Patent Publication Number: US-2005131614-A1

Title: Vehicle driving system and method

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application is based on and incorporates herein by reference Japanese Patent Applications No. 2003-415335 filed Dec. 12, 2003 and No. 2004-307360 filed Oct. 21, 2004.  
     FIELD OF THE INVENTION  
      The present invention relates to a vehicle driving system and method that performs drive control of a vehicle according to a target travel speed.  
     BACKGROUND OF THE INVENTION  
      Conventionally, a system that performs a drive control such as a cruise control of a vehicle according to a set target travel speed is known. Such a system operates in two operation modes: a set mode of newly setting a target travel speed and starting drive control of a vehicle according to the set target travel speed; and a resume mode of reading the target travel speed having been set in the set mode and restarting the drive control of a vehicle according to the read target travel speed.  
      Once a driver has set the target travel speed in the set mode, the driver can restart the drive control with the already set target travel speed by using the resume mode next time and later rounds without newly setting a target travel speed again.  
      However, with the conventional system, the drive control may be restarted at an inappropriate target travel speed. For example, suppose that a driver sets a high target travel speed and makes the system control driving of the vehicle while the automobile is traveling on a highway at a high speed. When the driver restarts the drive control by the resume mode due to the driver&#39;s misunderstanding or misapprehension after the vehicle left the highway to be on an ordinary road, the drive control of the vehicle will be restarted with the high target travel speed having been set during highway running.  
      Moreover, for example, when the driver starts or restarts the drive control in the set mode or in the resume mode without noticing a change in a speed limit of a road on which the vehicle is traveling, the drive control is not performed with a target travel speed matching the speed limit of the road.  
     SUMMARY OF THE INVENTION  
      This invention has an object to provide a vehicle driving system and method capable of starting or restarting the drive control of a vehicle with an appropriate target travel speed.  
      A vehicle driving system and method of this invention acquires a travel speed recommended in a road on which the vehicle is traveling. When the drive control of the vehicle is restarted in the resume mode, the vehicle driving system compares the target speed having been set in the set mode with a recommended travel speed, and sets the smaller speed as a target speed used for performing the drive control of the vehicle. Accordingly, for example, even in the case where the drive control of a vehicle was performed using the set mode during highway running and subsequently the drive control in the resume mode is restarted during ordinary road running due to the driver&#39;s misunderstanding or misapprehension, and when the already set target speed is larger than a recommended travel speed in the road, the drive control of the vehicle is restarted with the recommended travel speed. That is, the drive control of a vehicle can be restarted with an appropriate target speed.  
      Moreover, when the drive control of a vehicle is started or restarted either in the set mode or in the resume mode, the vehicle driving system determines a difference between a road attribute of the road on which the vehicle was traveling at the time of performing the drive control of the vehicle last time and a road attribute of the road on which the vehicle is traveling at the time of starting or restarting the drive control of the vehicle this time.  
      Further, when the road attributes are different, the vehicle driving system compares the target speed being set this time or the target speed having been set in the set mode with the recommended travel speed included in the road attributes, and sets the smaller speed as the target speed used for performing the drive control of the vehicle. Thereby, even in the case where the driver starts or restarts the drive control of a vehicle in the set mode or in the resume mode, being not conscious of a change in the speed limit of the road on which the vehicle is traveling, and when the target speed being set this time or the target speed having been set in the set mode is larger than the recommended travel speed, the drive control of the vehicle is started or restarted with the recommended travel speed. That is, the drive control of the vehicle can be started or restarted with an appropriate target speed.  
      Still Further, even if, for example, the drive control of a vehicle was performed using the set mode during highway running and subsequently the driver directs the vehicle driving system to start or restart the drive control in the resume mode during ordinary road running due to the driver&#39;s misunderstanding or misapprehension, the vehicle driving system prohibits the restart of the drive control of the vehicle because the road attributes changed. Thereby, it is possible to prevent the drive control of the vehicle from being restarted according to an inappropriate target speed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings:  
       FIG. 1  is a block diagram showing a vehicle driving system in a first embodiment of this invention;  
       FIG. 2  is a schematic view showing a button and a lever provided in an operation switch in the first embodiment;  
       FIG. 3  is a flowchart of a process in which the vehicle driving system performs the drive control of a vehicle in the first embodiment;  
       FIG. 4  is a flowchart of a process in which the vehicle driving system determines the operation content of an operation switch in the first embodiment;  
       FIG. 5  is a flowchart of a process in which the vehicle driving system sets various settings related to the drive control of the vehicle in the first embodiment;  
       FIG. 6  is a flowchart of a process in which the vehicle driving system adjusts the target travel speed in the first embodiment;  
       FIG. 7  is a flowchart of a process in which the vehicle driving system sets various settings related to the drive control of the vehicle in a second embodiment of this invention;  
       FIG. 8  is a flowchart of a process in which the vehicle driving system sets various settings related to the drive control of the vehicle in a third embodiment of this invention;  
       FIG. 9  is a flowchart of a process in which the vehicle driving system sets various settings related to the drive control of the vehicle in a fourth embodiment of this invention;  
       FIG. 10  is a flowchart of a process in which the vehicle driving system sets various settings related to the drive control of the vehicle in a fifth embodiment of this invention; and  
       FIG. 11  is a flowchart of a process in which the vehicle driving system of sets various settings related to the drive control of the vehicle in a sixth embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     First Embodiment  
      Referring first to  FIG. 1 , a vehicle driving system  1  is connected with a radar device  2  for detecting other vehicle traveling in front of the vehicle. Moreover, through an in-vehicle LAN  3  provided in the vehicle, the vehicle driving system  1  is connected with an engine control system  4  for controlling a engine of the vehicle, a vehicle operation detecting device  5  for detecting an operating status of the vehicle, a car navigation apparatus  6  for performing various navigation operations, and a meter control device  7  for controlling the operation of a meter  71 . Communication between the vehicle driving system  1  and each apparatus/device is performed according to a CAN communication protocol.  
