System for controlling a driving device of a vehicle

A system for controlling running of a vehicle comprises a switch for setting a slow-run mode, a sensor for detecting that the brake is being applied, a driving force control device mechanically separated from the accelerator for controlling a driving force of the vehicle, a sensor for detecting the running speed of the vehicle, a switch for setting a slow-run speed command signal. A target speed is generated on the basis of information concerning the brake activation and, the running speed of the vehicle and the slow-run speed command signal value. In the slow-run mode, when the brake is not applied, the driving force control device is driven to cause the vehicle to run at the target slow-run speed, and when the brake is applied, the driving force control device is driven so that no driving force is produced at least in the direction of travel of the vehicle.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a system for controlling a driving apparatus of a 
vehicle which includes an engine, a transmission for receiving the engine 
output to drive the vehicle for controlling the running thereof, and, more 
particularly, to a system for controlling the running of a vehicle 
suitable when the vehicle negotiates a congested road at low speed and is 
repeatedly starting and stopping and when the vehicle cruises on a 
relatively busy highway, and on an ordinary suburban road. 
2. Prior Art 
Hitherto, a speed of a motor vehicle has been controlled by opening or 
closing the throttle valve by pulling a wire, such as a piano wire, by 
means of an accelerator pedal for adjusting an engine output. Recently, as 
disclosed in "Isuzu Technical Report" No. 72, pages 7-20, a system has 
been proposed which includes an accelerator pedal disconnected from a 
throttle valve opened and closed by a sensor for sensing the depression of 
the accelerator pedal, and an electric motor for driving the throttle 
valve and a controller. This system is arranged so that, as shown in FIG. 
1, the output of an accelerator pedal 50 is detected by a computer unit 
52, the detected signal being processed to reflect the driver's intention. 
On the basis of information derived from a selector position sensor 54, a 
cooling water temperature sensor 56, an engine rotation sensor 58 for 
detecting the rotation of the engine EN, a gear position sensor 60, a 
vehicle speed sensor 62, and a clutch position sensor 64, a motor MT for 
regulating the opening of a throttle valve SV is operated, a clutch CL is 
connected and disconnected and a gear position of a transmission TM is 
controlled, whereby easy drive and low fuel consumption can be 
established. 
A conventional vehicle control system being constructed such as described 
above, when a road is crowded causing consecutive motor vehicles to 
repeatedly start and stop, the accelerator pedal and the brake have to be 
depressed alternately. Under these conditions, a driver must constantly 
control an accelerator pedal being careful not to depress it excessively. 
As such, a driver will become fatigued over a period of time, thus 
increasing the probability of an accident. 
SUMMARY OF THE INVENTION 
The present invention is intended, to solve the above-mentioned problems 
and provide a system for controlling a driving apparatus of a motor 
vehicle capable of preventing driver fatigue when a vehicle runs at low 
speed (a running condition not containing substantial acceleration and 
deceleration such as a rapid acceleration on sudden stopping is 
hereinafter referred to as "slow-run"), thereby providing safe running of 
a vehicle. 
According to one aspect of the invention, a system for controlling a 
driving apparatus of the vehicle comprises means for setting a slow-run 
mode, means for detecting that the brake is being applied, means 
mechanically separated from the accelerator for controlling a driving 
force of the vehicle, means for detecting a running speed of the vehicle, 
means for setting a slow-run speed command signal, means receiving 
information concerning the brake activation, the running speed of the 
vehicle and the slow-run speed command signal for generating a target 
slow-run speed. In the slow-run mode, when the brake is not applied, the 
means for controlling the driving force is driven so as to allow the 
running speed of the vehicle to become the target slow-run speed, and, 
when the brake is applied, no driving force is produced at least in the 
direction of travel of the vehicle. 
Thus, when the slow-run mode is set, the start, stop and running operation 
of the vehicle without abrupt acceleration and deceleration can be 
attained by operating one pedal, that is, the brake. Thus, the driver 
fatigue is minimized even when the vehicle is travelling on a heavily 
congested road where the driver repeatedly starts and stops the vehicle, 
and the time required for operating the brake to avoid an obstacle, a 
person or a vehicle suddenly appearing in front of the vehicle can be 
minimized whereby highly safe driving is possible. 
