine controller or a transmission controller, which can provide a signal indicative of the vehicle velocity. The vehicle speed sensor  30  provides an output to the controller  60 . 
     The driver assisting system  1  further includes a vehicle speed database  40 . The vehicle speed database  40  contains vehicle upper speed limits incorporated, for example, in a navigation system, not illustrated. The controller  60  retrieves the vehicle speed database  40  using a current position of the vehicle, obtained from the navigation system, to find restriction on traffic speed for the current position of the vehicle. The current position of the vehicle is determined by the navigation system after calculation based on information indicated by a GPS signal. 
     The controller  60  responsible for information processing within the driver assisting system  1  may contain a microprocessor including a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM). The controller  60  receives distance information from the laser radar  10  and image information around the vehicle from the front camera  20  to determine running environment around the vehicle and state of obstacles around the vehicle. To determine the state of obstacles around the vehicle, the controller  60  receives the image data from the front camera  20  for image-processing, including filtering and pattern recognition. The state of obstacles around the vehicle includes a distance to a preceding vehicle in front, presence or absence of and degree of any vehicle running in the adjacent next lane, the degree of how much the vehicle has approached the vehicle running in the adjacent next lane, position of the vehicle relative to the lane marker and guard rail (distance and angle to the left and right lane markers and guard rails), and the configuration of the lane markers and the guard rails. 
     The controller  60  calculates a risk perceived RP by the vehicle driver from each of the obstacles based on the determined state of the obstacles around the vehicle, and regulates reaction force on an accelerator pedal within the vehicle cabin. The controller  60  calculates a vehicle speed standard, which may be set for a road the vehicle is running on. The controller  60  calculates an excess by which the vehicle speed exceeds the vehicle speed standard and provides an output to keep the vehicle driver informed of the calculated excess by subjecting the driver to reaction force pulses (click reaction force) from the accelerator pedal. The vehicle speed standard is herein used to mean a vehicle speed value calculated based on a recommended vehicle speed, which the vehicle is recommended to travel at, taking into account the speed limit for the road, an error of the vehicle speed sensor  30  and fuel economy. 
       FIG. 3  shows a driver controlled input device, in the form of an accelerator pedal  82 . For understanding the accelerator pedal, reference should be made to US 2003/0236608 A1 (published Dec. 25, 2003) and also to US 2003/0233902 A1 (published Dec. 25, 2003), both of which have been hereby incorporated by reference in their entireties. As shown in  FIG. 3 , the accelerator pedal  82  has a link mechanism including a servo motor  81  and an accelerator pedal stroke sensor  83 . The servo motor  81  can provide torque having varying magnitudes under control of an accelerator pedal reaction force control unit  80 . The accelerator pedal reaction force control unit  80  controls the servo motor  81  so that the torque produced by the servo motor  81  varies in magnitude with different values of a command from the controller  60 . As the servo motor  81  can provide torque having a desired one of the magnitudes to cause generation of a desired reaction force under the control if the servo motor reaction force control unit  80 , it is easy to alter, in a desired manner, the magnitude of manual effort, with which a driver steps on the accelerator pedal  82 . As the link mechanism converts a magnitude of manual operation for the accelerator pedal  82  to an angle of rotation of the servo motor  81 , the accelerator pedal stroke sensor  83  measures this angle of rotation of the servo motor  81  to detect the magnitude of manual operation for the accelerator pedal  82 . 
     When the above-described reaction force control is not carried out, the accelerator pedal  82  shows an ordinary reaction force characteristic by which the reaction force increases linearly as the accelerator pedal is depressed deeply. This ordinary reaction force characteristic is accomplished by a spring force provided by a torque spring  84  arranged at the center of rotational movement of the accelerator pedal  82 . 
     The following describes operation of this exemplary implementation of driver assisting system  1 . The controller  60  recognizes the state of obstacles around the vehicle from a vehicle speed at which the vehicle travels, a relative position between the vehicle and the preceding vehicle in front and/or the preceding vehicle in the next lane (a distance to the preceding vehicle in front and/or a distance to the preceding vehicle in the next lane), a direction of travel of each of the preceding vehicles, and a position of the vehicle relative to lane markers or guard rails. Based on the recognized state of obstacles around the vehicle, the controller  60  calculates risk perceived RP by the driver from each of the obstacles. The controller  60  calculates a control amount by which the magnitude of risk perceived RP by the driver from each of the obstacles may be communicated to the driver. For example, the controller  60  calculates a reaction force control amount by which the reaction force of the accelerator pedal  82  varies. 
     Based on information of restriction on vehicle speed for a road which the vehicle is traveling on, the controller  60  calculates a vehicle speed standard, which is to be taken into account during driving the vehicle. Based on the calculated vehicle speed standard, the controller  60  sets a train of additional pulses of reaction force, called a click reaction force. The controller  60  adds the click reaction force to the accelerator pedal reaction force control amount for the RP to give an accelerator pedal reaction force instruction value and provides the accelerator pedal reaction force instruction value to the accelerator pedal reaction force control unit  80 . The accelerator pedal reaction force control unit  80  carries out accelerator pedal reaction force control in response to the accelerator pedal reaction force instruction value. 
     Referring to  FIG. 4  and  FIGS. 5(   a ) to  5 ( d ), the following describes reaction force control employed by the first exemplary implementation of the present invention.  FIG. 4  illustrates a flow chart of a driver assisting control program executed by the controller  60  used in the first exemplary implementation of the present invention.  FIGS. 5(   a ) to  5 ( d ) are time charts illustrating time variations of vehicle speed Vf, accelerator pedal position Sp, click reaction force Fc and accelerator pedal reaction force instruction value FA, respectively. The execution of the program illustrated in  FIG. 4  is repeated at regular intervals for example, every 50 msec. 
     In  FIG. 4 , at step S 101 , the controller  60  performs a reading operation of output signals from the laser radar  10 , front camera  20  and vehicle speed sensor  30  to obtain information on the running environment around the vehicle. At step S 102 , the controller  60  performs analysis of the obtained information to recognize the environment state around the vehicle by calculating a distance D to each of the obstacles, and a relative speed Vr to each obstacle. If an obstacle is a preceding vehicle, the relative speed Vr is given by subtracting a vehicle speed Vf of the vehicle (illustrated in  FIG. 2 ) operated by a driver from a vehicle speed Vp of the preceding vehicle (Vr=Vp−Vf). 
     At step S 103 , using the distance D, relative speed Vr, and vehicle speed Vf, the controller  60  calculates a risk perceived RP by the driver from each obstacle within the environment around the vehicle. In this exemplary implementation, the controller  60  calculates a risk perceived RP by the driver from the preceding vehicle. The risk perceived RP may be expressed by two concepts, namely, a time to collision TTC and a time headway THW. 
     The TTC is a measure of time from a current moment to a future moment when the distance D would become zero if the relative speed Vr to the preceding vehicle remains unaltered. The TTC may be expressed as:
 
 TTC=−D/Vr   (Eq. 1)
 
     The smaller the value of TTC, the more imminent is the collision and the larger is the value of an extent the vehicle has approached the preceding vehicle. In a traffic situation where a vehicle is following the preceding vehicle, most vehicle drivers perceive a high degree of risk and initiate deceleration to avoid collision well before the TTC becomes less than 4 seconds. To some extent, the TTC is a good indication for predicting a future behavior the vehicle driver might take. However, when it comes to quantifying the degree of risk, which the vehicle driver actually perceives, there is a discrepancy between the TTC and the degree of risk. Thus, the TTC alone is insufficient to quantify the degree of risk. 
     Such discrepancy may be confirmed by considering a traffic situation where the relative speed Vr is zero. In this case, the TTC is infinite irrespective of how narrow the distance D is. However, the vehicle operator perceives an increase in the degree of risk in response to a reduction in the distance D, accounting for an increase in how much an unpredictable drop in a vehicle speed of the preceding vehicle might influence the TTC. 
     To remedy the above-mentioned discrepancy, the concept of time headway THW has been introduced to quantify an increase how much an unpredictable drop in the vehicle speed of the preceding vehicle might influence the TTC in a traffic situation where the vehicle is following the preceding vehicle with the distance D kept constant. The THW is a measure of a timer that is set to count when the preceding vehicle reaches a point on a road and will be subsequently reset when the following vehicle reaches the same point. The THW is expressed as,
 
 THW=D/Vf   (Eq. 2)
 
     In the case where the vehicle is following the preceding vehicle, the vehicle speed of the preceding vehicle may be used instead of the vehicle speed Vf in equation 2. 
     The relationship between the two concepts TTC and THW is such that when a change in vehicle speed, if any, of the preceding vehicle results in a small change in the TTC when the THW is long, but the same change in vehicle speed of the preceding vehicle results in a large change in the TTC when the THW is short. 
     In the exemplary implementation, the risk perceived RP is expressed as a sum of a first extent and a second extent. The first extent represents how much the vehicle has approached the preceding vehicle. The second extent represents how much an unpredictable change in vehicle speed of the preceding vehicle might influence the vehicle. The first extent is determined as a function of the reciprocal of time to collision TTC, and the second extent is determined as a function of the reciprocal of time headway THW. 
     In the first exemplary implementation, as mentioned above, the reciprocal of TTC determines the first extent, and the reciprocal of THW determines the second extent. The risk perceived RP is expressed as,
 
 RP=a/THW+bTTC   (Eq. 3)
 
     where: b and a (b&gt;a) are parameters weighting 1/TTC and 1/THW, respectively, such that 1/THW is less weighted than 1/TTC. The values of b and a are optimized after accounting for statistics of values of THW and TTC collected in a traffic situation including one vehicle following another vehicle. In this exemplary implementation, b=8 and a=1. 
     At step S 104 , responsive to the risk perceived RP calculated at step S 103 , the controller  60  calculates a stimulus to the driver in the form of an accelerator pedal reaction force increment dF. In the first exemplary implementation, the reaction force increment dF is proportional to the risk perceived RP and may be expressed as:
 
 dF=k 1 ·RP   (Eq. 4)
 
where: k 1  is a constant previously set as an appropriate value.
 