      The radar device  2  is made up of, for example, a millimeter wave transceiver, transmits a millimeter wave of a predetermined frequency ahead of the vehicle, and receives a reflected wave reflected from the other vehicle that is traveling in front of the vehicle and irradiated by the millimeter wave. Then, based on a phase difference between the transmitted millimeter wave and the received reflected wave and an attenuation level, a relative distance and relative direction or bearing to the other vehicle traveling in front of the vehicle are measured. A relative distance and relative direction to the other vehicle traveling in front of the vehicle may be measured using an ultrasonic wave or a laser.  
      A speed sensor  41  is for detecting rotation of a wheel, and calculates the travel speed from the number of revolution of a wheel of the vehicle. A travel speed of the vehicle may be calculated based on an output of an acceleration sensor for detecting acceleration of the vehicle or an axle sensor for detecting rotation of the axle of the vehicle.  
      The engine control system  4  is equipped with a CAN communication circuit and controls the engine of the vehicle by adjusting the opening of a throttle valve and shifting a transmission based on a position signal from an accelerator pedal, an engine speed from the engine rotation sensor, and the travel speed of the vehicle. Moreover, the engine control system  4  transmits a travel speed of the vehicle calculated by the speed sensor  41  and the engine speed from the engine rotation sensor to the vehicle driving system  1 .  
      Once the engine control system  4  has acquired the drive-control-start signal from the vehicle driving system  1 , the engine control system  4  performs engine control of the vehicle according to a control signal transmitted from the vehicle driving system  1  after that. When having acquired a drive-control-termination signal from the vehicle driving system  1 , the engine control system  4  will perform engine control of the vehicle again based on the position signal of the accelerator pedal etc.  
      A steering sensor  51  is equipped with, for example, a plurality of mechanical gears, and measures the steering angle of a steering wheel of the vehicle based on the number of revolution of the gears. The steering angle of the steering may be measured using an infrared ray or a laser.  
      A yaw rate sensor  52  is equipped with, for example, a gyro sensor and detects the yaw angular velocity (rotation angular velocity) of the vehicle based on an output of the gyro sensor. The yaw angular velocity may be detected based on an output from an earth magnetism sensor.  
      A brake pedal switch  53  consists of, for example, a switching sensor for detecting a contact, detects whether a brake pedal of the vehicle is depressed, and outputs a brake status signal. Detection of whether the brake pedal of the vehicle is depressed may be done by using a non-contact type sensor that uses a laser, etc.  
      A vehicle operation detecting device  5  is equipped with a CAN communication circuit and transmits the steering angle of the steering measured by the steering sensor  51 , the yaw angular velocity detected by the yaw rate sensor  52 , and the brake status signal outputted from the brake pedal switch  53  to the vehicle driving system  1 . Other signals than these signals, such as traveling acceleration of the vehicle, may be transmitted to the vehicle driving system  1 .  
      A GPS antenna  61  is a small vehicle-mounted antenna for receiving a GPS signal for location measurement transmitted from a GPS Satellite that is an artificial satellite.  
      A map data memory  62  is made up of, for example, a hard disk drive, stores map information including road information, building information, address information of zones, postal code number information, etc. as the map data, and also stores image data for map image display. Furthermore, the map data memory  62  stores speed limits of roads in urban areas and the travel speeds that are recommended in performing the drive control of the vehicle on the respective roads (recommended travel speeds). These pieces of data may be stored in CD-ROM, DVD-ROM, etc.  
      The car navigation apparatus  6  calculates the longitude and latitude of a point where the vehicle is traveling (present location of the vehicle) from the GPS signal received by the GPS antenna  61  and performs various navigation operations. Specifically, the car navigation apparatus  6  reads map image data from the map data memory  62 , generates a map image such that a mark indicating the vehicle is superimposed on the map data, and performs navigation display. Moreover, the car navigation apparatus  6  identifies a road on which the vehicle is traveling from the calculated location of the vehicle, reads the speed limit and recommended travel speed in the road from the map data memory  62 , and transmits them to the vehicle driving system  1 .  
      Moreover, when receiving the travel speed of the vehicle, the car navigation apparatus  6  also overwrites the recommended travel speed stored in the map data memory  62  with the travel speed of the vehicle as a recommended travel speed of the road on which the vehicle is traveling. The latter recommended travel speed is determined in advance based on the speed limit, and stored in the map data memory  62 .  
      The meter control device  7  acquires the vehicle speed signal indicating the travel speed of the vehicle and the engine speed signal indicating the rotational speed of the engine, and makes the meter  71  operate according to them, and thereby notifies the driver of the travel speed and the engine speed of the vehicle.  
      The vehicle driving system  1  has two operation modes, the set mode and the resume mode, and performs the drive control of the vehicle by one of the operation modes.  