The control system may further comprise means for measuring a distance 
between the driver's vehicle and the vehicle in front, and means for 
generating a target slow-run speed generated in response to information 
concerning the brake operation, the inter-vehicle distance and the running 
speed of the vehicle, and a slow-run speed command signal. This control 
system will result in decreasing the number of braking operations and thus 
increasing driving safety. 
The control system may further comprise means for selecting either one of 
two different control modes, i.e., an inter-vehicle distance predominating 
control mode or a running speed predominating control mode, and the means 
for generating the target slow-run speed can generate a target slow-run 
speed in accordance with the selected control mode. Thus, the driver can 
either maintain an inter-vehicle distance (a distance between the driver's 
vehicle and the vehicle in front) at a proper value or drive the vehicle 
at a desired running speed in accordance with the driver's preference, 
thereby running the vehicle without adjusting an amount of accelerator 
depression. 
According to another aspect of the invention, a system for controlling a 
driving apparatus of a vehicle comprises means for setting a slow-run 
mode, means for detecting that the brake is being applied, means 
mechanically separated from the accelerator for controlling a driving 
force of the vehicle, means for sensing a running speed of the vehicle, 
means for measuring a distance between the driver's vehicle and the 
vehicle in front, means for setting a slow-run speed command signal, 
means, in response to information concerning the brake activation, the 
inter-vehicle distance and the running speed of the vehicle and the 
slow-run speed command signal, for generating a target slow-run speed, and 
decision means, in response to information concerning the inter-vehicle 
distance and the running speed of the vehicle, for calculating a speed 
approaching the vehicle in front to make a decision as to whether the 
measured inter-vehicle distance is within a proper distance value so as to 
output a warning signal to the driver and a braking command signal to the 
driving force control device. In the slow-run mode, if the braking 
commanding signal is output from the proper inter-vehicle distance 
decision means, the driving force control means is driven in accordance 
with the braking command signal. If there is no such braking command 
signal, when the brake is not applied, the driving force control means is 
driven to cause the vehicle to run at the target slow-run speed, and when 
the brake is applied, the driving force control means is driven such that 
no driving force is produced at least in the direction of travel of the 
vehicle. 
Thus, safe driving can be achieved in the slow-run mode in various running 
conditions including low speed running on a congested road and cruising on 
a suburban road or highway. 
The proper inter-vehicle distance decision means may output a warning 
signal when the product of a speed approaching the vehicle in front and a 
first set value is greater than a current inter-vehicle distance, and 
output a braking command signal when the product of a speed approaching 
the vehicle in front and a second set value is larger than a current 
inter-vehicle distance. This operation can minimize a risk of collision 
between vehicles. 
In an embodiment of the control system of the invention, once a vehicle is 
set in the slow-run mode by a driver, the driving force generating device 
and the transmission of the vehicle are controlled to cause the vehicle to 
run at a target slow-run speed generated on the basis of information of a 
slow-run speed command signal set by the driver and a running speed of the 
vehicle, without depressing the accelerator further and adjusting an 
amount of accelerator depression, thereby enabling the vehicle to be 
driven with the brake only. The running speed is supervised by the 
computer unit with a signal from the vehicle speed sensor. The computer 
unit operates to variably control the opening of the engine throttle or a 
current supplied to the electric motor and adjust a speed regulating 
mechanism and a load located in the driving force transmitting path, 
thereby maintaining a vehicle speed substantially corresponding to a 
sequentially generated target slow-run speed. 
If a target slow-run speed is generated on the basis of information 
concerning an inter-vehicle distance in addition to the information 
described above, the brake pedal is less frequently depressed and safety 
is enhanced. 
The target slow-run speed can be generated in accordance with either of two 
different modes, i.e., vehicle speed predominating control mode or 
inter-vehicle distance predominating control mode, which is selected by 
the driver. 
