Why is it possible for some cars to regulate their own speed?
Cruise control allows your car to maintain or alter its speed without the manual compression of the pedals. It is usually controlled by buttons on a steering wheel. Cruise control systems control a vehicle in the same way that the pedals do, namely by adjusting the throttle position of the engine. A throttle valve controls how much air an engine takes in and ultimately its speed.
Cruise control systems control this by using an actuator attached to a cable. This cable is attached to a pivot that controls the throttle valve and also the pedal of the car. As the speed is increased using the buttons, the actuator (powered by an engine vacuum) pulls the cable, which in turn releases more air into the engine by altering the pivot and also pulls the acceleration pedal towards the floor, as if you were pressing it with your foot.
The opposite is true for deceleration. Cruise control systems adjust the throttle to accelerate towards the desired speed at a decelerating rate, so that it does not overshoot.