      An operation switch  11 , as shown in  FIG. 2 , is a switch equipped with a lever  1 B added with a main button  1 A. When the main button  1 A is pressed, power ON and power OFF signals will be outputted alternately. Moreover, when the lever  1 B is bowed upward, an upward operation signal will be outputted; when it is bowed downward, a downward operation signal will be outputted. When it is bowed toward the driver, a pull operation signal is outputted. The outputs of the operation signals may be outputted by a remote controller equipped with a cursor key or the like.  
      An alarm  12  has, for example, a buzzer, and outputs an alarm tone from the buzzer. The alarm tone may be outputted from speakers of audio system etc. mounted on the vehicle.  
      A drive control ECU  13  performs the drive control of the vehicle by determining a travel speed that is a target in performing the drive control of the vehicle (hereinafter referred to as target travel speed) and the adjustment of the target travel speed during the drive control of the vehicle, etc., based on a drive control flag stored in internal memory of the ECU, an operation signal outputted from the operation switch  11 , and a relative distance and relative direction that are outputted from the radar device  2 .  
      Specifically, in the case where the drive control flag is kept reset (no cruise drive control), when the ECU  13  acquires the downward operation signal from the operation switch  11 , the drive control ECU  13  will determine that a set operation was done, and start the drive control of the vehicle in the set mode. That is, the drive control ECU  13  transmits the drive control start signal to the engine control system  4 , and receives the travel speed of the vehicle received from the engine control system  4 .  
      Next, the drive control ECU  13  transmits the received travel speed of the vehicle to the car navigation apparatus  6 , and makes the car navigation apparatus  6  overwrite the recommended travel speed of the road on which the vehicle is traveling that is stored in the map data memory  62  with the travel speed of the vehicle.  
      Furthermore, the drive control ECU  13  stores the received travel speed of the vehicle in the internal memory as a set travel speed, determines the received travel speed of the vehicle as the target travel speed, and generates control signals for adjusting opening of a throttle valve and for shifting the transmission so that the travel speed of the vehicle received from the engine control system  4  agrees with the target travel speed. Further, it transmits the generated control signal to the engine control system  4 , and starts the drive control of the vehicle.  
      In the case where the drive control flag is kept reset, when the upward operation signal is acquired from the operation switch  11 , the drive control ECU  13  determines that a resume operation was done, and restart the drive control of the vehicle in the resume mode. In this situation, the drive control ECU  13  transmits the drive control start signal to the engine control system  4 , reads the set travel speed from an internal memory, and receives a recommended travel speed from the car navigation apparatus  6 .  
      Next, the drive control ECU  13  compares the acquired set travel speed with the recommended travel speed. When the set travel speed is smaller than the recommended travel speed, it determines the set travel speed as the target travel speed. On the other hand, when the recommended travel speed is smaller than the set travel speed, the drive control ECU  13  reads a speed limit change flag from the internal memory.  
      It should be noted that in the above two cases (case 1: the set travel speed is smaller than the recommended travel speed; and case 2: the recommended travel speed is smaller than the set travel speed) are referred to, omitting an similar case just for simplicity, and that the equal case may be included in either of the two cases. Further, when the flag is kept set, the drive control ECU  13  determines the recommended travel speed as the target travel speed; when the flag is kept reset, it determines the set travel speed as the target travel speed.  
      The drive control ECU  13  generates the control signals for adjusting the opening of the throttle valve and for shifting the transmission so that the target travel speed determined by the above procedure agrees with the travel speed of the vehicle outputted from the engine control system  4 , transmits it to the engine control system  4 , and restarts the drive control of the vehicle.  
      On the other hand, in the case where the drive control flag is kept set (in cruise drive control), when the upward operation signal is acquired from the operation switch  11 , the drive control ECU  13  determines that an acceleration operation was done and will increase the target travel speed by a predetermined amount. In that case, when the downward operation signal is acquired from the operation switch  11 , the drive control ECU  13  determines that a deceleration (coasting) operation was done and will decrease the target travel speed by a predetermined amount.  
      Moreover, in the case where the drive control flag is kept set, when the drive control ECU  13  acquires the pull operation signal from the operation switch  11  or determines that the brake pedal was depressed from a brake status signal received from the vehicle operation detecting device  5 , the drive control ECU  13  transmits the drive-control-termination signal to the engine control system  4  and resets the drive control flag.  
      The drive control ECU  13  directs the alarm  12  to output the alarm tone in addition to the above operations related to the drive control of the vehicle. Specifically, when the drive control ECU  13  determines that the vehicle and the other vehicle will come closer than the predetermined distance from the relative distance to other vehicle traveling in front of the vehicle and the relative direction to them received from the radar device  2  and the steering angle of the steering and the yaw angular velocity received from the vehicle operation detecting device  5 , it directs the alarm  12  to output the alarm tone.  
      Furthermore, the drive control ECU  13  always monitors the speed limit of the road on which the vehicle is traveling that is transmitted by the car navigation apparatus  6 . When the above speed limit changes, the drive control ECU  13  will set the speed limit change flag stored in the internal memory. The flag is reset at the time of start and restart of the drive control of the vehicle.  
      In addition, the drive control ECU  13  generates the speed signal indicating the travel speed of the vehicle and the engine speed signal indicating the engine speed, and transmits them to the meter control device  7 . When the operation switch  11  outputs power ON and power OFF signals, the drive control ECU  13  will turn on and off the power source of the vehicle driving system  1 , respectively.  
      The drive control ECU  1  is programmed to execute a control process shown in FIGS.  3  to  6 . This process is executed for every predetermined time.  