Thus, according to an embodiment of the invention, when the brake pedal is 
depressed, the computer unit detects this fact immediately to cease the 
generation of the driving force, or cause the driving force control means 
to operate to vary the transmission such that a braking force is further 
generated. When the driver stops applying the brake, the vehicle 
accelerates again to run at a target slow-run speed. As a result, it is 
unnecessary for the driver to shift his foot from the brake pedal to the 
accelerator pedal. 
In a control system according to another embodiment of the invention, the 
computer unit calculates speed approaching the vehicle in front on the 
basis of information concerning a running speed of the vehicle and a 
distance between the vehicle and the vehicle in front. If such a distance 
becomes less than a proper value, then a warning signal is issued to draw 
the driver's attention and a braking force is produced through the driving 
force control means to ensure safer driving.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A first embodiment of the invention will now be described with reference to 
the drawings. In FIG. 2, a brake switch 1 for detecting the depression of 
a brake by the driver or a brake pedal sensor 1 for generating an electric 
signal corresponding to a brake depression amount, a switch 2 located to 
be easily operated by the driver for setting the vehicle in a slow-run 
mode, and an inter-vehicle distance sensor 3 which employs a radar system 
or a triangulation system using picture images are connected to a computer 
unit 4 comprising a well-known microcomputer including a ROM, a RAM, a 
CPU, etc., A/D converter circuits and input and output interface circuits 
having pulse waveform shaping circuits and executing a main operation in 
accordance with a software program. In addition to the above-described 
brake activation detecting signal, a slow-run mode setting signal and an 
inter-vehicle distance signal, an electric signal indicative of a vehicle 
speed is input from a vehicle speed sensor 5 to the computer unit 4, and 
an electric signal corresponding to a speed command in the slow-run mode 
is also input from a speed command inputting device 6 to the computer unit 
4. A circuit for generating a target slow-run speed is provided within the 
computer unit 4 and includes a microcomputer, a ROM, etc. The computer 
unit 4 is further connected with a display device 7 which informs the 
driver that the slow-run mode has been set in response to a signal output 
by the computer unit 4 as well as informing the driver of a commanded 
slow-run speed, an accelerator pedal sensor 8 for detecting an amount of 
accelerator depression by the driver, a selector switch 9 for selecting a 
control mode in the slow-run mode as an input, and a driving force control 
device 10 for generating a driving force of the vehicle in response to a 
command signal from the computer unit 4. (It is noted that the driving 
force control device is mechanically separated from the accelerator). FIG. 
3 schematically illustrates an example of the driving force control device 
10. As seen in FIG. 3, an engine 20 for generating a driving force is 
coupled to a clutch section 21 which is attached to the output shaft of 
the engine to transmit and control an engine torque. The clutch section 21 
may be a disk clutch or a hydraulic torque converter. A transmission 22 
receives a rotational driving force from the clutch section 21 and 
appropriately converts the number of rotation and an amount of torque to 
transmit the driving force to the final driving section 23 which comprises 
a reduction gear set and a differential gear train. Wheels 24 actually 
drive the vehicle using the rotational force of the final driving section 
23. The transmission 22 is adapted to change a combination of gear sets 
having different gear ratios through a hydraulic or electric actuator. An 
actuator driving circuit 11 drives a gear change actuator in the 
transmission 22 in response to a command signal from the computer unit 4. 
The output of the engine 20 is varied by the degree of opening of a 
throttle valve 12, and the opening of the throttle valve is electrically 
controlled by a throttle actuator 13 including an electric motor. In order 
to open and close the throttle valve 12 in response to the command signal 
from the computer unit 4, a driving circuit 14 generates a signal for 
driving the throttle actuator 13. 
An operation of the vehicle run control system will now be described. 