     At step S 111 , the controller  60  calculates a vehicle speed standard Vt. The vehicle speed standard Vt is a vehicle speed determined for a current area of a road which the vehicle is running through. Firstly, the controller  60  receives information on a current position of the vehicle that is continuously detected by an appropriate system, for example, a navigation system and accesses the vehicle speed information database  40  to obtain a vehicle speed limit for the area of the road which the vehicle is running through. Accounting for the vehicle speed limit, error in the vehicle speed sensor  30  and fuel economy, the controller  60  sets a recommended vehicle speed Vt 0 . The controller  60  may access a database to obtain an appropriate vehicle speed value for use as the recommended vehicle speed Vt 0 . Such database may contain appropriate vehicle speed values, which may be used as the recommended vehicle speed Vt 0 , arranged against different values of vehicle speed limit and different kinds of road. 
     Using the recommended vehicle speed Vt 0 , the controller  60  calculates the vehicle speed standard Vt. The vehicle speed standard Vt may be expressed as:
 
 Vt=Vt 0+α  (Eq. 5)
 
     In the equation 5, α(alpha) indicates a predetermined value for setting an appropriate value as the vehicle speed standard Vt against a given value of the recommended vehicle speed Vt 0 . The predetermined value of α should be set to give an appropriate value as the vehicle speed standard Vt against a given value of the recommended vehicle speed Vt 0  accounting for an inevitably occurring error in the vehicle speed sensor  30 . Thus, as shown by the equation 5, adding the predetermined value of α to the recommended vehicle speed Vt 0  gives the vehicle speed standard Vt. This vehicle speed standard Vt may be regarded as an index for estimating a potential risk which might be actualized within the environment around the vehicle in the future. For example, if the vehicle is running at a vehicle speed faster than the vehicle speed standard Vt, an estimate that a risk may grow in the future is justified. 
     At step S 112 , the controller  60  compares the vehicle speed Vf detected at step S 101  to the vehicle speed standard Vt calculated at step S 111  by determining whether or not the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt. If the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt, the logic goes to step S 113 . At step S 113 , the controller  60  calculates a time elapsed T —over  from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. At the next step S 114 , the controller  60  determines whether or not occurrence of a click reaction force is counted by a click frequency counter called COUNT. Specifically, at step S 114 , it is determined whether or not the content of the click frequency counter COUNT is equal to 0 (zero). 
     Determining, at step S 114 , that the content of the click frequency counter COUNT is zero (COUNT=0) means that no click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic proceeds to step S 115 . At step S 115 , the controller  60  determines whether or not the time elapsed T —over  has exceeded a predetermined click starting period t 0 . The predetermined click starting period t 0  is herein used to mean a fixed or a variable period, which is set immediately after the vehicle speed Vf has become equal to or greater than the vehicle speed standard Vt and kept set as long as the vehicle speed Vf stays equal to or greater than the vehicle speed standard Vt. If, at step S 115 , the time elapsed T —over  has exceeded the predetermined click starting period t 0 , the logic continues to step S 117  and onwards for generation of a click reaction force. 
     Determining, at step S 114 , that the content of the click frequency counter COUNT is equal to or greater than 1 (COUNT≠0) means that a click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic proceeds to step S 116 . At step S 116 , the controller  60  determines whether or not a click time T —on , which is a time elapsed from a moment immediately after occurrence of a click reaction force, has exceeded a predetermined click interval t 1 . The predetermined click interval is an interval between the adjacent two click reaction forces. If, at step S 116 , it is determined that the click time T —on  has exceeded the predetermined click interval t 1  (T —on &gt;t 1 ), the logic goes to step S 117  and onwards for generation of another click reaction force. 
     At step S 117 , the controller  60  resets the click time T —on . At the next step S 118 , the controller  60  updates the click frequency counter COUNT. Subsequently, at step S 119 , the controller resets a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. At the next step S 120 , the controller  60  sets a click reaction force Fc which is herein used to mean a one-shot pulse-like additional force to the reaction force increment dF calculated at step S 104 . Thus, the click reaction force Fc has a magnitude (comparable to a pulse height) and a duration (comparable to a pulse width) during which the magnitude of force is continuously applied. In this exemplary implementation, both the magnitude and duration are predetermined fixed values, respectively. 
     If, at step S 116 , the click time T —on  is equal to or less than the predetermined click interval t 1 , the logic continues to step S 121 . At step S 121 , the controller  60  performs updating by increasing the click time T —on  by unit amount of time, and the logic proceeds to step S 122 . The logic also continues to step S 122  from step S 115  if, at step S 115 , the time elapsed T —over  is equal to or less than the predetermined click starting period t 0 . At step S 122 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0) to prevent occurrence of such click reaction force Fc. 
     After setting the click reaction force Fc at step S 120  or S 122 , the logic moves to step S 131 . At step S 131 , the controller  60  calculates an accelerator pedal reaction force instruction value FA, i.e., a value indicative of a magnitude of a reaction force instructed to be applied to an accelerator pedal using the reaction force increment dF calculated at step S 104  and the click reaction force Fc set at step S 120  or S 122 . The accelerator pedal reaction force instruction value FA may be expressed as:
 
 FA=dF+Fc   (Eq. 6)
 