      At step  301 , the drive control ECU  13  acquires the operation signal outputted from the operation switch  11 . At step  302 , the travel speed and the engine speed of the vehicle are received from the engine control system  4 . At step  303 , the steering angle of the steering, the yaw angular velocity, and the brake status signal are received from the vehicle operation detecting device  5 . At step  304 , the speed limit of the road on which the vehicle is traveling and the recommended travel speed are received from the car navigation apparatus  6 . At step  305 , the relative distance to the other vehicle traveling in front of the vehicle and the relative direction to it are received from the radar device  2 .  
      At step  306 , an operation content determining process of determining the operation content of an operation signal acquired at step  301  is performed. The details of the process in this step will be described later.  
      At step  307 , whether the drive control flag stored in the internal memory is kept set is determined. When the drive control flag is kept set, the processing proceeds to step  308 , where the control signals for adjusting the opening of the throttle valve and for shifting the transmission is generated so that the travel speed of the vehicle received at step  302  agrees with the target travel speed, and transmits them to the engine control system  4 . When the drive control flag is kept reset, the processing proceeds to step  309 .  
      At step  309 , whether the speed limit received at step  304  of the road on which the vehicle is driving changed is determined. When the speed limit of the road on which the vehicle is traveling changed, the processing proceeds to step  310 , where the speed limit change flag stored in the internal memory is kept set. When it is not so, the processing proceeds to step  311 .  
      At step  311 , when the drive control ECU  13  determines that the vehicle and the other vehicle will come closer than the predetermined distance determining from the steering angle of the steering and the yaw angular velocity received at step  303  and the relative distance to the other vehicle traveling in front of the vehicle and the relative direction to it received at step  305 , the drive control ECU  13  directs the alarm  12  to output the alarm tone.  
      At step  312 , the speed signal and the engine speed signal are generated from the travel speed of the vehicle and the engine speed received at step  302 , transmitted them to the meter control device  7 , and the process is completed.  
      Here, both when the operation signal is not outputted from the operation switch  11  and when the brake status signal indicating depressing of the brake pedal from the brake pedal switch  53  is not acquired, the operation content determination process at step  306  is skipped.  
      The processing at step  306  is shown in detail in  FIG. 4 . At step  401 , the drive control ECU  13  determines whether the brake pedal was depressed for vehicle braking from the brake status signal received from the vehicle operation detecting device  5 . When it is determined that the brake pedal was depressed, the processing proceeds to step  407 . When it is not so, the processing proceeds to step  402 .  
      At step  402 , whether the operation signal acquired from the operation switch  11  is the power OFF signal is determined. When it is determined to be the power OFF signal, the processing proceeds to step  407 . When it is not so, the processing proceeds to step  403 .  
      At step  403 , whether the operation signal acquired from the operation switch  11  is the pull operation signal is determined. When it is determined to be the pull operation signal, the processing proceeds to step  407 . When it is not so, the processing proceeds to step  404 .  
      At step  404 , the drive control flag stored in the internal memory is read, and whether the flag is kept set is determined. When the drive control flag is kept set, the processing proceeds to step  405 . When the drive control flag is not being set, the processing proceeds to step  406 .  
      At step  405 , when the operation signal acquired from the operation switch  11  is an upward operation signal, the operation content is determined to be the acceleration operation; when it is a downward operation signal, the operation content is determined to be a deceleration operation to reduce the vehicle speed, and the process is completed. On the other hand, at step  406 , when the operation signal acquired from the operation switch  11  is an upward operation signal, the operation content is determined to be the resume operation; when it is a downward operation signal, the operation content is determined to be the set operation, and the process is completed.  
      On the other hand, at step  407 , the drive control flag stored in the internal memory is reset, and the process is completed. In addition, in the process of this flowchart, when it is determined that the power OFF signal is acquired at step  402 , the power source of the vehicle driving system  1  will be turned off after completing the process of this process.  
      The vehicle driving system  1  sets various settings related to the drive control of the vehicle as shown in  FIG. 5 . This process is executed when it is determined that the operation content of the operation switch  11  is determined to be the set operation or the resume operation in the process shown in  FIG. 4 .  
      At step  501 , the drive control ECU  13  checks whether the operation content is the set operation. When it is the set operation, the processing proceeds to step  502 . When it is not the set operation, the processing proceeds to step  504 .  
      At step  502 , the present travel speed of the vehicle received from the engine control system  4  is set as the target travel speed. At step  503 , the travel speed of the vehicle received from the engine control system  4  is stored in the internal memory as the set travel speed. The received travel speed of the vehicle is transmitted to the car navigation apparatus  6 . The recommended travel speed of the road on which the vehicle is traveling that is stored in the map data memory  62  is overwritten or updated with the travel speed of the vehicle. Thereby, the travel speed of the vehicle can be assigned to the recommended travel speed of the road on which the vehicle is traveling. Then, the processing proceeds to step  510 .  
      At step  504 , whether the operation content is the resume operation is checked. When it is checked as the resume operation, the processing proceeds to step  505 . When it is not so, the process is completed.  
      At step  505 , the set travel speed stored in the internal memory and the recommended travel speed transmitted from the car navigation apparatus  6  are acquired. At step  506 , the set travel speed and the recommended travel speed that were acquired at step  505  are compared. Then, whether the recommended travel speed is smaller than the set travel speed is determined. When the recommended travel speed is smaller than the set travel speed, the processing proceeds to step  507 . When the set travel speed is smaller than the recommended travel speed, the processing proceeds to step  509 .  