When the slow-run mode setting switch 2 is in its off-position and the 
vehicle is in the normal driving mode, the computer unit 4 feeds a command 
through the driving circuit 14 to the throttle actuator 13 so as to open 
the throttle valve 12 basically in proportion to an amount of accelerator 
depression. When the vehicle is caught in a traffic jam and forced to 
repeatedly start and stop and the mode switch 2 is turned on by the 
driver, the computer unit 4 detects the signal from the mode switch 2. For 
safety, however, it is desirable that running control in the slow-run mode 
does not immediately commence, but is permitted to be set when certain 
predetermined conditions are satisfied. It is preferable for such 
predetermined conditions to be as simple as possible and necessary to at 
least maintain the safety desired. For example, the vehicle can actually 
enter in the slow-run mode when the brake pedal is operated or when the 
distance between the driver's vehicle and the vehicle in front is larger 
than that preset with respect to the running speed of the driver's 
vehicle. Such an operation is performed by the computer unit 4 which 
receives a brake operation detection signal and a signal output from the 
inter-vehicle distance sensor and processes these signals using a stored 
program. When the slow-run mode is set, the computer unit 4 informs the 
driver through the display device 7 visibly or audibly that the slow-run 
mode has been set so that the driver can always recognize this fact during 
the period of time when the slow-run mode is being set. When the slow-run 
mode is entered, the computer unit 4 sequentially generates a target 
slow-run speed in accordance to data input from the speed command 
inputting device 6, information as to whether the brake is applied and 
data input from the inter-vehicle distance sensor 3 and the vehicle speed 
sensor 5, so that the vehicle speed is set to the target slow-run speed. 
At this time, a selected one of two different target slow-run speeds is 
generated. More specifically, the switch 9 selects either of two control 
modes, that is, the mode in which the running speed of the driver's 
vehicle predominantly becomes a commanded slow-run speed input from the 
input device 6 (running speed predominating control mode) or the mode in 
which the distance between the driver's vehicle and the vehicle running in 
front becomes a predetermined value (inter-vehicle distance predominating 
control mode). The speed in the selected control mode is output and set as 
a target slow-run speed used in an actual operation, as described below. 
Even when the inter-vehicle distance predominating control mode is 
selected, when a signal output from the inter-vehicle sensor 3 shows no 
vehicle in front, the running speed predominating control mode is entered 
immediately or after a lapse of a predetermined time, so that the running 
condition does not change abruptly. The reverse shift from the running 
speed predominating control mode to the inter-vehicle distance 
predominating control mode when there is the vehicle in front is similarly 
performed. In the low-speed running mode, operating only the brake can 
achieve accelerating and constant speed running operations by controlling 
the throttle valve 12 of the engine 20 and the transmission 22 in response 
to a drive command signal output through the throttle actuator driving 
circuit 14 and the gear ratio change actuator driving circuit 11 in 
accordance with a control process programmed in the computer unit 4 in 
such a way that the vehicle runs at the target slow-run speed without the 
accelerator being operated. 
As described above, in the running speed predominating control mode the 
vehicle runs to maintain a commanded speed in a so-called cruising 
condition in which the vehicle runs without acceleration or deceleration. 
In the inter-vehicle distance predominating control mode the vehicle runs 
to always maintain a commanded distance between the vehicle in front or a 
proper distance automatically set by the computer on the basis of a 
current vehicle speed. Thus, during a transient running condition reaching 
the cruising, even in the running speed predominating control mode, a 
target slow-run speed is sequentially set in consideration of the distance 
to the vehicle in front. In the inter-vehicle distance predominating 
control mode, as long as the driver does not operate the brake, a target 
slow-run speed is sequentially set in consideration of the distance to the 
vehicle in front, even when the vehicle is accelerated or cruising. Either 
of these two control modes can be selected in accordance with a driver's 
preference and current traffic conditions, which makes the present system 
effective in various situations. 
FIG. 4 shows an example of acceleration patterns when a vehicle runs slowly 
in the running speed predominating control mode. As shown in this figure, 
a target slow-run speed value is time-sequentially set to allow the 
driving force control device to be operated in response thereto. During 
running in the slow-run mode, when the driver observes an obstacle or 
detects an abrupt decrease in the distance to the vehicle in front because 
it has decelerated, and the driver therefore depresses the brake pedal, 
the brake switch 1 outputs a signal informing the computer unit 4 that the 
brake has been operated. In response to this signal, the computer unit 4 
outputs a command signal to stop the operation of the throttle actuator 
driving circuit 14 and the gear ratio change actuator driving circuit 11 
so as to minimize a driving force of the vehicle or cause the power 
transmission system to operate as a brake. Thus, the throttle valve 13 is 
closed by a maximum extent within which range the engine does not stall. 
