Equal Opportunity
In a car the current speed is generally considered an output.
If you have cruise control you will want to input the speed in some way.
The obvious choice is to use the current speed.
Notes:
In a car the current speed is an output - the result of a combination of controls applied by the user, like the accelerator, and external conditions such as the the slope of the road. In a car with cruise control, you want some means to tell the cruise control system the speed at which you want to travel. The designer could provide a dial which the user turns to select the desired speed. Equal opportunity suggests that the value that is already available is used - the current speed of the car. This provides ease of input, and is also safer, since we have eliminated the possibility of the driver entering a speed dramatically different form the current speed.
Of course this mechanism would still work if there were no actual speedo acting as an output of the current speed. The value of the speed is still a valid input, since it is know to the system, and the user has a perception of the value from the motion of the car.