      At step  507 , the speed limit change flag stored in the internal memory is read, and whether the flag is kept set is determined. When the speed limit change flag is kept set, the processing proceeds to step  508 . When it is kept reset, the processing proceeds to step  509 .  
      At step  508 , the recommended travel speed acquired at step  505  is set as the target travel speed, and the processing proceeds to step  510 . On the other hand, at step  509 , the set travel speed acquired at step  505  is set as the target travel speed, and the processing proceeds to step  510 .  
      At step  510 , the speed limit change flag stored in the internal memory is reset, the drive control flag similarly stored in the internal memory is set, and the process is completed.  
      The vehicle driving system  1  adjusts the target travel speed as shown in  FIG. 6 . This process is executed when it is determined that the operation content of the operation switch  11  is either the acceleration operation or the deceleration operation in the process shown in  FIG. 4 .  
      At step  601 , the drive control ECU  13  checks whether the operation content is the acceleration operation. When it is the acceleration operation, the processing proceeds to step  602 , where the target travel speed is incremented by a predetermined amount, and the process is completed. When it is not the acceleration operation, the processing proceeds to step  603 .  
      At step  603 , whether the operation content is a deceleration operation is checked. When it is a deceleration operation, the processing proceeds to step  604 , where the target travel speed is decremented by a predetermined amount, and the process is completed. When it is not the deceleration operation, the process is completed. Thereby, the target travel speed can be adjusted during the drive control of the vehicle.  
      In this way, the vehicle driving system  1 , in restarting the drive control of the vehicle in the resume mode, compares the previously stored set travel speed with the recommended travel speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed. Thereby, for example, even if the driver starts or restarts the drive control in the resume mode during ordinary road running due to the driver&#39;s misunderstanding or misapprehension after the drive control of the vehicle was performed using the set mode during highway running, when the set travel speed is larger than the recommended speed, the drive control of the vehicle is restarted with the recommended speed. That is, the drive control of a vehicle can be restarted with an appropriate target travel speed.  
      The process at step  507  is for confirming that a reason of the recommended speed being lower than the set travel speed is a difference between the speed limit of the road on which the drive control executed in the past and the speed limit of the road on which the vehicle is traveling now. Therefore, it may be omitted.  
     Second Embodiment  
      Next, a vehicle driving system  1  according to a second embodiment will be described. In this second embodiment, in starting or restarting the drive control of the vehicle, when the speed limit of a road on which the vehicle is traveling changed compared with the speed limit of a road on which the vehicle performed its drive control last time, the set travel speed of the vehicle or the stored set travel speed is compared with the recommended speed of the road on which the vehicle is traveling. Then the smaller speed is set as the target travel speed.  
      The drive control ECU  13  in this embodiment checks whether the speed limit change flag stored in the internal memory is kept set in starting or restating the drive control of the vehicle. When the flag is kept set, the drive control ECU  13  compares the travel speed of the vehicle received from the engine control system  4  or the set travel speed stored in the internal memory with the presently recommended travel speed received from the car navigation apparatus  6 , sets the smaller speed as the target travel speed, and starts or restarts the drive control of the vehicle. In the case where the speed limit change flag is kept reset, the drive control ECU  13  does not perform the above operations, but starts or restarts the drive control of the vehicle according to the present travel speed of the vehicle received from the engine control system  4  or the set travel speed stored in the internal memory.  
      Other configurations and operations are the same as those of the first embodiment, and hence their explanations are omitted.  
       FIG. 7  shows a process in which the vehicle driving system of this embodiment sets various settings related to the drive control of the vehicle. This process is executed when it is determined that the operation content of the operation switch  11  is either the set operation or the resume operation shown in  FIG. 4 .  
      At step  701 , the drive control ECU  13  checks whether the operation content is the set operation. When it is the set operation, the processing proceeds to step  702 . When it is not so, the processing proceeds to step  708 .  
      At step  702 , the speed limit change flag stored in the internal memory is read, and whether the flag is kept set is determined. When the speed limit change flag is kept set, the processing proceeds to step  703 . When it is kept reset, the processing proceeds to step  706 .  
      At step  703 , the travel speed of the vehicle transmitted from the engine control system  4  and the recommended travel speed transmitted from the car navigation apparatus  6  are acquired. At step  704 , the travel speed of the vehicle acquired at step  703  is compared with the recommended travel speed to determine whether the recommended travel speed is smaller than the travel speed of the vehicle. When the recommended travel speed is smaller than the set travel speed, the processing proceeds to step  705 . When the travel speed of the vehicle is smaller than the recommended travel speed, the processing proceeds to step  706 .  
      At step  705 , the recommended travel speed acquired at step  703  is set as the target travel speed, and the processing proceeds to step  714 . At step  706 , the travel speed of the vehicle acquired at step  703  is set as the target travel speed.  
      At step  707 , the travel speed of the vehicle is stored in the internal memory as the set travel speed, and transmitted to the car navigation apparatus  6 , and the recommended travel speed of the road on which the vehicle is traveling that is stored in the map data memory  62  is overwritten with the travel speed of the vehicle. Thereby, the travel speed of the vehicle can be assigned as the recommended travel speed of the road on which the vehicle is traveling. Then, the processing proceeds to step  714 .  
      On the other hand, at step  708 , whether the operation content is the resume operation is checked. When it is the resume operation, the processing proceeds to step  709 . When it is not so, the process is completed. At step  709 , the speed limit change flag stored in the internal memory is read, and whether the flag is kept set is determined. When the speed limit change flag is kept set, the processing proceeds to step  710 . When it is kept reset, the processing proceeds to step  713 .  