     At the next step S 132 , the controller  60  provides, as an output, the reaction force instruction value FA to an accelerator pedal reaction force control unit  80 . In response to the reaction force instruction value FA provided by the controller  60 , the accelerator pedal reaction force control unit  80  performs regulation of reaction force for the accelerator pedal  82 , applying different forms of tactile stimulus to the vehicle driver for keeping the driver informed of the risk perceived RP around the vehicle and how the vehicle deviates from the vehicle speed standard Vt. 
     If, at step S 12 , the vehicle speed Vf is less than the vehicle speed standard Vt, the logic proceeds to step S 123 . At step S 123 , the controller  60  calculates a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. At the next step S 124 , the controller  60  determines whether or not the time elapsed T —under  has exceeded a predetermined reset period t 2 . If, at step S 124 , the time elapsed T —under  has exceeded the predetermined reset period t 2 , the logic goes to step S 125 . At step S 125 , the controller  60  resets the click frequency counter COUNT. At the next step S 126 , the controller  60  resets the time elapsed T —over . At the next step S 127 , the controller  60  resets or clears the click time T —on  (T —on =0). Then, the logic moves to step S 122 . At step S 122 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
     If, at step S 124 , the time elapsed T —under  is less than the predetermined reset period t 2 , the logic goes directly to step S 122  without resetting the click frequency counter COUNT (step S 125 ), the time elapsed T —over  (step S 126 ), and the click time T —on  (step S 127 ). At step S 122 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
       FIGS. 5(   a )- 5 ( d ) illustrate how the first exemplary implementation of a driver assisting system  1  operates. As illustrated in  FIGS. 5(   a )- 5 ( d ), a click reaction force Fc occurs upon expiration of the predetermined click starting period t 0  after the vehicle speed Vf became greater than or equal to the vehicle speed standard Vt. Subsequently, a train of click reaction forces Fc occur at the predetermined click interval t 1  that is constant as long as the vehicle speed Vf remains greater than or equal to the vehicle speed standard Vt. At the same time, the reaction force increment dF for the risk perceived RP occurs together with the train of click reaction forces Fc at the accelerator pedal  82 . Thus, the driver perceives a continuous variation of the reaction force for the accelerator pedal  82  to receive information on the calculated risk perceived RP and a discontinuous and distinct temporary change of the reaction force for the accelerator pedal  82  to receive information on how the vehicle deviates from the vehicle speed standard Vt. 
     Referring back to steps S 117 , S 121  and S 127 , it will be readily understood that the controller  60  does not reset but stops increasing the click time T —on  as long as the time elapsed T —under  is less than the predetermined reset period t 2  (step S 124 ). If the time elapsed T —under  is less than the predetermined reset period t 2 , the controller  60  resumes increasing the click time T —on  (step S 121 ) immediately after the vehicle speed Vf has become greater than or equal to the vehicle speed standard Vt again. In other words, before or upon expiration of the predetermined reset period t 2  beginning with the moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt, the controller  60  may resume increasing the click time T —on  immediately after the vehicle speed Vt has become greater than or equal to the vehicle speed standard Vt again. Subsequently, regardless of expiration of the predetermined click starting period t 0 , the controller  60  may allow a click reaction force Fc to occur when the click time T —on  exceeds the predetermined click interval t 1 . 
     After expiration of the predetermined reset period t 2 , the controller  60  resets the click frequency counter COUNT (step S 232 ), the time elapsed T —over  (step S 233 ) and the click time T —on  (step S 234 ). Thus, expiration of the predetermined click starting period t 0  is needed before the controller  60  allows a click reaction force Fc to occur after the vehicle speed Vt has become greater than or equal to the vehicle speed standard Vt again. 
     The above-mentioned first exemplary implementation gives effects as follows: 
     (1) The controller  60  calculates a risk perceived RP by a driver around a vehicle operated by the driver and information on how the vehicle deviates from a vehicle speed standard Vt. This information is considered to represent a potential risk, which is a risk not yet perceived by the driver but should be notified to the drive. The information is provided to the driver by applying to the driver different forms of tactile stimulus derived from the same instrumentality the driver usually keeps in contact with during driving the vehicle. Thus, the driver is kept informed of such pieces of information by paying attention to the different forms of tactile stimulus derived from the single instrumentality. As the different forms of tactile stimulus are used, the driver quickly understands different bits of information. As the different forms of tactile stimulus are derived from the single instrumentality, the overall system may be simplified. 
     (2) The driver perceives different forms of tactile stimulus derived from an accelerator pedal  82  to obtain information on a risk perceived RP by the driver from an obstacle, such as a preceding vehicle, and information on how much the vehicle deviates from the vehicle speed standard Vt. Thus, the driver is kept informed of an actual risk, perceived by the driver, within a current environment around the vehicle and a potential risk that might become an actual risk in the future. 
     (3) The driver perceives a continuous variation of the reaction force from the accelerator pedal  82  to continuously get information on a risk perceived RP by the driver from an obstacle, such as a preceding vehicle because the reaction force represents the risk perceived RP. The driver perceives a discontinuous and distinct temporary change of the reaction force (a click reaction force) from the accelerator pedal  82  and is urged to pay attention to the fact that the vehicle speed Vf exceeds the vehicle speed standard Vt because the click reaction force represents how much the vehicle speed Vf exceeds the vehicle speed standard Vt. 
     (4) The driver is warned of the fact that the vehicle speed Vf exceeds the vehicle speed standard Vt because the controller  60  calculates how much the vehicle speed Vf exceeds from the vehicle speed standard Vt for production of a train of click reaction forces when the vehicle speed Vf exceeds the vehicle speed standard Vt. 
     Second Exemplary Implementation of the Invention 
     With continuing reference to  FIGS. 1 to 3 , a second exemplary implementation of a driver assisting system according to the present invention is substantially the same in hardware as the first exemplary implementation. However, as shown in phantom line in  FIG. 1 , the second exemplary implementation is different from the first exemplary implementation in that a controller  60  monitors an accelerator pedal stroke sensor  83  to get an accelerator pedal position Sp of an accelerator pedal  82  as illustrated by phantom line in  FIG. 1 . As the discussion proceeds, one may understand that the accelerator pedal position Sp is used in calculating a click reaction force Fc. 
     The second exemplary implementation of the present invention informs a vehicle driver of the fact that a vehicle speed Vf exceeds a vehicle speed standard Vt more positively than the first exemplary implementation does. To inform the driver more positively, the vehicle speed standard Vt, a click starting period t 0 , a click interval t 1 , and a reset period t 2  are altered. A click reaction force Fc is altered in magnitude, too. 
     Referring to  FIG. 6  and  FIGS. 7(   a ) to  7 ( d ), the following describes on reaction force control employed by the second exemplary implementation of the present invention.  FIG. 6  illustrates a flow chart of a driver assisting control program executed by a controller  60  used in the second exemplary implementation of the present invention.  FIGS. 7(   a ) to  7 ( d ) are time charts illustrating time variations of vehicle speed Vf, accelerator pedal position Sp, click reaction force Fc and accelerator pedal reaction force instruction value FA, respectively. The execution of the program illustrated in  FIG. 6  is repeated at regular intervals, for example, every 50 msec. 
     In  FIG. 6 , at step S 201 , the controller  60  performs a reading operation of output signals from the accelerator pedal stroke sensor  83 , a vehicle speed sensor  30 , a laser radar  10 , and a front camera  20  to obtain information on running environment around the vehicle. At step S 202 , the controller  60  performs analysis of the obtained information to recognize the state of the environment around the vehicle by calculating a distance D to a preceding vehicle and a relative speed Vr to the preceding vehicle. The relative speed Vr is given by subtracting a vehicle speed Vf of the vehicle operated by a driver from a vehicle speed Vp of the preceding vehicle (Vr=Vp−Vf). 
     After step S 202 , the logic goes to steps S 203  and S 204 . At step S 203 , the controller  60  calculates a risk perceived RP. At step S 204 , the controller  60  calculates an accelerator pedal reaction force increment dF. For brevity, further description on what the controller  60  performs at steps S 203  and S 204  is hereby omitted because the steps S 203  and S 204  correspond to steps S 103  and S 104  of the flow chart in  FIG. 4 , respectively. 
     At step S 211  following step S 201 , the controller  60  calculates a vehicle speed standard Vt. Because the step S 211  corresponds to step S 111  of the flow chart in  FIG. 4 , further description on what the controller  60  performs at step S 211  is hereby omitted for brevity. 
     At the next step S 212 , the controller  60  corrects the vehicle speed standard Vt. Specifically, the controller  60  sets a value given at step S 211  as an initial vehicle speed standard value Vti, and corrects the initial vehicle speed standard value Vti based on a time elapsed T —over  that is a time elapsed from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt as corrected. The fully drawn line in  FIG. 8  illustrates an exemplary relationship between the vehicle speed standard Vt as corrected and the time elapsed T —over . In  FIG. 8 , the one-dot-chain line illustrates the initial vehicle speed standard value Vti that is the value given at step S 211 , and the dotted line illustrates the minimum (or lowest) vehicle speed standard value Vtm. 
     As illustrated in  FIG. 8 , the controller  60  determines the vehicle speed standard Vt by lowering the vehicle speed standard Vt from the initial vehicle speed standard value Vti toward the minimum vehicle speed standard value Vtm as the time elapsed T —over  increases. Lowering the vehicle speed standard Vt continues until a moment when the time elapsed T —over  reaches a predetermined value T —over   1 . Upon and after the moment when the time elapsed T —over  reaches the predetermined value T —over   1 , the controller  60  sets the minimum vehicle speed standard value Vtm as the vehicle speed standard Vt. The vehicle speed standard Vt drops as the time elapsed T —over  increases, causing an increase in frequency of a click reaction force Fc as will be described later. 
     At step S 213 , the controller  60  determines whether or not the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt as corrected at step S 212 . If the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt, the logic moves to step S 214 . At step S 214 , the controller  60  calculates the above-mentioned time elapsed T —over , which is a time elapsed from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. At the next step S 215 , the controller  60  determines whether or not the content of a click frequency counter COUNT is equal to 0 (zero). 
     Determining, at step S 215 , that the content of the click frequency counter COUNT is zero (COUNT=0) means that no click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic moves to step S 216 . At step S 216 , the controller  60  determines a click starting period t 0 . In this second exemplary implementation, the controller  60  determines the click starting period t 0  based on an excess ΔV by which the vehicle speed Vf exceeds the vehicle speed standard Vt (ΔV=Vf−Vt). In  FIG. 9 , the fully drawn line illustrates an exemplary varying of the click starting period t 0  with different values of the excess ΔV. 
     In  FIG. 9 , the reference character t 01  indicates an initial value of the click starting period t 0 . As indicated by the fully drawn line in  FIG. 9 , the controller  60  determines the click starting period t 0  by shortening the click starting period from the initial value t 0   i  toward a minimum value of, for example 0 (zero), as the excess ΔV increases if the excess ΔV is less than a predetermined excess value ΔV 1 . If the excess ΔV is equal to or greater than the predetermined excess value ΔV 1 , the controller  60  sets the minimum value of 0 as the click starting period t 0 . As a result, the controller  60  allows a click reaction force Fc to occur quickly after the vehicle speed Vf has greatly exceeded the vehicle speed standard Vt by shortening the click starting period t 0 . 
     At step S 217 , the controller  60  determines whether or not the time elapsed T —over  has exceeded the click starting period t 0 . If, at step S 217 , the time elapsed T —over  has exceeded the click starting period t 0 , the logic goes to step S 220 . 
     Determining, at step S 215 , that the content of the click frequency counter COUNT is equal to or greater than 1 (0) means that a click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic moves to step S 218 . At step S 218 , the controller  60  determines a click interval t 1  between the adjacent two click reaction forces. In this second exemplary implementation, the controller  60  determines the click interval t 1  based on the excess ΔV. In  FIG. 10 , the fully drawn line illustrates an exemplary varying of the click interval t 1  with different values of the excess ΔV. 
     In  FIG. 10 , the reference characters t 1   i  and t 1   m  indicate an initial value and a minimum value of click interval t 1 , respectively. As indicated by the fully drawn line in  FIG. 10 , the controller  60  determines the click interval t 1  by shortening the click interval t 1  from the initial value t 1   i  toward the minimum value t 1   m  as the excess ΔV increases if the excess ΔV is less than a predetermined excess value ΔV 2 . If the excess ΔV is equal to or greater than the predetermined excess value ΔV 2 , the controller  60  sets the minimum value t 1   m  as the click interval t 1 . As a result, the controller  60  allows an increase in the number of click reaction forces Fc occurring within a unit time by shortening the click interval t 1  if the vehicle speed Vf greatly exceeds the vehicle speed standard Vt. 
     At step S 219 , the controller  60  determines whether or not a click time T —on , which is a time elapsed from a moment immediately after occurrence of a click reaction force, has exceeded the click interval t 1  determined at step S 218 . If, at step S 219 , the click time T —on  has exceeded the click interval t 1  (T —on &gt;t 1 ), the logic moves to step S 220  and onwards for generation of another click reaction force. 
     At step S 220 , the controller  60  resets the click time T —on . At the next step S 221 , the controller  60  updates the click frequency counter COUNT. Subsequently, at step S 222 , the controller resets a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. 
     At step S 223 , the controller  60  determines a first correction coefficient k 1  based on the excess ΔV. The fully drawn line in  FIG. 11  illustrates an exemplary varying of the first correction coefficient k 1  with different values of the excess ΔV. 
     In  FIG. 11 , the reference character k 1   mx  indicates a maximum value of the first correction coefficient k 1 . As indicated by the fully drawn line in  FIG. 11 , the controller  60  determines the first correction coefficient k 1  by increasing the first correction coefficient K 1  from an initial value of 1 (one) toward the maximum value k 1   mx  as the excess ΔV increases if the excess ΔV is less than a predetermined excess value ΔV 3 . If the excess ΔV is equal to or greater than the predetermined excess value ΔV 3 , the controller  60  sets the maximum value k 1   mx  as the first correction coefficient k 1 . 
     At step S 224 , the controller  60  determines a second correction coefficient k 2  based on the risk perceived RP calculated at step S 203 . The fully drawn line in  FIG. 12  illustrates an exemplary varying of the second correction coefficient k 2  with different values of the risk perceived RP. As indicated by the fully drawn line in  FIG. 12 , the controller  60  determines the second correction coefficient k 2  by increasing the second correction coefficient k 2  from an initial value of 1 (one) as the risk perceived RP increases. 
     At step S 225 , the controller  60  determines a third correction coefficient k 3  based on the accelerator pedal position Sp. The fully drawn line in  FIG. 13  illustrates an exemplary varying of the third correction coefficient k 3  with different values of the accelerator pedal position Sp. As indicated by the fully drawn line in  FIG. 13 , the controller  60  determines the third correction coefficient k 3  by increasing the third correction coefficient k 3  from an initial value of 1 (one) as the accelerator pedal position Sp increases. 
     At the next step S 226 , using the first, second and third correction coefficients k 1 , k 2  and k 3 , the controller  60  calculates a magnitude of click reaction force Fc, which is expressed as:
 
 Fc=k 1 ·k 2 ·k 3 ·Fcr   (Eq. 7)
 
where: Fcr represents a standard value of the magnitude of a click reaction force Fc.
 
