MY SON (2.5-YEAR-OLD, 32LBS) WAS SITTING IN HIS CHILD SEAT WHICH WAS ON RIGHT REAR SEAT. THE CHILD SEAT WAS SECURED WITH LAP&SHOULDER SEAT BELT. WHILE I WAS DRIVING ON A CURVY FREEWAY ON-RAMP AT ABOUT 25MPH, THE SEAT BELT UNBUCKLED ITSELF AND MY SON FELL WITH HIS CHILD SEAT ONTO THE FLOOR. HIS HEAD HIT THE SEAT BACK OF THE DRIVER SEAT. THE SPEED POST ON THAT RAMP IS 25MPH. ACTUALLY, THIS IS THE 2ND OCCURRENCE OF THE SAME PROBLEM. THE FIRST ACCIDENT HAPPENED ON 11/20/2010 ON ANOTHER CURVY FREEWAY ON-RAMP. MY KIDS HAVE BEEN SITTING ON THAT CHILD SEAT ON THE SAME SEAT FOR THE PAST 5 YEARS. THERE WAS NO ISSUE UNTIL 11/20/2010. ALSO, MY SON WAS VERY TIGHTLY SECURED IN HIS CHILD SEAT. HIS HAND WAS AT LEAST 10-15" FROM THAT BUCKLE. SO THERE WAS NO WAY HE COULD UNBUCKLE HIMSELF. I CALLED LEXUS, BUT WAS TOLD TO VISIT A LOCAL DEALER. I TOOK THE CAR TO THE SERVICE DEPARTMENT OF A LOCAL DEALER. THE SERVICE GUY DID SOME SIMPLE TESTS OF THAT SEAT BELT, BUT COULD SEE NOTHING WRONG. HE THEN SUGGESTED THE ONLY SOLUTION IS TO GET A NEW CAR.