COLLISION OCCURRED ON E DENNY WAY IN SEATTLE, APPROX 3:20 PM ON A DOWNHILL SLOPE IN STOP-AND-GO TRAFFIC AFTER ABOUT 20 MINUTES OF CONSTANT BRAKE USE TO HOLD BACK THE CAR. ABOUT 2/3 OF THE WAY DOWNHILL THE BRAKES FAILED, NO RESISTANCE WHATSOEVER AND THE CAR ROLLED INTO THE VEHICLE IN FRONT, A DISTANCE OF ABOUT 15 FEET. THIS WAS A ONE-OFF INCIDENT AND DID NOT RECUR SO FAR. GM AND 3RD PARTY INSPECTORS FOUND NO ERROR CODES, BUT THE BRAKES INDEED FAILED, WHICH MAKES ME SUSPECT A DESIGN PROBLEM IN THE BRAKE-BY-WIRE SYSTEM THAT CONTROLS THE PEDAL PRESSURE AND MODULATED REGENERATIVE BRAKING. IT WAS LIKE SLIDING ON ICE, NO TRACTION WHATSOEVER.