     If, at step S 219 , the click time T —on  is equal to or less than the click interval t 1 , the logic goes to step S 227 . At step S 227 , the controller  60  carries out updating by increasing the click time T —on  by unit amount of time, and the logic moves to step S 228 . The logic also moves to step S 228  from step S 217  if, at step S 217 , the time elapsed T —over  is equal to or less than the click starting period t 0 . At step S 228 , the controller  60  sets the magnitude of click reaction force Fc equal to 0 (Fc=0) to prevent occurrence of such click reaction force Fc. 
     After setting the click reaction force Fc at step S 226  or S 228 , the logic goes to step S 241 . At step S 241 , the controller  60  calculates an accelerator pedal reaction force instruction value FA using the reaction force increment dF calculated at step S 204  and the click reaction force Fc determined at step S 226  or S 228 . The accelerator pedal reaction force instruction value FA is expressed by equation 6. 
     At the next step S 242 , the controller  60  provides, as an output, the reaction force instruction value FA to an accelerator pedal reaction force control unit  80 . In response to the reaction force instruction value FA provided by the controller  60 , the accelerator pedal reaction force control unit  80  performs regulation of reaction force for the accelerator pedal  82 , applying different forms of tactile stimulus to the vehicle driver for keeping the driver informed of the risk perceived RP around the vehicle and how the vehicle deviates from the vehicle speed standard Vt. 
     If, at step S 213 , the vehicle speed Vf is less than the vehicle speed standard Vt, the logic continues to step S 229 . At step S 229 , the controller  60  calculates a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. At step S 230 , the controller  60  determines a reset period t 2  based on the time elapsed T —over . The fully drawn line in  FIG. 14A  illustrates an exemplary varying of the reset period t 2  with different values of time elapsed T —over . 
     In  FIG. 14A , the reference characters t 2   i  and t 2   mx  indicate an initial value and a maximum value of the reset period t 2 . As indicated by the fully drawn line in  FIG. 14A , the controller  60  determines the reset period t 2  by increasing the reset period t 2  from the initial value of t 2   i  toward the maximum value t 2   mx  as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined value T —over   2 . If the time elapsed T_over is equal to or greater than the predetermined value T —over   2 , the controller  60  sets the maximum value t 2   imx  as the reset period t 2 . 
     As the discussion proceeds, it will be understood that the contents of click frequency counter COUNT, time elapsed T —over , and click time T —on  are held during the reset period t 2 , which increases with an increase in the time elapsed T —over . Thus, if the vehicle speed Vf becomes equal to or greater than the vehicle speed standard Vt again before expiration of the reset period t 2 , the controller  60  allows occurrence of click reaction forces Fc having the previously set magnitude and frequency. 
     Referring to  FIG. 14B , as the content of the click frequency counter COUNT increases with an increase in the time elapsed T —over , the content of the click frequency counter COUNT may be used to retrieve the relationship as indicated by the fully drawn in  FIG. 14B  to determine an appropriate value of the reset period t 2 . The relationship illustrated in  FIG. 14B  is substantially the same as the illustrated relationship in  FIG. 14A . 
     At the next step S 231 , the controller  60  determines whether or not the time elapsed T —under  has exceeded the reset period t 2  that was determined at step S 230 . If, at step S 231 , the time elapsed T —under  has exceeded the reset period t 2 , the logic goes to step S 232 . At step S 232 , the controller  60  resets the click frequency counter COUNT. At the next step S 233 , the controller  60  resets the time elapsed T —over . At the next step S 234 , the controller  60  resets the click time T —on . Then, the logic goes to step S 228 . At step S 228 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
     If, at step S 231 , the time elapsed T —under  is less than the reset period t 2 , the logic goes directly to step S 228  without resetting the click frequency counter COUNT (step S 232 ), the time elapsed T —over  (step S 233 ), and the click time T —on  (step S 234 ). At step S 228 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
       FIGS. 7(   a )- 7 ( d ) illustrate how the second exemplary implementation of the driver assisting system  1  operates. As illustrated in  FIGS. 7(   a )- 7 ( d ), a click reaction force Fc occurs upon expiration of the click starting period t 0  that varies in response to the excess ΔV after the vehicle speed Vf became greater than or equal to the vehicle speed standard Vt. Subsequently, a train of click reaction forces Fc occur at the click interval t 1  that varies in response to the excess ΔV when the vehicle speed Vf remains greater than or equal to the vehicle speed standard Vt. The magnitude of the click reaction force Fc is determined in response to the excess ΔV, risk perceived RP, and accelerator pedal position Sp. The frequency of click reaction forces Fc increases in response to an increase in the excess ΔV. The magnitude of click reaction forces Fc increases in response to an increase in the excess ΔV, an increase in the risk perceived RP, and an increase in the accelerator pedal position Sp. 
     A sum given by adding the train of click reaction forces Fc to the reaction force increment dF representative of the risk perceived RP appears in the magnitude of a reaction force, which the driver perceives from the accelerator pedal  82 . Thus, the driver perceives a continuous variation of the reaction force from the accelerator pedal  82  to obtain information on the calculated risk perceived RP and a discontinuous and distinct temporary change of the reaction force from the accelerator pedal  82  to obtain information on how the vehicle deviates from the vehicle speed standard Vt. The driver perceives a change in the magnitude of click reaction forces Fc to obtain information on a change in the risk perceived RP and a change in the interval between the adjacent two click reaction forces to get information on a change in the excess ΔV. An increase in potential risk is positively brought to the attention of the driver via the drivers perception of an increase in frequency of the click reaction forces Fc caused by a drop in the vehicle speed standard Vt when the vehicle speed Vf exceeds the vehicle speed standard Vt over extended period of time. 
     Referring back to steps S 220 , S 227  and S 234  of the flow chart illustrated in  FIG. 6 , it will be readily understood that the controller  60  does not reset but stops increasing the click time T —on  as long as the time elapsed T —under  is less than the reset period t 2  (step S 231 ). If the time elapsed T —under  is less than the reset period t 2 , the controller  60  resumes increasing the click time T —on  (step S 227 ) immediately after the vehicle speed Vf has become greater than or equal to the vehicle speed standard Vt again. In other words, before or upon expiration of the reset period t 2  beginning with the moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt, the controller  60  may resume increasing the click time T —on  immediately after the vehicle speed Vt has become greater than or equal to the vehicle speed standard Vt again. Subsequently, regardless of expiration of the click starting period t 0 , the controller  60  may allow a click reaction force Fc to occur when the click time T —on  exceeds the click interval t 1 . Thus, the level of risk may be continuously and positively brought to the attention of the driver. 
     First Modification of the Second Exemplary Implementation 
     In the flow chart of  FIG. 6 , at step S 218 , the controller  60  determines the click interval t 1  using the varying of click interval t 1  with different values of the excess ΔV illustrated in  FIG. 10 . The controller  60  may determine the click interval t 1  based on the time elapsed T —over  using the illustrated relationship in  FIG. 15A . In  FIG. 15A , the fully drawn line illustrates an exemplary varying of the click interval t 1  with different values of the time elapsed T —over . 
     In  FIG. 15A , the reference characters t 1   i  and t 1   m  indicate an initial value and a minimum value of the click interval t 1 , respectively. As indicated by the fully drawn line in  FIG. 15A , the controller  60  determines the click interval t 1  by shortening the click interval t 1  from the initial value t 1   i  toward the minimum value t 1   m  as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined time value T —over   3 . If the time elapsed T —over  is equal to or greater than the predetermined time value T —over   3 , the controller  60  sets the minimum value t 1   m  as the click interval t 1 . 
     The controller  60  may determine the click interval t 1  based on the content of the counter COUNT instead of the time elapsed T —over  using the illustrated relationship in  FIG. 15B . In  FIG. 15B , the fully drawn line illustrates an exemplary varying of the click interval t 1  with different values of the content of the counter COUNT. 
     In  FIG. 15B , the reference characters t 1   i  and t 1   m  indicate an initial value and a minimum value of the click interval t 1 , respectively. As indicated by the fully drawn line in  FIG. 15B , the controller  60  determines the click interval t 1  by shortening the click interval t 1  from the initial value t 1   i  toward the minimum value t 1   m  as the content of the counter COUNT increases if the content of the counter COUNT is less than a predetermined counter value N 1 . If the content of the counter COUNT is equal to or greater than the predetermined counter value N 1 , the controller  60  sets the minimum value t 1   m  as the click interval t 1 . 
     Second Modification of the Second Exemplary Implementation 
     In the flow chart of  FIG. 6 , at step S 223 , the controller  60  determined the first correction coefficient k 1  using the varying of the first correction coefficient k 1  with different values of the excess ΔV illustrated in  FIG. 11 . The controller  60  may determine the first correction coefficient k 1  based on the time elapsed T —over  using the illustrated relationship in  FIG. 16A . In  FIG. 16A , the fully drawn line illustrates an exemplary varying of the first correction coefficient k 1  with different values of the time elapsed T —over . 
     In  FIG. 16A , the reference character k 1   mx  indicates a maximum value of the first correction coefficient k 1 . As indicated by the fully drawn line in  FIG. 16A , the controller  60  determines the first correction coefficient k 1  by increasing the first correction coefficient k 1  from an initial value of 1 (one) toward the maximum value k 1   mx  as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined time value T —over   4 . If the time elapsed T —over  is equal to or greater than the predetermined time value T —over   4 , the controller  60  sets the maximum value k 1   mx  as the first correction coefficient k 1 . 
     The controller  60  may determine the first correction coefficient k 1  based on the content of the counter COUNT instead of the time elapsed T —over  using the illustrated relationship in  FIG. 16B . In  FIG. 16B , the fully drawn line illustrates an exemplary varying of the first correction coefficient k 1  with different values of the content of the counter COUNT. 
     In  FIG. 16B , the reference character k 1   mx  indicates a maximum value of the first correction coefficient k 1 . As indicated by the fully drawn line in  FIG. 16B , the controller  60  determines the first correction coefficient k 1  by increasing the first correction coefficient k 1  from an initial value of 1 (one) toward the maximum value k 1   mx  as the content of the counter COUNT increases if the content of the counter COUNT is less than a predetermined counter value N 2 . If the content of the counter COUNT is equal to or greater than the predetermined counter value N 2 , the controller  60  sets the maximum value k 1   mx  as the first correction coefficient k 1 . 
     In addition to the effects provided by the first exemplary implementation, the above-mentioned second exemplary implementation gives further effects as follows: 
     (1) The vehicle driver can perceive intuitively how much the vehicle speed Vf exceeds the vehicle speed standard Vt by feeling varying of the click interval t 1  and/or varying, in magnitude, of click reaction forces Fc with different values of the excess, in state or amount, by which the vehicle speed Vf exceeds the vehicle speed standard Vt because the controller  60  calculates the excess, in state or amount, to alter the click interval t 1  and/or the magnitude of click reaction forces Fc. The driver can perceive positively the fact that the vehicle speed Vf exceeds the vehicle speed standard Vt by feeling an increase in frequency of click reaction forces Fc caused when the excess ΔV or the time elapsed t —over  is great. 
     (2) The driver can get information on the risk perceived RP from an obstacle as well as the vehicle speed standard Vt by feeling varying, in magnitude, of click reaction forces Fc with different levels of the risk perceived RP because the controller  60  alters the magnitude of click reaction forces Fc based on the risk perceived RP from the obstacle. 
     (3) The driver is urged to pay attention to the fact that the vehicle speed Vf exceeds the vehicle speed standard Vt when the driver depresses the accelerator pedal  82  deeply by increasing the magnitude of click reaction forces Fc because the controller  60  alters the magnitude of click reaction forces Fc based on the accelerator pedal position Sp. 
     Third Exemplary Implementation of the Invention 
     With continuing reference to  FIGS. 1 to 3 , a third exemplary implementation of a driver assisting system according to the present invention is substantially the same in hardware as the first exemplary implementation. However, as different from the first exemplary implementation, the third exemplary implementation permits a vehicle driver to identity how much a vehicle speed Vf exceeds a vehicle speed standard Vt with a number of times a click reaction force Fc is repeated within a limited span of time. 
     Referring to  FIG. 17  and  FIGS. 18(   a ) to  18 ( d ), the following provides description on reaction force control employed by the second exemplary implementation of the present invention.  FIG. 17  illustrates a flow chart of a driver assisting control program executed by a controller  60  used in the third exemplary implementation of the present invention.  FIGS. 18(   a ) to  18 ( d ) are time charts illustrating time variations of vehicle speed Vf, accelerator pedal position Sp, click reaction force Fc and accelerator pedal reaction force instruction FA, respectively. The execution of the program illustrated in  FIG. 17  is repeated at regular intervals, for example, every 50 msec. 
     In  FIG. 17 , at step S 301 , the controller  60  performs a reading operation of output signals from the accelerator pedal stroke sensor  83 , a vehicle speed sensor  30 , a laser radar  10 , and a front camera  20  to obtain information on running environment around the vehicle. At step S 302 , the controller  60  performs analysis of the obtained information to recognize the state of environment around the vehicle by calculating a distance D to a preceding vehicle and a relative speed Vr to the preceding vehicle. The relative speed Vr is given by subtracting a vehicle speed Vf of the vehicle operated by a driver from a vehicle speed Vp of the preceding vehicle (Vr=Vp−Vf). 
     After step S 302 , the logic goes to steps S 303  and S 304 . At step S 303 , the controller  60  calculates a risk perceived RP. At step S 304  the controller  60  calculates an accelerator pedal reaction force increment dF. For brevity, further description on what the controller  60  performs at steps S 303  and S 304  is hereby omitted because the steps S 303  and S 304  correspond to steps S 103  and S 104  of the flow chart in  FIG. 4 , respectively. 
     At step S 311  following the previously mentioned step S 301 , the controller  60  determines a vehicle speed standard Vt. Because the step S 211  corresponds to step S 111  of the flow chart in  FIG. 4 , further description on what the controller  60  performs at step S 311  is hereby omitted for brevity. 
     At step S 312 , the controller  60  determines whether or not the vehicle speed Vf is equal to or greater than a predetermined vehicle speed standard Vt. If the vehicle speed Vf is equal to or greater than the predetermined vehicle speed standard Vt, the logic goes to step S 313 . 