As a result, no driving force is generated. If the vehicle runs at 
relatively high speed, the engine brake is operated. During such an 
operation, there is no change in function and operation of the brake, 
irrespective of whether the slow-run mode is maintained or not. 
Such running control of a vehicle as described above enables the driver to 
drive on a congested road without operating the accelerator. Although it 
is necessary for the driver to respond to obstacles and detect an approach 
to the vehicle in front, the control system of the present invention 
eliminates the necessity of shifting from the accelerator pedal to the 
brake pedal, so that a time delay from the driver's recognition of need 
for brake operation to the driver's actual braking operation can not only 
be shortened, but also the accelerator pedal, the brake pedal or the 
clutch pedal will not be depressed incorrectly, whereby safe driving can 
be achieved. 
In order to perform the above-described control operation, the computer 
unit 4 is programmed to operate in accordance with a flowchart shown in 
FIGS. 5A and 5B. 
At a step S1, a decision is made as to whether the slow-run mode setting 
switch 2 is ON or OFF. If the switch 2 is OFF, the program proceeds to a 
step S2 and the vehicle runs in the normal driving mode. If the switch 2 
is ON, then the slow-run mode has been set (step S3), and at a step S4 a 
decision is made as to whether or not the brake is being operated. If yes, 
a generated driving force is decreased to zero (step S5), and if no, a 
suitable inter-vehicle distance is determined and set based on a current 
running speed V.sub.C (step S6). Then, the program makes a decision at a 
step S7 as to whether or not the selector switch 9 has selected the 
running speed predominating control mode, and if yes, a decision is made 
as to whether or not a commanded speed V.sub.O has been input (step S8). 
At a step S9, a decision is made as to whether or not the current speed 
V.sub.C has reached the commanded speed V.sub.O. If no commanded speed is 
input at step S8, a speed of 40 km/h is set as the commanded speed V.sub.O 
at a step S10, and the program proceeds to step S9. If V.sub.C has reached 
the commanded speed V.sub.O, the program sets target slow-run speed 
V.sub.T based on the current speed V.sub.C and the commanded speed V.sub.O 
(step S11), and then outputs a driving force control signal (step S12). If 
V.sub.C has not reached V.sub.O at step S9, i.e., the vehicle is in a 
transient running condition reaching the cruising, a decision is made as 
to whether or not an inter-vehicle distance is detected at a step S13. If 
no, that is, there is no vehicle in front, the program proceeds to a step 
S11 and the target speed V.sub.T is obtained from the current speed 
V.sub.C and the commanded speed V.sub.O. If an inter-vehicle distance is 
detected, the target slow-run speed V.sub.T is set at a step S14 from the 
current speed V.sub.C, a current inter-vehicle distance D.sub.C and a 
commanded inter-vehicle distance (or a proper inter-vehicle distance) 
D.sub.O, and then the program proceeds to step S12. If the inter-vehicle 
distance predominating control mode is selected at step S7, a decision is 
made as to whether or not the commanded inter-vehicle distance D.sub.O has 
been input (step S15), and if no, at a step S16 the proper inter-vehicle 
distance determined at step S6 is set as a commanded inter-vehicle 
distance. Then, the program proceeds to step S13 at which, as described 
above, a decision is made as to whether an inter-vehicle distance is 
detected. If there is no vehicle in front, the running control is 
automatically shifted to the running speed predominating control mode, and 
the program proceeds to step S11. If an inter-vehicle distance is 
detected, the program proceeds to step S14. 
