The invention relates to a vehicle speed control system which compares a prevailing vehicle speed against a target speed and automatically regulates the attitude of speed controlling means of a drive source in accordance with a difference therebetween. 
When a vehicle speed control system of the kind described is applied to an automobile, the current speed of the automobile is compared against a target speed, and the opening of a throttle valve of a carburetor, which supplies a fuel to an engine, is automatically regulated in accordance with a difference therebetween.  This is effective to maintain the vehicle speed equal to the target speed, and hence is advantageously utilized to alleviate a driver's effort when running at a constant speed over a relatively long distance as when running on a highway.  In fact, there is an increasing number of automobiles on which is mounted a system for effecting a constant speed running control of the kind described. 
A driver of an automobile which is provided with a constant speed running system of this kind will initiate a constant speed running control mode well in advance when running at a constant speed over a relatively long time interval is expected according to his driving schedule.  However, the termination of such control is not always performed with a margin in time.  Many drivers will depress a brake pedal whenever it is urgently required to reduce the speed of the automobile, rather than operating a constant speed running device.  When considered in terms of control system, this represents an extreme limit on the feedback, causing a wind-up phenomenon.  To prevent this, a constant speed running device is often operable to terminate a constant speed running control mode in response to the depression of the brake pedal.  This would be a proper choice in view of the fact that the depression of the brake pedal indicates the driver's desire to change or reduce the vehicle speed. 
On the other hand, the depression of a brake pedal is frequently detected by utilizing a switch.  Thus, a switch is provided which is opened and closed in response to the operation of the brake pedal so that the depression may be detected by an on/off condition thereof.  However, it will be appreciated that the frequency of use of a brake pedal is very high in an automobile while the durability of a switch is not as high as desired.  This means that the switch degrades with time, eventually resulting in a failure to detect the depression of the brake pedal.  Thus, there is a need to inspect such a switch periodically and to change it as required.  A problem then arises that the inspection and exchange of the switch may sometimes take place with an improper period. 
To provide one solution to this problem, Japanese Laid-Open Patent Application No.  271,131/1986 discloses a constant speed running system including a pair of switches, each capable of detecting the depression of a brake pedal, so that the constant speed running control may be terminated whenever either one of the switches has detected the depression. 
However, the provision of a pair of switches each capable of detecting the depression of a brake pedal has merely increased the length of time until the degradation of the switch results in its loss of functioning to the durable period of either switch, whichever is the longer, but does not overcome the problem of the incapability to detect the depression of the brake pedal which may be caused by an exceptional failure of a switch occurring out of the period.  While the probability that such exception occurs for all of the switches may be reduced by providing an increased number of detecting switches to overcome the problem presented above, this would merely complicate the associated mechanism and possibly cause another failure unless the periodic inspection and exchange of the switches are observed in a predetermined manner. 
As another solution, Japanese Laid-Open Patent Application No.  128,433/1983 discloses a constant speed running system which terminates a constant speed running control mode when the ratio of the current vehicle speed to a target vehicle speed is equal to or less than a given value.  Thus, if a failure of a switch which is provided to detect the depression of a brake pedal occurs, resulting in a failure to detect the depression of a brake pedal, the constant speed running control mode is terminated when the current vehicle speed decreases until the ratio reduces to or below the given value. 
It is appreciated that this would enhance the safety of the constant speed running system.  However, the driver is only capable of recognizing the result that the constant speed running control mode has been terminated as a result of depressing the brake pedal, but cannot know a failure of the switch which is provided to detect the depression of the brake pedal.  Accordingly, he will again utilize the constant speed running control.  In such instance, because the constant speed running control mode has been terminated in response to the ratio of the prevailing vehicle speed to the target vehicle speed, the wind-up phenomenon in the control system mentioned above may not be avoided depending upon the magnitude of the deceleration. 
In view of the foregoing, it is an object of the invention to provide a vehicle speed control system of enhanced safety and reliability in which a vehicle speed control is enabled only when means commanding the termination of a vehicle speed control, such as a switch detecting the depression of a brake pedal, is functioning properly.