TRAVELING EAST BOUND ON TEXAS HIGHWAY 380 IN DENTON COUNTY, TX. TRAFFIC SLOWED AS IT APPROACHED ENTRANCE RAMP TO I35 SOUTHBOUND. I APPLIED PRESSURE TO THE BRAKES TO DISENGAGE THE CRUISE CONTROL AND SLOW VEHICLE, NOTHING HAPPENED. DID THIS SEVERAL TIMES AND ALSO TRIED TO TURN CRUISE OFF FROM STEERING COLUMN LEVER. CRUISE DID NOT DISENGAGE, CAR DID NOT SLOW, MY VEHICLE RAN INTO THE REAR END OF THE ROCK HAULER TRUCK IN FRONT OF ME. *TR