DRIVING ON FREEWAY, A CAR WAS STOPPED AHEAD, HIT BRAKES, AUTOMATIC BRAKING SYSTEM DID NOT ENGAGE, HIT REAR OF CAR AHEAD DIRECTLY WITHOUT MUCH SLOWDOWN COMPARED TO EXPECTATION, POSSIBLY SKIDDING STRAIGHT, FOR WHAT SEEMED LIKE 2-3 SECONDS (LONG ENOUGH TO BE PUZZLED) OR FOREVER. HAVE NOT HAD AUTOMATIC BRAKING KICK IN A LOT OF TIMES IN PAST, NEVER HAD AN ACCIDENT LIKE THIS. NO FLUID LEAKAGES SEEN ON GROUND BEFORE OR AFTER ACCIDENT. CAR WAS DRIVABLE BUT TOTALED, NO INJURY REPORTED. TIRES 1 YEAR OLD. HAD JUST GOTTEN ON FREEWAY, WHICH HAD TURNED TO STRAIGHTAWAY, SO MIGHT HAVE BEEN GOING AROUND 50 WHEN BRAKING STARTED, MAYBE 15-20 ON IMPACT. PASSENGER SIDE AIR BAG WENT OFF.