     At step S 313 , the controller  60  determines whether or not the vehicle speed Vf, which has been found to be equal to or greater than the vehicle speed standard Vt at step S 312 , is increasing. In particular, the controller  60  calculates time differential dVf/dt of the vehicle speed Vf and determines whether or not the calculated time differential dVf/dt is greater than 0 (zero). The controller  60  determines that the vehicle speed Vf is increasing if the time differential dVf/dt is greater than 0, and it determines that the vehicle speed is constant or decreasing if the time differential dVf/dt is equal to or less than 0. If, at step S 313 , the time differential dVf/dt is greater than 0 (the vehicle speed Vf increasing), the logic moves to S 314 . At step  314 , the controller  60  updates a click time T —on , which, in this exemplary implementation, is a time elapsed from a moment immediately after or upon the first occurrence of a click reaction force Fc within a predetermined time frame having span of time ta, which is later described later in connection with  FIG. 19 . 
     At the next step S 315 , the controller  60  determines, based on the click time T —on  determined at step S 314  and a current excess level, which the vehicle speed Vf belongs to, whether or not occurrence of a click reaction force Fc is permitted. The excess level is a level of an excess by which the vehicle speed Vf exceeds the vehicle speed standard Vt. Referring to  FIG. 18(   a ), in this exemplary implementation, the controller  60  has four excess levels, which represent four band regions with the same width Va given after dividing a range of vehicle speeds exceeding the vehicle speed standard Vt. The four excess levels are an excess level  1 , an excess level  2 , an excess level  3  and an excess level  4 . The excess level  1  represents a first band region consisting of vehicle speed values equal to or greater than the vehicle speed standard Vt but less than a vehicle speed (Vt+Va). The excess level  2  represents a second band region consisting of vehicle speed values equal to or greater than the vehicle speed (Vt+Va) but less than a vehicle speed (Vt+2 Va). The excess level  3  represents a third band region consisting of vehicle speed values equal to or greater than the vehicle speed (Vt+2 Va) but less than a vehicle speed (Vt+3 Va). Finally, the excess level  4  represents a fourth band region consisting of vehicle speed values equal to or greater than the vehicle speed (Vt+3 Va) but less than a vehicle speed (Vt+4 Va). In this exemplary implementation, the controller  60  determines which one of the four excess levels the vehicle speed Vf belongs to. 
     At step S 315 , in determining whether or not occurrence of a click reaction force Fc is permitted, the controller  60  may use one of two examples of logic as illustrated in  FIG. 19  in this exemplary implementation. Referring to  FIG. 19 , the horizontal axis represents the click time T —on  and has thereon the before-mentioned time frame ta, and a predetermined interval tb (tb&gt;ta). The reference character “LOG. A” indicates logic illustrated by a lower half in  FIG. 19 . The logic “LOG. A” prohibits occurrence of a click reaction force Fc upon a change in the excess level until expiration of the time frame ta, thus permitting occurrence of a click reaction force Fc upon a change in the excess level upon or after expiration of the time frame ta. The logic “LOG. A” also provides that, if the excess level remains unchanged upon or after expiration of the time frame ta, no click reaction force Fc will occur. The reference character “LOG. B” indicates complementary logic to the logic “LOG. A”. This complementary logic “LOG. B” permits occurrence of a click reaction force Fc upon expiration of the predetermined interval tb even if the excess level remains unchanged. It will now be noted that the logic “LOG. A” alone constitute the one of the above-mentioned two examples of logic, which the controller  60  may use in performing job at step S 315 . The logic “LOG. A” as complemented by the logic “LOG. B” constitutes other example of logic, which the controller  60  uses in performing job at step S 315  according to this exemplary implementation. 
     According to this exemplary implementation, within a range where the vehicle speed Vf is equal to or greater than the vehicle speed standard, the controller  60  permits occurrence of a click reaction force Fc upon a change in the excess level upon or after expiration of the time frame ta. Upon or after expiration of the predetermined interval tb (tb&gt;ta), the controller  60  permits occurrence of a click reaction force Fc even if the excess level remains unchanged. According to this exemplary implementation, if the vehicle speed Vf exceeds the vehicle speed standard Vt, the controller  60  permits occurrence of a click reaction force Fc immediately after the vehicle speed Vf has exceeded the vehicle speed standard Vt because the vehicle speed Vf enters the excess level  1 . As will be later described, the controller  60  permits a single click reaction force Fc to occur immediately after the vehicle speed Vf has exceeded the vehicle speed standard Vt into the excess level  1 . 
     If, at step S 315 , the occurrence of a click reaction force Fc is permitted, the logic moves to step S 316 . At step S 316 , the controller  60  determines a number Nc of times a click reaction force Fc is repeated based on the excess level, which the vehicle speed Vf belongs to.  FIG. 20  illustrates varying of number Nc of times a click reaction force Fc is repeated with different excess levels  1 ,  2 ,  3  and  4 . As is readily seen from  FIG. 20 , the controller  60  sets one (1) as the number Nc when the vehicle speed Vf belongs to the excess level  1 , permitting one click reaction force Fc to occur within the time frame ta. When the vehicle speed Vf belongs to the excess level  2 , the controller  60  sets two (2) as the number Nc, permitting two click reaction forces Fc to occur within the time frame ta. When the vehicle speed Vf belongs to the excess level  3 , the controller  60  sets three (3) as the number Nc, permitting three click reaction forces Fc to occur within the time frame ta. When the vehicle speed vf belongs to the excess level  4 , the controller  60  sets four (4) as the number Nc, permitting four click reaction forces Fc to occur within the time frame ta. The setting is such that the magnitude of a click reaction forces Fc is predetermined. 
     At the next step S 317 , the controller  60  resets the click time T —on  (T — =0). 
     If, at step S 312 , the vehicle speed Vf is less than the vehicle speed standard Vt, or if, at step S 313 , the vehicle speed Vf is decreasing (df/dt&lt;0), or if, at step S 315 , the logic goes to step S 318 . At step S 318 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0) to prevent occurrence of such click reaction force Fc. 
     After step S 317  or S 318 , the logic goes to step S 331 . At step S 331 , the controller  60  calculates an accelerator pedal reaction force instruction value FA using the reaction force increment dF calculated at step S 304  and the click reaction force Fc. The accelerator pedal reaction force instruction value FA is expressed by the equation 6 mentioned before. 
     At the next step S 332 , the controller  60  provides, as an output, the reaction force instruction value FA to an accelerator pedal reaction force control unit  80 . In response to the reaction force instruction value FA provided by the controller  60 , the accelerator pedal reaction force control unit  80  performs regulation of reaction force for the accelerator pedal  82 . 
       FIGS. 18(   a )- 18 ( d ) illustrate how the third exemplary implementation of the driver assisting system operates. As illustrated in  FIGS. 18(   a )- 18 ( d ), a single click reaction force Fc is permitted to occur upon the vehicle speed Vf exceeding the vehicle speed standard Vt. Subsequently, upon a shift of the vehicle speed Vf from the excess level  1  to the excess level  2 , two click reaction forces Fc are permitted to occur. Upon a shift of the vehicle speed Vf from the excess level  2  to the excess level  3 , three click reaction forces Fc are permitted to occur. Upon a shift of the vehicle speed Vf from the excess level  3  to the excess level  4 , four click reaction forces are permitted to occur. No click reaction force Fc is permitted to occur upon a change in the excess level if the vehicle speed Vf is decreasing. The vehicle driver can perceive how the vehicle speed Vf varies after exceeding the vehicle speed standard Vt by feeling varying of the number Nc of times a click reaction force Fc is repeated with different excess levels when the vehicle speed Vf is increasing in excess of the vehicle speed standard Vt. 
     If the vehicle speed Vf continues to stay in the excess level  1  after occurrence of single click reaction force Fc upon the vehicle speed Vf exceeding the vehicle speed standard Vt, another single click reaction force Fc is permitted to occur upon expiration of the predetermined interval tb. If the vehicle speed Vf continues to stay in the excess level  2 , two click reaction forces Fc are permitted to occur upon expiration the predetermined interval tb. In this manner, since the corresponding number of times a click reaction force Fc is repeated to one excess level is repeated regularly at the predetermined interval tb if the vehicle speed Vf continues to stay in the same excess level, the vehicle driver is urged to pay attention to how much the vehicle speed Vf exceeds the vehicle speed standard Vt. 
     In addition to the effects provided by the first and second exemplary implementations, the above-mentioned third exemplary implementation gives further effects as follows: 
     The controller  60  alters the number Nc of times a click reaction force Fc is repeated based on the excess ΔV from the vehicle speed standard Vt. The vehicle driver is urged to pay attention to how greatly the vehicle speed Vf exceeds the vehicle speed standard Vt because the number Nc of times a click reaction force Fc is repeated increases. 
     Fourth Exemplary Implementation of the Invention 
     With continuing reference to  FIGS. 1 to 3 , a fourth exemplary implementation of a driver assisting system according to the present invention is substantially the same in hardware as the first exemplary implementation. However, as shown in phantom line in  FIG. 1 , the fourth exemplary implementation is different from the first exemplary implementation in that a controller  60  monitors an accelerator pedal stroke sensor  83  to obtain an accelerator pedal position Sp of an accelerator pedal  82  as illustrated by phantom line in  FIG. 1 . As the discussion proceeds, one may understand that the accelerator pedal position Sp is used in calculating a click reaction force Fc. 
     The fourth exemplary implementation of the present invention keeps a vehicle driver informed of the fact that a vehicle speed Vf exceeds a vehicle speed standard Vt without causing the driver to feel an objection to receiving such information. To accomplish this object, the vehicle speed standard Vt and a reset period t 2  are altered. A click reaction force Fc is altered in magnitude, too. 
     Referring to  FIG. 21 , the following describes reaction force control employed by the fourth exemplary implementation of the present invention.  FIG. 21  illustrates a flow chart of a driver assisting control program executed by a controller  60  used in the fourth exemplary implementation of the present invention. The execution of the program illustrated in  FIG. 21  is repeated at regular intervals, for example, every 50 msec. 
     In  FIG. 21 , at step S 401 , the controller  60  performs a reading operation of output signals from the accelerator pedal stroke sensor  83 , a vehicle speed sensor  30 , a laser radar  10 , and a front camera  20  to obtain information on running environment around the vehicle. At step S 402 , the controller  60  performs analysis of the obtained information to recognize the state of the environment around the vehicle by calculating a distance D to a preceding vehicle and a relative speed Vr to the preceding vehicle. The relative speed Vr is given by subtracting a vehicle speed Vf of the vehicle operated by a driver from a vehicle speed Vp of the preceding vehicle (Vr=Vp−Vf). 
     After step S 402 , the logic moves to steps S 403  and S 404 . At step S 403 , the controller  60  calculates a risk perceived RP. At step S 404  the controller  60  calculates an accelerator pedal reaction force increment dF. For brevity, further description on what the controller  60  performs at steps S 403  and S 404  is hereby omitted because the steps S 403  and S 404  correspond to steps S 103  and S 104  of the flow chart in  FIG. 4 , respectively. 
     At step S 411  following the previously mentioned step S 401 , the controller  60  calculates a vehicle speed standard Vt. Because the step S 211  corresponds to step S 111  of the flow chart in  FIG. 4 , further description on what the controller  60  performs at step S 411  is hereby omitted for brevity. 
     At the next step S 412 , the controller  60  corrects the vehicle speed standard Vt. Specifically, the controller  60  sets a value given at step S 411  as an initial vehicle speed standard value Vti, and corrects the initial vehicle speed standard value Vti based on a time elapsed T —over  that is a time elapsed from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt as corrected. The fully drawn line in  FIG. 22  illustrates an exemplary relationship between the vehicle speed standard Vt as corrected and the time elapsed T —over . In  FIG. 22 , the one-dot-chain line illustrates the initial vehicle speed standard value Vti that is the value given at step S 411 , and the dotted line illustrates the maximum (or highest) vehicle speed standard value Vtmx. 
     As illustrated in  FIG. 22 , the controller  60  determines the vehicle speed standard Vt by increasing the vehicle speed standard Vt from the initial vehicle speed standard value Vti toward the maximum vehicle speed standard value Vtmx as the time elapsed T —over  increases. Increasing the vehicle speed standard Vt continues until a moment when the time elapsed T —over  reaches a predetermined value T —over   5 . Upon and after the moment when the time elapsed T —over  reaches the predetermined value T —over   5 , the controller  60  sets the maximum vehicle speed standard value Vtmx as the vehicle speed standard Vt. The vehicle speed standard Vt increases as the time elapsed T —over  increases, causing a reduction in frequency of a click reaction force Fc. The reduction in frequency of the click reaction force Fc is considered to be effective in preventing the driver from feeling objection to receiving the click reaction force Fc. 
     At step S 413 , the controller  60  determines whether or not the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt as corrected at step S 412 . If the vehicle speed Vf is equal to or greater than the vehicle speed standard Vt, the logic goes to step S 414 . At step S 414 , the controller  60  calculates the above-mentioned time elapsed T —over , which is a time elapsed from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. At the next step S 415 , the controller  60  determines whether or not the content of a click frequency counter COUNT is equal to 0 (zero). 
     Determining, at step S 415 , that the content of the click frequency counter COUNT is zero (COUNT=0) means that no click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic moves to step S 416 . At step S 416 , the controller  60  determines whether or not the time elapsed t —over  has exceeded a predetermined click starting period t 0 . If, at step S 416 , the time elapsed t —over  has exceeded the predetermined click starting period t 0 , the logic continues to step S 418  and onwards for generation of another click reaction force. 
     Determining, at step S 415 , that the content of the click frequency counter COUNT is equal to or greater than 1 (0) means that a click reaction force has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic moves to step S 417 . At step S 417 , the controller  60  determines whether or not a click time T —on , which is a time elapsed from a moment immediately after occurrence of a click reaction force, has exceeded a predetermined click interval t 1 . If, at step S 417 , the click time T —on  has exceeded the predetermined click interval t 1  (T —on &gt;t 1 ), the logic moves to step S 418  and onwards for generation of another click reaction force. 
     At step S 418 , the controller  60  resets the click time T —on . At the next step S 419 , the controller  60  updates the click frequency counter COUNT. Subsequently, at step S 420 , the controller resets a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. 
     At step S 421 , the controller  60  determines a correction coefficient k 3 * based on the accelerator pedal position Sp. The fully drawn line in  FIG. 23  illustrates an exemplary varying of the correction coefficient k 3 * with different values of the accelerator pedal position Sp. As indicated by the fully drawn line in  FIG. 23 , the controller  60  sets a value 0 (zero) as the correction coefficient k 3 * when the accelerator pedal position Sp is less than a predetermined value Sp 1 , for preventing generation of a click reaction force Fc. When the accelerator pedal position Sp is equal to or greater than the predetermined value Sp 1 , the controller  60  sets a value 1 as the correction coefficient k 3 *, for permitting occurrence of a click reaction force Fc. 
     At the next step S 422 , using the correction coefficient k 3 *, the controller  60  calculates a magnitude of click reaction force Fc, which is expressed as:
 
