I PULLED INTO A RELATIVE'S GRAVEL DRIVEWAY WHICH HAS A VERY LARGE PARKING AREA TO THE SIDE. I PULLED UP TO ABOUT TWO FEET FROM THEIR GRASSY AREA, GOT INTO POSITION TO BACK UP INTO A PARKING SPACE, STOPPED, AND PUT MY 2011 SIENNA INTO REVERSE, AND AS SOON AS I GENTLY TOUCHED THE EXCELERATOR TO BEGIN BACKING UP THE VAN SPED FORWARD AT A HIGH RATE OF SPEED CRASHING INTO AND OVER AN 8 INCH RAILROAD TIE THAT HAD A STEELBEAM ON TOP THAT SEPARATED THE DRIVEWAY FROM THEIR YARD. I HAD TO SLAM BOTH FEET ONTO THE BRAKE TO GET IT TO STOP WHICH IT FINALLY DID. I THOUGHT I HAD NOT PUT IT IN REVERSE BUT I LOOKED AT THE GEARSHIFT AND IT WAS INDEED IN REVERSE. I JIGGLED THE GEARSHIFT TO MAKE SURE IT WAS SECURED IN THE SLOT AND IT WAS. I THEN PUT IT IN PARK AND SAT THERE FOR A MINUTE OR TWO TRYING TO GAIN COMPOSURE TRYING TO FIGURE OUT WHAT JUST HAPPENED, I WAS IN SHOCK AS NOTHING LIKE THIS HAS EVER HAPPENED TO ME BEFORE. AFTER THAT MINUTE OR TWO I DECIDED I COULDN'T LET THE VEHICLE SIT THERE AND NEEDED TO MOVE IT. SCARED, BUT I PUT IT IN REVERSE AGAIN, READY TO POUNCE ON THE BRAKES WITH BOTH FEET AND BARELY TOUCHING THE EXCELERATOR I STARTED MOVING IN REVERSE. IT WAS WORKING FINE THEN. I THEN GOT OUT TO CHECK FOR DAMAGE AND THE BUMPER ON THE DRIVER'S SIDE WAS DAMAGED FROM THE IMPACT. THE AIRBAG DID NOT DEPLOY, I WAS NOT HURT HOWEVER HAD ANY OF MY RELATIVES BEEN IN THAT AREA AT THAT MOMENT IT COULD'VE RESULTED IN AN INJURY OR FATALITY. I REPORTED WHAT HAPPENED TO THE LOCAL DEALERSHIP WHERE I B