I WAS DRIVING AT A STEADY SPEED OF APPROX. 45 -50MPH ON RT.219 IN ORCHARD PARK, N Y, WHEN MY 1988 MERCURY GRAND MARQUIS WAGON STARTED TO ACCELERATE ON ITS OWN. THE CRUISE CONTROL WAS NOT ENGAGED. THE CAR CONTINUED TO ACCELERATE ON LEVEL GROUNG TO APPROX. 60 MPH. CONTINUOUS BRAKING ONLY SLOWED THE CAR TEMPORARILY. I MANAGED TO BRAKE THE CAR ENOUGH TO GET OFF THE EXIT RAMP AND ONLY WAS ABLE TO STOP IT BY RIDING THE BRAKE PEDAL AND PUTTING THE CAR INTO N. SMOKE AND FLAMES WERE COMING FROM UNDER THE CAR. THERE WERE NO PRIOR WARNINGS OF ANY MECHANICAL PROBLEMS. THE SPEED PEDAL WAS NOT "STUCK" DOWN. I HAVE NEVER ENCOUNTERED THIS PROBLEM BEFORE. IN MORE TRAFFIC OR IN A DIFFERENT LOCATION THERE WOULD HAVE BEEN A MAJOR ACCIDENT. *AK