 Fc=k 3 *·Fcr   (Eq. 8)
 
where: Fcr represents a standard value of the magnitude of a click reaction force Fc.
 
     If, at step S 417 , the click time T —on  is equal to or less than the click interval t 1 , the logic moves to step S 423 . At step S 423 , the controller  60  carries out updating by increasing the click time T —on  by an unit amount of time, and the logic moves to step S 424 . The logic also moves to step S 424  from step S 416  if, at step S 416 , the time elapsed T —over  is equal to or less than the predetermined click starting period t 0 . At step S 424 , the controller  60  sets the magnitude of click reaction force Fc equal to 0 (Fc=0) to prevent occurrence of such click reaction force Fc. 
     After setting the click reaction force Fc at step S 422  or S 424 , the logic proceeds to step S 441 . At step S 441 , the controller  60  calculates an accelerator pedal reaction force instruction value FA using the reaction force increment dF calculated at step S 404  and the click reaction force Fc determined at step S 422  or S 424 . The accelerator pedal reaction force instruction value FA is expressed by equation 6. 
     At the next step S 442 , the controller  60  provides, as an output, the reaction force instruction value FA to an accelerator pedal reaction force control unit  80 . In response to the reaction force instruction value FA provided by the controller  60 , the accelerator pedal reaction force control unit  80  performs regulation of reaction force for the accelerator pedal  82 , applying different forms of tactile stimulus to the vehicle driver for keeping the driver informed of the risk perceived RP around the vehicle and how the vehicle deviates from the vehicle speed standard Vt. 
     If, at step S 413 , the vehicle speed Vf is less than the vehicle speed standard Vt, the logic proceeds to step S 425 . At step S 425 , the controller  60  calculates a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. At step S 426 , the controller  60  determines a reset period t 2  based on the time elapsed T —over . The fully drawn line in  FIG. 24A  illustrates an exemplary varying of the reset period t 2  with different values of time elapsed T —over . 
     In  FIG. 24A , the reference characters t 2   i  and t 2   im  indicate an initial value and a minimum value of the reset period t 2 . As indicated by the fully drawn line in  FIG. 24A , the controller  60  determines the reset period t 2  by shortening the reset period t 2  from the initial value of t 2   i  toward the minimum value t 2   m  as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined value T —over   6 . If the time elapsed T —over  is equal to or greater than the predetermined value T —over   6 , the controller  60  sets the minimum value t 2   m  as the reset period t 2 . 
     Referring to  FIG. 24B , as the content of the click frequency counter COUNT increases with an increase in the time elapsed T —over , the content of the click frequency counter COUNT may be used to retrieve the relationship as indicated by the fully drawn in  FIG. 24B  to determine an appropriate value of the reset period t 2 . The relationship illustrated in  FIG. 24B  is substantially the same as the illustrated relationship in  FIG. 24A . 
     At the next step S 427 , the controller  60  determines whether or not the time elapsed T —under  has exceeded the reset period t 2  that was determined at step S 426 . If, at step S 427 , the time elapsed T —under  has exceeded the reset period t 2 , the logic proceeds to step S 428 . At step S 428 , the controller  60  resets the click frequency counter COUNT. At the next step S 429 , the controller  60  resets the time elapsed T —over . At the next step S 430 , the controller  60  resets the click time T —on . Then, the logic moves to step S 424 . At step S 424 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
     If, at step S 427 , the time elapsed T —under  is less than the reset period t 2 , the logic goes directly to step S 228  without resetting the click frequency counter COUNT (step S 428 ), the time elapsed T —over  (step S 429 ), and the click time T —on  (step S 430 ). At step S 424 , the controller  60  sets the magnitude of a click reaction force Fc equal to 0 (Fc=0). 
     If the time elapsed T —under  or the click frequency counter COUNT counted up until the vehicle speed Vf dropped below the vehicle speed standard Vt becomes longer, the reset period t 2 , during which the time elapsed T —under  and the click frequency counter COUNT stop counting up, becomes shorter. Thus, if the vehicle speed Vf becomes equal to or greater than the vehicle speed standard Vt again before expiration of the reset period t 2 , the rule available until the vehicle speed Vf dropped below the vehicle speed standard Vt will govern appearance of click reaction force Fc. 
     First Modification of the Fourth Exemplary Implementation 
     The controller  60  may determine the click interval t 1  based on the time elapsed T —over  using the illustrated relationship in  FIG. 25A . In  FIG. 25A , the fully drawn line illustrates an exemplary varying of the click interval t 1  with different values of the time elapsed T —over . 
     In  FIG. 25A , the reference characters t 1   i  and t 1   mx  indicate an initial value and a maximum value of the click interval t 1 , respectively. As indicated by the fully drawn line in  FIG. 25A , the controller  60  determines the click interval t 1  by elongating the click interval t 1  from the initial value t 1   i  toward the maximum value t 1   mx  as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined time value T —over   7 . If the time elapsed T —over  is equal to or greater than the predetermined time value T —over   7 , the controller  60  sets the maximum value t 1   mx  as the click interval t 1 . Thus, when the time elapsed T —over  is long, elongating the click interval t 1  reduces the number of click reaction forces Fc, preventing the driver from feeling objection to receiving the click reaction forces Fc. Setting the click interval t 1  in this manner based on the time elapsed T —over  provides more effective setting of the frequency of click reaction forces Fc if the reset period t 2  is set using the illustrated relationship in  FIG. 24A  or  24 B. 
     Referring to  FIG. 25B , as the content of the click frequency counter COUNT increases with an increase in the time elapsed T —over , the content of the click frequency counter COUNT may be used to retrieve the relationship as indicated by the fully drawn in  FIG. 25B  to determine an appropriate value of the click interval t 1 . The relationship illustrated in  FIG. 25B  is substantially the same as the illustrated relationship in  FIG. 25A . 
     Second Modification of the Fourth Exemplary Implementation 
     At step S 422 , using a correction coefficient k 1 * in addition to the correction coefficient k 3 *, the controller  60  may calculate a magnitude of click reaction force Fc, which is expressed as:
 
 Fc=k 1 *·k 3*· Fcr   (Eq. 9)
 
