ON DEC 18, 2015, DRIVING AT ABOUT 20 MPH ON A NEIGHBORHOOD STREET WHICH CURVED LEFT, THE STEERING WHEEL ON MY 2002 PRIUS BEGAN SHAKING VIOLENTLY. I LOST CONTROL OF THE CAR AND STRUCK A LARGE LANDSCAPING ROCK. THE CAR HAD $2233 IN DAMAGES TO THE RIGHT FRONT BUMPER, FENDER, WHEEL AND TIE ROD. THERE WERE NO INJURIES. AFTER REPAIR, THE MECHANIC COULD NOT REPLICATE THE SHAKING. PRIOR TO THE ACCIDENT, THE PRIUS ON RARE OCCASION HAD BRIEF EPISODES OF STEERING WHEEL SHAKING, ONLY WHILE BACKING OUT OF THE DRIVEWAY ON COLD MORNINGS. IN EARLY JAN 2016 THE STEERING VIBRATION OCCURRED AGAIN WHILE DRIVING ON NEIGHBORHOOD STREETS AND COULD BE STOPPED ONLY BY TURNING THE CAR OFF. WE CEASED DRIVING THE CAR. I TALKED TO OUR TOYOTA DEALER AND WERE ADVISED THE PROBABLE CAUSE WAS A DEFECT IN THE ELECTRONIC POWER STEERING SYSTEM AND TO CONTACT TOYOTA. I COMMUNICATED BY PHONE, EMAIL AND LETTER WITH TOYOTA, PROVIDED A VIDEO OF THE SHAKING, AND REQUESTED REPLACEMENT OF THE POWER STEERING SYSTEM. I GOT A CASE NUMBER AND WAS SCHEDULED FOR AN INSPECTION BY TOYOTA AT THE DEALER. TOYOTA BY LETTER ON 3/22/16 STATED 'WE DID NOT FIND ANY EVIDENCE THAT THIS INCIDENT WAS THE RESULT OF ANY TYPE OF MANUFACTURING OR DESIGN DEFECT.' WHEN I PICKED UP THE VEHICLE, THE VIOLENT STEERING SHAKING OCCURRED IN THE DEALER'S PARKING LOT AND WAS WITNESSED BY THE TECHNICIANS. THIS PROBLEM IS WELL KNOWN TO TOYOTA. IN 2011 WE RECEIVED BOTH A SAFETY RECALL NOTICE TO REPLACE THE EPS PINION NUTS AND A NOTICE OF WARRANTY EXT