      At step  710 , the set travel speed stored in the internal memory and the recommended travel speed transmitted from the car navigation apparatus  6  are acquired. At step  711 , the set travel speed and the recommended travel speed acquired at step  710  are compared, and whether the recommended travel speed is smaller than the set travel speed is determined. When the recommended travel speed is smaller than the set travel speed, the processing proceeds to step  712 . When the set travel speed is smaller than the recommended travel speed, the processing proceeds to step  713 .  
      At step  712 , the recommended travel speed acquired at step  710  is set as the target travel speed, and the processing proceeds to step  714 . At step  713 , the set travel speed acquired at step  710  is set as the target travel speed, and the processing proceeds to step  714 .  
      At step  714 , the speed limit change flag stored in the internal memory is reset, the drive control flag similarly stored in the internal memory is set, and the process is completed. Thereby, only when the drive control of the vehicle is started or restarted for the first time after the speed limit change flag has been set, it is allowed to compare the travel speed of the vehicle or the set travel speed with the recommended speed and determine the smaller speed as the target travel speed.  
      In this way, the vehicle driving system of this embodiment checks the status of the speed limit change flag in starting or restarting the drive control of the vehicle in the set mode or in the resume mode. When the flag is kept set, the travel speed of the vehicle or the stored set travel speed is compared with the recommended speed of the road on which the vehicle is traveling, and the smaller speed is set as the target travel speed.  
      Thereby, when the driver fails to notice that the speed limit of the road on which the vehicle is traveling has changed and starts or restarts the drive control of the vehicle in the set mode or in the resume mode, and when the travel speed of the vehicle or the set travel speed is larger than the recommended speed, the drive control of the vehicle will be started or restarted with the recommended speed. That is, the drive control of the vehicle can be started or restarted with an appropriate target travel speed.  
     Third Embodiment  
      Next, a vehicle driving system  1  in a third embodiment will be described. The vehicle driving system  1  of this embodiment, in starting or restarting the drive control of the vehicle, checks whether the road type (classification) of a road on which the vehicle is traveling differs from the road type of a road on which the vehicle performed its drive control last time. When each road type differs from that of another, the vehicle driving system compares the travel speed of the vehicle or the stored set travel speed with the recommended speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed.  
      In this embodiment, instead of the speed limit change flag that is set when the speed limit of the road on which the vehicle is traveling changed in the second embodiment described above, a road type change flag that is set when the road type of the road on which the vehicle is traveling changed is provided.  
      When starting or restarting the drive control of the vehicle in the set mode or in the resume mode, the drive control ECU  13  of this embodiment checks whether the road type change flag stored in the internal memory is kept set. In the case where the flag is kept set, the drive control ECU  13  compares the travel speed of the vehicle received from the engine control system  4  or the set travel speed stored in the internal memory with the recommended speed received from the car navigation apparatus  6 , sets the smaller speed as the target travel speed, and starts or restarts the drive control of the vehicle. In the case where the road type change flag is kept reset, the drive control ECU  13  does not perform the above operations, but starts or restarts the drive control of the vehicle according to the travel speed of the vehicle received from the engine control system  4  or the set travel speed stored in the internal memory.  
      Other configurations and operations are the same as those of the second embodiment described above.  
       FIG. 8  shows a process of fixing various settings related to the drive control of the vehicle. In this process, a step of determining whether the road type change flag is kept set is provided instead of the step of determining whether the speed limit change flag is kept set in the second embodiment shown in  FIG. 7 . Moreover, a process of resetting the road type change flag and setting the drive control flag is provided instead of a process of setting the drive control flag. That is, the process steps  801  to  814  except for steps  802 ,  809  and  814  are the same as those steps  701  to  714  of the process shown in  FIG. 7 .  
      At step  802 , the drive control ECU  13  reads the road type change flag stored in the internal memory, and determines whether the flag is kept set. When the road type change flag is kept set, the processing proceeds to step  803 . When the flag is kept reset, the processing proceeds to step  806 .  
      At step  809 , the road type change flag stored in the internal memory is read, and whether the flag is kept set is determined. When the road type change flag is kept set, the processing proceeds to step  810 . When the flag is kept reset, the processing proceeds to step  813 .  
      At step  814 , the road type change flag stored in the internal memory is reset, the drive control flag stored similarly in the internal memory is set, and the process is completed.  
      In this way, in the case where the road type change flag is kept set, when the drive control of the vehicle is started or restarted in the set mode or in the resume mode, the vehicle driving system  1  of this embodiment compares the travel speed of the vehicle or the stored set travel speed with the recommended speed of the road on which the vehicle was traveling, and sets the smaller speed as the target travel speed. Thereby, even when the driver starts or restarts the drive control of the vehicle in the set mode or in the resume mode without noticing that the road type of the road on which the vehicle is traveling changed, it is possible to start or restart the drive control of the vehicle with an appropriate target travel speed.  
     Fourth Embodiment  
      Next, a vehicle driving system  1  in a fourth embodiment will be described. The vehicle driving system  1  of this embodiment differs from the second embodiment described above in that, when the drive control of the vehicle is started or restarted within a predetermined time after the speed limit of a road on which the vehicle was traveling changed compared with the speed limit of a road on which the vehicle performed its drive control last time, the vehicle driving system compares the travel speed of the vehicle or the stored set travel speed with the recommended speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed.  