     In this modification, the controller  60  may determine the correction coefficient k 1 * based on the time elapsed T —over  using the illustrated relationship in  FIG. 26A . In  FIG. 26A , the fully drawn line illustrates an exemplary varying of the correction coefficient K 1 * with different values of the time elapsed T —over . 
     In  FIG. 26A , the reference character k 1 *mx indicates a minimum value of the correction coefficient k 1 *mx. As indicated by the fully drawn line in  FIG. 26A , the controller  60  determines the correction coefficient k 1 * by decreasing the correction coefficient k 1 * from an initial value of 1 (one) toward the minimum value k 1 *m as the time elapsed T —over  increases if the time elapsed T —over  is less than a predetermined time value T —over   8 . If the time elapsed T —over  is equal to or greater than the predetermined time value T —over   8 , the controller  60  sets the minimum value k 1 *m as the correction coefficient k 1 *. Using the correction coefficient k 1 *, the magnitude of a click reaction force Fc becomes smaller as the time elapsed T —over  becomes longer, thus preventing the driver from feeling an objection to receiving the click reaction force Fc. 
     The controller  60  may determine the correction coefficient k 1 * based on the content of the counter COUNT instead of the time elapsed T —over  using the illustrated relationship in  FIG. 26B . In  FIG. 26B , the fully drawn line illustrates an exemplary varying of the correction coefficient k 1 * with different values of the content of the counter COUNT. The illustrated relationship in  FIG. 26B  is substantially the same as the illustrated relationship in  FIG. 26A . 
     Third Modification of the Fourth Exemplary Implementation 
     At step S 421 , the controller  60  may determine the correction coefficient k 3 * based on the accelerator pedal position Sp using the illustrated relationship in  FIG. 27 . The fully drawn line in  FIG. 27  illustrates an exemplary varying of the correction coefficient k 3 * with different values of the accelerator pedal position Sp. As indicated by the fully drawn line in  FIG. 27 , the correction coefficient k 3 * gradually increases from 0 (zero) to 1 as the accelerator pedal position Sp increases when the accelerator pedal position Sp is less than a predetermined value Sp 2 . When the accelerator pedal position Sp is equal to or greater than the predetermined value Sp 2 , the correction coefficient k 3 * is fixed to 1. 
     In addition to the effects provided by the first, second, and third exemplary implementations, the above-mentioned fourth exemplary implementation provides further effects as follows: 
     (1) The vehicle driver is kept informed of how much the vehicle speed Vf exceeds the vehicle speed standard Vt without feeling an objection to receiving click reaction forces Fc because the controller  60  alters the click interval and the magnitude of a click reaction force based on the results of calculation of the excess, in state or in amount, by which the vehicle speed Vf exceeds the vehicle speed standard Vt. Further, the frequency of occurrence of click reaction forces Fc becomes lower as the time elapsed T —over  becomes longer, keeping the driver informed of the excess by which the vehicle speed Vf exceeds the vehicle speed standard Vt without feeling an objection to receiving the click reaction forces Fc. 
     (2) The driver is prevented from feeling an objection to receiving click reaction forces Fc because the controller  60  alters the magnitude of click reaction forces Fc based on the accelerator pedal position Sp. Concretely, the magnitude of click reaction forces Fc is held 0 (zero) or small when the accelerator pedal position Sp is small. 
     Fifth Exemplary Implementation of the Invention 
     With continuing reference to  FIG. 2 , the illustrated vehicle may be installed with a fifth exemplary implementation of a driver assisting system  2  according to the present invention. Referring to  FIGS. 28 to 31 ,  FIG. 28  is a block diagram showing elements constituting the driver assisting system  2 , and  FIGS. 29 to 31  illustrate a driver&#39;s seat for the vehicle illustrated in  FIG. 2 . 
     The fifth exemplary implementation is substantially the same as the first exemplary implementation in that there are a laser radar  10 , a front camera  20 , a vehicle speed sensor  30 , and a vehicle speed database  40 . However, instead of the accelerator pedal reaction force control unit  80 , the driver assisting system  2  uses a seat pressure control unit  90  and a seat vibration control unit  100 . The seat pressure control unit  90  performs regulation of pressure within an air bag embedded in the driver&#39;s seat. The seat vibration control unit  100  controls vibration generated at the driver&#39;s seat. 
     In the fifth exemplary implementation, the hardness and elevation (or height) of the driver&#39;s seat are adjustable by regulating pressure within the air bag in dependence on a risk perceived RP by the vehicle driver from environment around the vehicle. In the fifth exemplary implementation, vibration of the driver&#39;s seat keeps the vehicle driver informed of how the vehicle speed Vf exceeds the vehicle speed standard Vt. Further, altering interval and/or magnitude of vibration provides the driver with information on the relationship with the vehicle speed standard Vt. 
     Referring to  FIGS. 29-30 , one example of the driver&#39;s seat is generally indicated by the reference numeral  110 .  FIG. 29  is a perspective view of the driver&#39;s seat  110 , which is controlled by the seat pressure control unit  90  and the seat vibration control unit  100 . The driver&#39;s seat  110  includes a headrest  111 , a seat back  112 , and a seat cushion  113 .  FIG. 30  is a cross section of the seat cushion  113  taken through the line  30 - 30  in  FIG. 29  in an uninflated state.  FIG. 31  is the same cross section of the seat cushion  113  in an inflated state. At a front edge portion, the cushion portion  113  has embedded therein an air bag  91  and a plurality of vibrators  101 . The air bag  91  is under the control of the seat pressure control unit  90 . The vibrators  101  are under the control of the seat vibration control unit  100 . As shown in  FIG. 31 , when the air bag  91  is inflated, the surface of the seat cushion  113  is expanded and stretched, allowing the vehicle driver to feel a change in pressure within the air bag  91  through the femoral region. 
     Referring to  FIG. 32  and  FIGS. 33(   a ) to  33 ( c ), the following provides description on operation of the fifth exemplary implementation of driver assisting system  2 .  FIG. 32  illustrates a flow chart of a driver assisting control program executed by a controller  61  used in the fifth exemplary implementation of the present invention.  FIGS. 33(   a ) to  33 ( c ) are time charts illustrating time variations of vehicle speed Vf, seat vibration, and seat pressure. The execution of the program illustrated in  FIG. 6  is repeated at regular intervals, for example, every 50 msec. 
     In  FIG. 32 , at step S 501 , the controller  61  performs a reading operation of output signals from the accelerator pedal stroke sensor  83 , vehicle speed sensor  30 , laser radar  10 , and front camera  20  to obtain information on the running environment around the vehicle. At step S 502 , the controller  61  performs analysis of the obtained information to recognize the state of the environment around the vehicle by calculating a distance D to a preceding vehicle and a relative speed Vr to the preceding vehicle. The relative speed Vr is given by subtracting a vehicle speed Vf of the vehicle operated by a driver from a vehicle speed Vp of the preceding vehicle (Vr=Vp−Vf). 
     After step S 502 , the logic moves to steps S 503  and S 504 . At step S 503 , the controller  61  calculates a risk perceived RP. At step S 204 , the controller  61  calculates a seat pressure P based on the risk perceived RP. The seat pressure P is a pressure to build up within the air bag  91  embedded in the driver&#39;s seat  110 . In the fifth exemplary implementation, the seat pressure P is proportional to the risk perceived RP. 
     Because steps S 511 , S 512 , S 513 , and S 514  correspond to steps S 211 , S 212 , S 213 , and S 214  of the flow chart illustrated in  FIG. 6 , further description on what the controller  61  performs at steps S 511 , S 512 , S 513 , and S 514  is hereby omitted for brevity. At step S 515 , the controller  61  determines whether or not the content of a vibration frequency counter COUNT_V is equal to 0 (zero). 
     Determining, at step S 515 , that the content of the vibration frequency counter COUNT_V is zero (COUNT_V=0) means that no driver&#39;s seat vibration has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic proceeds to step S 516 . At step S 516 , the controller  61  determines a vibration starting period t 0 . In this fifth exemplary implementation, the controller  61  determines the vibration starting period t 0  based on an excess ΔV by which the vehicle speed Vf exceeds the vehicle speed standard Vt (ΔV=Vf−Vt) using the illustrated relationship in  FIG. 9 . 
     At step S 517 , the controller  61  determines whether or not a time elapsed T —over , which is a time elapsed from a moment immediately after the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt, has exceeded the vibration starting period t 0 . If, at step S 517 , the time elapsed T —over  has exceeded the vibration starting period t 0 , the logic goes to step S 520 . 
     Determining, at step S 515 , that the content of the vibration frequency counter COUNT_V is equal to or greater than 1 (COUNT_V≠0) means that driver&#39;s seat vibration has occurred since the vehicle speed Vf became equal to or greater than the vehicle speed standard Vt. If this is the case, the logic proceeds to step S 518 . At step S 518 , the controller  61  determines a vibration interval t 1  between the adjacent two vibrations. In this fifth exemplary implementation, the controller  61  determines the vibration interval t 1  based on an excess ΔV, by which the vehicle speed Vf exceeds the vehicle speed standard Vt using the illustrated relationship in  FIG. 10 . In  FIG. 10 , the fully drawn line illustrates an exemplary varying of the vibration interval t 1  with different values of the excess ΔV. 
     At step S 519 , the controller  61  determines whether or not a vibration time T —on , which is a time elapsed from a moment immediately after occurrence of vibration, has exceeded the vibration interval t 1  determined at step S 518 . If, at step S 519 , the vibration time T —on  has exceeded the vibration interval t 1  (T —on &gt;t 1 ), the logic goes to step S 520  and onwards for generating vibration of the driver&#39;s seat  110 . 
     At step S 520 , the controller  61  resets the vibration time T —on . At the next step S 521 , the controller  61  updates the vibration frequency counter COUNT_V. Subsequently, at step S 522 , the controller resets a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. 
     At step S 523 , the controller  61  determines a first correction coefficient k 1  based on the excess ΔV using the illustrated relationship in  FIG. 11 . The first correction coefficient k 1  is used to calculate amplitude Fv of vibration. 
     At step S 524 , the controller  61  determines a second correction coefficient k 2  based on the risk perceived RP calculated at step S 503  using the illustrated relationship in  FIG. 12 . The second correction coefficient k 2  is used to calculate the amplitude Fv of vibration. 
     At step S 525 , using the first and second correction coefficients k 1  and k 2 , the controller  61  calculates the amplitude Fv, which is expressed as:
 
 Fv=k 1 ·k 2 ·Fvr   (Eq. 10)
 
where: Fvr represents a standard value of the amplitude of vibration.
 