A procedure of obtaining the target slow-run speed V.sub.T from the current 
running speed V.sub.C, the commanded running speed V.sub.O, the current 
inter-vehicle distance D.sub.C and the commanded inter-vehicle distance 
(or proper inter-vehicle distance) D.sub.O will now be described by way of 
a practical example. FIG. 6 shows an example of a change in the target 
slow-run speed V.sub.T from the time when a vehicle is at rest to the time 
when the vehicle reaches a commanded speed, for example, 65 km/h and is 
cruising in the running speed predominating control mode. A dot-and-dash 
curve shows the case in which the speed is linearly increased from zero at 
a time t=0 to 65 km/h at a time t=18 sec and thereafter maintained at 65 
km/h. This indicates that there is no vehicle in front and that a 
prestored acceleration pattern is used to sequentially set a target 
slow-run speed. The speed V.sub.T shown by a solid curve shows the case in 
which there is a vehicle in front when the vehicle is accelerated. In this 
case, the target slow-run speed is sequentially corrected and set on the 
basis of the current inter-vehicle distance D.sub.C and the commanded 
inter-vehicle distance (or proper inter-vehicle distance) D.sub.O during 
the period from a time t=4 sec to a time t=14 sec, because excessive 
access to the vehicle ahead will be caused if the driver's vehicle runs in 
accordance with a prestored acceleration pattern for the running speed 
predominating control mode. The target slow-run speed V.sub.T is set at 
every predetermined time interval .DELTA.t (.DELTA.t=0.25 sec, for 
example) by a procedure shown in a flowchart of FIG. 7. The flowchart of 
FIG. 7 shows in detail the operation at step S14 in the flowchart of FIG. 
5B. 
A running speed at the time of t.sub.i taken at every interval of .DELTA.t 
is expressed by V.sub.C (i), a commanded running speed by V.sub.O (i), an 
inter-vehicle distance by D.sub.C (i) and a commanded inter-vehicle 
distance by D.sub.O (i), and these information values taken at the time 
t.sub.i-1 (prior to the time t.sub.i by .DELTA.t) are suffixed by (i-1). 
Thus, .DELTA.t=t.sub.i -t.sub.i-1. 
In FIG. 7, at a step S17 a decision is made as to which is larger, the 
inter-vehicle distance D.sub.C (i) or the commanded inter-vehicle distance 
D.sub.O (i). If D.sub.O (i)&lt;D.sub.C (i), i.e., there is a sufficient 
distance between the vehicle and the vehicle in front, then and at a step 
S18 a decision is made as to which mode has been selected by the selector 
switch. If V.sub.O (i).gtoreq.V.sub.C (i), the target slow-run speed 
V.sub.T is corrected in accordance with D.sub.O and D.sub.C (step S19). 
After checking with the selector switch, if the inter-vehicle distance 
predominating control mode is selected, the program proceeds to step S19, 
but if the running speed predominating control mode is selected, the 
program proceeds to a step S20 and the target slow-run speed V.sub.T is 
determined merely by V.sub.O and V.sub.C. At each of steps S20 and S19, 
the target slow-run speed V.sub.T is set by the following equations: 
##EQU1## 
where .alpha. is an acceleration preset on the basis of the commanded 
speed V.sub.O. For example, in the case of FIG. 6 in which the vehicle is 
accelerated to reach a commanded speed of 65 km/h after the lapse of 18 
sec, the acceleration .alpha. is 1 m/sec.sup.2. Equation 1 shows that the 
speed the vehicle attains after accelerated at the rate of .alpha. during 
.DELTA.t from the time of t.sub.i is set to a target speed to be attained 
at a time of t.sub.i+1. Equation 2 includes equation 1 as the first and 
second terms and, as a third term, a term for correction by the 
inter-vehicle distance. D.sub.C '(i) in equation 2 is a value of the 
inter-vehicle distance predicted at t.sub.i to be taken after .DELTA.t and 
is calculated by the following equation: 
EQU D.sub.C '(i)=2D.sub.C (i)-D.sub.C (i-1) (3) 
The speed is corrected such that a difference between the predicted value 
D.sub.C '(i) and the commanded inter-vehicle distance D.sub.O (i) is 
reduced to zero within the interval .DELTA.t so as to attain V.sub.T(i+1). 