      The drive control ECU  13  of this embodiment has an internal timer, and activates the internal timer when the speed limit change flag stored in the internal memory is set. In the case where the speed limit change flag is kept set, when starting or restarting the drive control of the vehicle in the set mode or in the resume mode, the drive control ECU  13  reads a value of the internal timer and checks whether the predetermined time has elapsed after the speed limit change flag was set.  
      When the predetermined time has not yet elapsed, the drive control ECU  13  compares the travel speed of the vehicle received from the engine control system  4  or the set travel speed stored in the internal memory with the recommended speed received from the car navigation apparatus  6 , sets the smaller speed as the target travel speed, and starts or restarts the drive control of the vehicle. When the predetermined time has already elapsed, the drive control ECU  13  does not perform the above operations, but starts or restarts the drive control of the vehicle according to the travel speed of the vehicle or the set travel speed stored in the internal memory.  
      Other configurations and operations are the same as those of the second embodiment described above.  
       FIG. 9  shows a process in which the vehicle driving system of this embodiment sets various settings related to the drive control of the vehicle. The process in  FIG. 9  is similar to the process shown in  FIG. 7  to which a step of determining whether a predetermined time has elapsed after setting of the speed limit change flag is newly added. Specifically, the process steps  901  to  916  except for steps  903  and  911  are the same as those steps  701  to  714  of the process shown in  FIG. 7 .  
      At step  903 , the drive control ECU  13  reads a value of the internal timer, and determines whether the predetermined time has elapsed. When the predetermined time has not yet elapsed, the processing proceeds to step  904 . When the predetermined time has already elapsed, the processing proceeds to step  907 .  
      At step  911 , the value of the internal timer is read, and whether the predetermined time has elapsed is determined. When the predetermined time has not yet elapsed, the processing proceeds to step  912 . When the predetermined time has already elapsed, the processing proceeds to step  915 .  
      In this way, when the drive control of the vehicle is started or restarted in the set mode or in the resume mode within a predetermined time after the speed limit of the road on which the vehicle is traveling changed compared with the speed limit of the road on which the vehicle performed its drive control last time, the vehicle driving system  1  of this embodiment compares the drive control of the vehicle or the stored set travel speed with the recommended speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed. Thereby, even when the driver starts or restarts the drive control of the vehicle immediately after the vehicle left a highway to be on an ordinary road, being conscious as if the vehicle is still traveling on a highway, the drive control of the vehicle can be started or restarted with an appropriate target travel speed.  
     Fifth Embodiment  
      Next, a vehicle driving system  1  in a fifth embodiment will be described. The vehicle driving system  1  of this embodiment differs from the second embodiment in that when the speed limit of a road on which the vehicle is traveling changed compared with the speed limit of a road on which the vehicle performed its drive control last time, measurement of a travel distance of the vehicle is started from that point of time. In the case where the travel distance of the vehicle under measurement is less than a predetermined distance, when the drive control of the vehicle is started or restarted, the vehicle driving system  1  compares the travel speed of the vehicle or the stored set travel speed with the recommended speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed.  
      The drive control ECU  13  of this embodiment has an internal memory, and when the speed limit stored in the memory is set, the drive control ECU  13  starts to measure a travel distance of the vehicle from that point of time based on the travel speed of the vehicle transmitted from the engine control system  4 . The measure travel distance of the vehicle is stored in the internal memory. In the case where the speed limit change flag is kept set, in starting or restarting the drive control of the vehicle in the set mode or in the resume mode, drive control ECU  13  reads a drive distance of the vehicle stored in the internal memory, and checks whether it is larger than a predetermined distance.  
      When the travel distance of the vehicle is less than a predetermined distance, the drive control ECU  13  compares the set travel speed received from the engine control system  4  or the set travel speed stored in the internal memory with the recommended speed received from the car navigation apparatus  6 , sets the smaller speed as the target travel speed, and starts or restarts the drive control of the vehicle. When the travel distance of the vehicle is more than the predetermined distance, the drive control ECU  13  does not perform the above operations, but starts or restarts the drive control of the vehicle according to the travel speed of the vehicle or the set travel speed stored in internal memory.  
      Other configurations and operations are the same as those of the second embodiment described above.  
       FIG. 10  shows a process in which the vehicle driving system  1  of this embodiment sets various settings related to the drive control of the vehicle. The process shown in  FIG. 10  is similar to that of the process shown in  FIG. 7  to which a step of determining whether the vehicle traveled a predetermined time after setting of the speed limit change flag is newly added. That is, the process steps  1001  to  1016  except for steps  1003  and  1011  are the same as those steps  701  to  714  of the process shown in  FIG. 7 .  
      At step  1003 , the drive control ECU  13  reads the travel distance of the vehicle stored in the internal memory, and determines whether the vehicle has traveled the predetermined distance. When the vehicle has not traveled the predetermined distance, the processing proceeds to step  1004 . When the vehicle has traveled the predetermined distance, the processing proceeds to step  1007 .  
      At step  1011 , the travel distance of the vehicle stored in the internal memory is read, and whether the vehicle has traveled the predetermined distance is determined. When the vehicle has not traveled the predetermined distance or more, the processing proceeds to step  1012 . When the vehicle has traveled the predetermined distance, the processing proceeds to step  1015 .  