     If, at step S 519 , the vibration time T —on  is equal to or less than the vibration interval t 1 , the logic proceeds to step S 526 . At step S 526 , the controller  61  carries out updating by increasing the vibration time T —on  by an unit amount of time, and the logic goes to step S 527 . The logic also proceeds to step S 527  from step S 517  if, at step S 517 , the time elapsed t —over  is equal to or less than the vibration starting period t 0 . At step S 527 , the controller  61  sets the amplitude Fv equal to 0 (Fv=0) to prevent occurrence of vibration. 
     After setting the amplitude Fv at step S 525  or S 527 , the logic moves to step S 541 . At step S 541 , the controller  61  provides, as an output, the seat pressure P calculated at step S 504  to the seat pressure control unit  90 . At the next step S 542 , the controller  61  provides, as an output, the amplitude Fv calculated at step S 525  or  527  to the seat vibration control unit  100 . In response to the output provided by the controller  61 , the seat pressure control unit  90  inflates the air bag  91  to accomplish the seat pressure P. In response to the output provided by the controller  61 , the seat vibration control unit  100  causes the vibrators  101  to accomplish the amplitude Fv. 
     If, at step S 513 , the vehicle speed Vf is less than the vehicle speed standard Vt, the logic continues to step S 528 . At step S 528 , the controller  61  calculates a time elapsed T —under  from a moment immediately after the vehicle speed Vf became less than the vehicle speed standard Vt. At step S 529 , the controller  61  determines a reset period t 2  based on the time elapsed t —over  using the illustrated relationship in  FIG. 14A  or  14 B. 
     At the next step S 530 , the controller  61  determines whether or not the time elapsed T —under  has exceeded the reset period t 2  that was determined at step S 529 . If, at step S 530 , the time elapsed T —under  has exceeded the reset period t 2 , the logic moves to step S 531 . At step S 531 , the controller  61  resets the vibration frequency counter COUNT_V. At the next step S 532 , the controller  61  resets the time elapsed T —over . At the next step S 533 , the controller  61  resets the vibration time T —on . Then, the logic proceeds to step S 527 . At step S 527 , the controller  61  sets the amplitude Fv equal to 0 (Fv=0). 
     If, at step S 530 , the time elapsed T —under  is less than the reset period t 2 , the logic proceeds directly to step S 527  without resetting the click frequency counter COUNT_V (step S 531 ), the time elapsed T —over  (step S 532 ), and the vibration time T —on  (step S 533 ). At step S 527 , the controller  61  sets the amplitude Fv equal to 0 (Fv=0). 
       FIGS. 33(   a )- 33 ( c ) illustrate how the fifth exemplary implementation of the driver assisting system  2  operates. As illustrated in  FIGS. 33(   a )- 33 ( c ), vibration occurs upon expiration of the vibration starting period t 0  that varies in response to the excess ΔV after the vehicle speed Vf became greater than or equal to the vehicle speed standard Vt. Subsequently, the vibration occurs at the vibration interval t 1  that varies in response to the excess ΔV when the vehicle speed Vf remains greater than or equal to the vehicle speed standard Vt. The amplitude Fv is determined in response to the excess ΔV and the risk perceived RP. Thus, the amplitude Fv of vibration increases in response to an increase in the excess ΔV and/or an increase in the risk perceived RP. 
     The driver&#39;s seat  110  provides varying seat pressure P with the calculated risk perceived RP and vibration indicative of information on the vehicle speed standard Vt. With this driver&#39;s seat  110 , the vehicle driver is kept informed of varying of the calculated risk perceived RP via continuous variations of the seat pressure P and also of the information on the vehicle speed standard Vt via interrupted vibration of the driver&#39;s seat  110 . Amplitude Fv of vibration may be altered to keep the driver informed of varying of the excess ΔV or varying of the magnitude of the calculated risk perceived RP. The frequency of vibration becomes higher when the vehicle speed Vf continues to stay longer above the vehicle speed standard Vt, amplifying effectiveness in prompting the driver to pay attention to the increased potential risk. 
     The amount of potential risk, which was forwarded to the driver, is immediately forwarded to the driver when the vehicle speed Vf exceeds the vehicle speed standard Vt again unless time during which the vehicle speed Vf stays temporarily below the vehicle speed standard Vt exceeds the reset period t 2  because the amplitude Fv and interval of vibration existing before the vehicle speed Vt dropped below the vehicle speed standard Vt are held for the reset period t 2 . Thus, forwarding the amount of potential risk to the driver resumes immediately after the vehicle speed Vf has exceeded the vehicle speed standard Vt again unless the time during which the vehicle speed Vf stayed below the preset period exceeds the reset period t 2 . 
     In the previous description, the relationship illustrated in  FIG. 10  was used for the controller  61  to calculate the interval t 1  of vibration, and the relationship illustrated in  FIG. 11  was used for the controller  61  to calculate the correction coefficient k 1 . Instead of using the relationship illustrated in  FIG. 10 , the controller  61  may use the relationship illustrated in  FIG. 15A  or  15 B to calculate the interval t 1 . Instead of using the relationship illustrated in  FIG. 11 , the controller  61  may use the relationship illustrated in  FIGS. 16A and 16B . 
     In the previous description, the driver&#39;s seat  110  (see  FIGS. 29 to 31 ) was equipped with the air bag  91  and vibrators  101  to generate pressure and vibration applied to the driver. Instead of the air bag  91  and vibrators  101 , a lifter mechanism for adjusting the height of a front edge portion of a cushion  113  of a driver&#39;s seat may be used. The lifter mechanism may be used to lift the front edge portion of the cushion  113  for the driver&#39;s seat to generate increased seat pressure P. It may be used to repeat vertical up and down movement of the front edge portion of the cushion  113  once or more for the driver&#39;s seat to generate vibration. This fifth exemplary implementation may be modified to cause the driver&#39;s seat to generate the seat pressure and vibration under the same conditions as those used in the third or fourth exemplary implementation. 
     In addition to the effects provided by the first to fourth exemplary implementations, the above-mentioned fifth exemplary implementation provides further effects as follows: 
     The controller  61  translates a change in the risk perceived RP from an obstacle into a continuous change in seat pressure of the driver&#39;s seat  110 , that is, a change in height and hardness, and information relating to the vehicle speed standard Vt into vibration generated by the driver&#39;s seat  110  to be forwarded to the driver. Thus, the driver is kept informed of a plurality of risks around the vehicle by perceiving different forms of tactile stimulus from the driver&#39;s seat. Altering the interval or amplitude of vibration generated by the driver&#39;s seat  110  in response to the amount or level of excess by which the vehicle speed Vf exceeds the vehicle speed standard Vt makes it possible to forward the amount or level of the excess to the driver. 
     Sixth Exemplary Implementation of the Invention 
     With continuing reference to  FIG. 2 , the illustrated vehicle may be installed with a sixth exemplary implementation of a driver&#39;s assisting system according to the present invention. This sixth exemplary implementation is substantially the same as any one of the previously described second to fourth exemplary implementation. However, the sixth exemplary implementation is different from the previously described exemplary implementation in that, instead of the accelerator pedal reaction force control unit  80  and servo motor  81 , a steering system is equipped with a driver mechanism to move a steering wheel back and forth. 
     In the sixth exemplary implementation, the driver mechanism moves the steering wheel toward or away from a driver in response to a change in risk perceived RP from the environments around the vehicle. The driver mechanism moves the steering wheel back and forth once within a short stroke or repeats such short stroke back-and-forth movement when the vehicle speed Vf exceeds the vehicle speed standard Vt. 
       FIGS. 34 and 35  illustrate a portion of the steering system equipped with the driver mechanism. As usual, the steering system includes a steering wheel  120  and a steering column  121 . The steering system is equipped with a motor unit  122  of the driver mechanism. As the risk perceived RP from environments around the vehicle grows bigger, the motor unit  122  extends the steering column  121  to move the steering wheel  120  toward the driver. When the vehicle speed Vf exceeds the vehicle speed standard Vt, the motor unit  122  carries out a short stroke back-and-forth movement of the steering wheel  120  or repeats the short stroke back-and-forth movement. Altering the interval and/or stroke of the short stroke back-and-forth movement makes it possible to forward information relative to the vehicle speed standard Vt to the driver. This sixth exemplary implementation may be modified to cause the back and forth movement of the steering wheel  120  under the same conditions as those used in the second or third or fourth exemplary implementations. 
     In addition to the effects provided by the first to fifth exemplary implementations, the above-mentioned fifth exemplary implementation provides further effects as follows: 
     As a change in risk perceived RP from an obstacle is translated into extension and retraction of the steering column  121 , the risk perceived RP is forwarded to the driver by a continuous change in position of the steering wheel  120 . Information that the vehicle speed Vf exceeds the vehicle speed standard Vt is forwarded to the driver by a short stroke back-and-forth movement of the steering column  121 . Altering the interval or stroke of the short stroke back-and-forth movement of the steering column  121  in response to the amount or level of excess by which the vehicle speed Vf exceeds the vehicle speed standard Vt makes it possible to forward the amount or level of the excess to the driver. 
     In the preceding description of the first to sixth exemplary implementations, a time to collision TTC and a time headway THW were used to give a risk perceived RP using the equation Eq. 3. This is just an example of calculation to give the risk perceived RP. Another example uses the reciprocal of time to collision TTC to give the risk perceived RP. In the preceding description, the reaction force increment dF was proportional to the risk perceived RP. This is just an example of giving the reaction force increment dF. Another example is setting the reaction force increment dF such that it increases exponentially as the risk perceived RP grows bigger. 
     In the preceding description, the second exemplary implementation was operable on the vehicle speed standard Vt, click starting period t 0 , click interval t 1 , click reaction force Fc, and reset period t 2 , all of which were variable. The second exemplary implementation, however, may be operable as long as at least one of the vehicle speed standard Vt, click starting period t 0 , click interval t 1 , click reaction force Fc, and reset period t 2  is variable. Referring back to Eq. 7, all of three correction coefficients k 1 , k 2 , and k 3  were calculated to give the magnitude of a click reaction force Fc. The second exemplary implementation, however, may be operable as long as at least one of these correction coefficients is calculated. 
     In the first to sixth embodiments, the laser radar  10 , front camera  20 , vehicle speed sensor  30 , vehicle speed information database  40 , and controller  60  or  61  cooperate with each other to input or obtain a running environment around the vehicle. The laser radar  10 , front camera  20 , and vehicle speed sensor  30  cooperate with each other to detect an obstacle. The vehicle speed information database  40  and controller  60  or  61  cooperate with each other to calculate a vehicle speed standard Vt. The controller  60  or  61  is operative to calculate a risk perceived RP and also to regulate tactile stimulus to be forwarded to a driver. The accelerator pedal reaction force control unit  80 , seat pressure control unit  90 , vibration control unit  100 , and motor unit  122  are used to forward the tactile stimulus to the driver. The accelerator pedal reaction force control unit  80  is operative to generate accelerator pedal reaction force. The seat pressure control unit  90  and seat vibration control unit  100  are operative to control the seat. The motor unit  122  is operative to control extension and retraction of the steering column. The hardware is not limited to them listed above in constituting the present invention. For example, the laser radar  10  might be replaced by a different type of millimeter wave radar to detect the obstacle. 
     In certain embodiments of the invention, the driver contact surfaces are those surfaces within the vehicle which the driver is expected to have substantially continuous contact to provide the driver with the best haptic channel information. For example, such a contact includes the driver&#39;s seat, the steering wheel, the accelerator pedal, etc. The invention is not limited to such surfaces, however, and can also be employed with other driver controlled input devices or surfaces, such as an armrest or turn signal lever, for example. Also, although the invention has been primarily described with the example of tactile stimulus, the invention is not limited to tactile stimuli, but other types of stimulus can be used to stimulate the driver. 
     Although the invention has been shown and described with respect to certain exemplary implementations, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon reading and understanding of the specification. The present invention includes all such equivalent alterations and modifications, and is limited only by the scope of the claims. 
     The present application claims the priority based on Japanese Patent Application No. 2003-391124, filed Nov. 20, 2003, the disclosure of which is hereby incorporated by reference in its entirety.