AFTER COMING TO COMPLETE STOP (AT A GAS PUMP), PUT TRANSMISSION INTO PARK, I STEPPED OUT OF VEHICLE, WITHIN 10 SECONDS THE VEHICLE BEGAN TO SLOWLY ROLLING BACKWARDS. MY VEHICLE STRUCK ANOTHER VEHICLE CAUSING DAMAGE TO BOTH VEHICLES. NOW BEING MORE AWARE OF THIS INCIDENT, I AM VERY CAREFUL TO ASSURE BOTH GEAR INDICATORS- SHIFT KNOB AND DISPLAY ON DASH- DISPLAY "P", AND CHECK VEHICLE PHYSICAL POSITION FOR MOVEMENT. I HAVE NOTICED THERE IS BOTH A LAG TO CHANGE DISPLAY AND NOT A POSITIVE INDICATION OF THE PARK POSITION WHEN CHANGING GEAR.