The second embodiment of a control system of the invention will now be 
described. FIG. 8 is a block diagram showing the construction of a control 
system of the second embodiment. Components similar to those shown in FIG. 
2 are denoted in FIG. 8 by the same reference numerals and an explanation 
thereof is omitted here. A sound generator 15 generates several kinds of 
voices (including signal tones) prestored and held therein, in response to 
a signal delivered from the computer unit 4. An input device 16 is 
connected to the computer unit 4 for inputting an inter-vehicle distance 
command to a target speed generating circuit 41 of the computer unit 4 for 
the purpose of reflecting driver's preference to the inter-vehicle 
distance control when the slow-run mode is selected and the vehicle is 
running in the inter-vehicle distance predominating control mode. The 
computer unit 4 further includes a proper inter-vehicle distance deciding 
circuit 42. Circuit 42 receives signals output from the vehicle speed 
sensor 5 and the inter-vehicle distance sensor 3 and data from the 
inter-vehicle distance command input device 16, and delivers a signal to 
the sound generator 15 for warning that the vehicle has come close to the 
vehicle in front and providing a braking signal to the driving force 
control device 10. 
The operation of the second embodiment is generally the same as that of the 
first embodiment, and some different operations will be described below. 
In the inter-vehicle distance predominating control mode or the running 
speed predominating control mode, when the vehicle in the slow-run mode 
comes close to the vehicle in front for some reason, the proper 
inter-vehicle distance deciding circuit 42 makes a decision as to whether 
or not a current inter-vehicle distance is within the proper range. More 
specifically, an approach speed Vap (corresponding to a difference (Vc-Va) 
and representing the rate of change in the inter-vehicle distance) is 
computed from sequentially input inter-vehicle distance information and 
running speed information. The product of that computation and a first 
preset value T.sub.1 is compared with current inter-vehicle distance 
information D.sub.C. And, if (Vap.times.T.sub.1)&gt;D.sub.C, then an approach 
signal is output. The sound generator 15 receives this approach signal and 
emits an alarm signal tone or voice to inform the driver of an excessive 
approach to the vehicle in front. It is desirable that the alarm produced 
becomes higher as the difference between (Vap.times.T.sub.1) and D.sub.C 
is larger. When such a situation continues and the product of Vap and a 
second preset value T.sub.2 becomes larger than D.sub.C, then the braking 
commanding signal is output. T.sub.1 and T.sub.2 are determined in 
consideration of various time delays such as the time required for the 
driver to decide if the brake should be applied when he notices the 
vehicle in front, the time required for the driver to manipulate the brake 
and the time required for the brake to initiate an actual braking 
operation. Preferably, T.sub.1 may be normally set to 2 to 5 seconds and 
T.sub.2 to 1,5 to 2 seconds. The driving force control device 10, upon 
receipt of such a braking command signal, forces the brake to operate just 
as the brake pedal is operated by the driver although the vehicle is in 
the slow-run mode, and the driving force is controlled such that the 
braking force is applied to the vehicle. Thus, the throttle valve 12 is 
completely closed to generate no driving force, and the transmission 22 is 
set to such a gear ratio that the engine rotation speed does not 
excessively rise thus braking the vehicle. 
A flowchart describing an operation of the second embodiment is identical 
to that of FIGS. 5A and 5B except that, after step S3 or before step S6 in 
the flowchart of FIG. 5, a step of detecting the above-described approach 
speed is provided. If the approach speed satisfies the above-described 
conditions, the approach signal calling the driver's attention and the 
braking command signal are output, and if not satisfied, the program 
proceeds to the subsequent steps, similar to the case described above in 
reference to FIGS. 5A and 5B. 
Although the above-described embodiments are arranged so that, during a 
transient running operation reaching the cruising, a target slow-run speed 
is sequentially set in consideration of an inter-vehicle distance even in 
the running speed predominating control mode, the target slow-run speed 
may be set without considering an inter-vehicle distance during the 
transient running operation.