      In this way, the vehicle driving system  1  of this embodiment, when the speed limit of the road on which the vehicle is traveling changed from the speed limit of the road on which the vehicle performed its drive control last time, measurement of travel distance of the vehicle is started from that point of time. In the case where the travel distance of the vehicle under measurement is less than a predetermined distance, when starting or restarting the drive control of the vehicle, the vehicle driving system compares the travel speed of the vehicle under measurement or a stored set travel speed with the recommended speed of the road on which the vehicle is traveling, and sets the smaller speed as the target travel speed. In this way, the drive control of the vehicle can be started or restarted with an appropriate target travel speed also by measuring the travel distance of the vehicle.  
     Sixth Embodiment  
      Next, a vehicle driving system  1  in a sixth embodiment will be described. The vehicle driving system  1  of this embodiment, in restarting the drive control of the vehicle in the resume mode, determines whether the road attributes of a road on which the vehicle is traveling, such as road type and a speed limit, differ from the road attributes of the road on which the vehicle performed its drive control in the past. Moreover, the vehicle driving system  1  is characterized in that, when it is determined that the road attributes are different, the restart of the drive control in the resume mode is prohibited.  
       FIG. 11  shows a process in which the vehicle driving system  1  of this embodiment sets various settings related to the drive control of the vehicle. Process steps  1101 - 1103  are the same as those of steps  501 - 503  in the first embodiment ( FIG. 5 ). However, instead of the process steps  501 - 503  in the first embodiment, any process selected among the process steps  701 - 707  in the second embodiment, the process steps  801 - 807  in the third embodiment, the process steps  901 - 908  in the third embodiment, and the processes steps  1001 - 1008  in the fifth embodiment may be used.  
      When the set travel speed is stored at step  1103 , the road attributes of the road on which the vehicle is traveling at the time of storing the set travel speed is acquired from the car navigation apparatus  6  and stored at step  1104 . The road attributes stored at this time include at least one of a speed limit of the road, road type, a width of the road and the numbers of lanes.  
      On the other hand, when it is determined that the set operation is not done at step  1101  and that the resume operation is done at step  1105 , the processing proceeds to step  1106 . At step  1106 , at the time when the resume operation is done, the road attributes of the road on which the vehicle is traveling are acquired from the car navigation apparatus  6 .  
      In the following step  1107 , the road attributes stored at step  1104  are compared with the road attributes acquired at step  1106 , and whether any change has arisen between the both is determined. In this determination process, when it is determined that the road attributes are not changed, the processing proceeds to step  1108 , where the stored set travel speed is set as the target travel speed. Subsequently, at step  1110 , the drive control flag stored in the internal memory is set, and the process is completed.  
      On the other hand, when it is determined that the road attributes changed at step  1107 , the processing proceeds to step  1109 , where the restart of the drive control by the resume operation is prohibited. For example, in this prohibition process, the resume operation is made invalid and a warning that the restart of the drive control by the resume operation cannot be done is sent out.  
      For example, the set travel speed stored during traveling on a highway is very likely to be inappropriate as the target travel speed on an ordinary road. In this way, it is highly likely that, when at least one of a speed limit, road type, a road width, and the number of lanes that constitute road attributes changes, an appropriate target speed may change.  
      Therefore, this embodiment stores the road attributes of the road on which the vehicle is traveling at the time of storing the set travel speed, and compares them with the road attributes of the road on which the vehicle was traveling when the resume operation was done. Then, when it is determined that the road attributes have changed, the restart of the drive control by the resume operation is prohibited. Thereby, an inappropriate target travel speed can be prevented from being set based on the set travel speed by the resume operation.  
      As described above, in this embodiment, the road attributes during traveling are stored at the time of storing the set travel speed, and compared with the road attributes during traveling at the time when the resume operation is done, whereby a change in the road attributes is determined. Therefore, in this embodiment, it is unnecessary to use the speed limit change flag unlike the first embodiment that requires its use. However, naturally it is possible to determine a change in the road attributes using the speed limit change flag or the road type change flag as in the cases of the above embodiments.  
      Conversely, in the above embodiments, it is also possible to store the road attributes at the time of the last drive control, as in the sixth embodiment, and determine whether the road attributes, such as a speed limit and road type, changed by comparing the last-time road attributes with the road attributes when the set operation or resume operation is done. Further, only one road attribute may be acquired to determine change of roads.  
      In the foregoing embodiments, the travel speed of the vehicle is set as the target travel speed in starting the drive control of the vehicle in the set mode. However, the above target travel speed may be fixed to a travel speed specified by a remote controller, etc.  
      Moreover, in the foregoing embodiments, when the travel speed of the vehicle is set as the target travel speed in the set mode, the recommended speed of the road on which the vehicle is traveling that is stored in the map data memory  62  is overwritten with the travel speed of the vehicle. However, the travel speed of the vehicle may be simply stored without overwriting the recommended speed of the road on which the vehicle is traveling. Thereby, it becomes possible to set the target travel speed considering also the travel speed of the vehicle that was set as the target travel speed in the past in restarting the drive control of the vehicle in the resume mode the next time and later rounds.  
      In the foregoing embodiments, in starting or restarting the drive control of the vehicle in the set mode or in the resume mode, the drive control of a vehicle is made to start or restart with an appropriate target travel speed. However, the drive control of a vehicle may be started or restarted so that an appropriate distance between vehicles is maintained. The drive control of a vehicle may be started or restarted by controlling an appropriate target travel speed and a distance between vehicles.  
      Other modifications and alterations are also possible to the above embodiments without departing from the spirit of the present invention.