DRIVING WEST IN A RENTAL CAR ON ST. CHARLES ROCK RD. I CHANGED LANES FROM THE RIGHT WESTBOUND LANE INTO THE L-WEST LANE AFTER CHECKING MY MIRRORS. WHILE I DID THAT I GOT HIT BY UNSEEN CAR (UNFAMILIAR BLIND SPOT?) IN THE REAR LEFT, WHICH CHANGED CAR'S HEADING TOWARDS THE LEFT (SOUTHWEST), ONTO A TRAJECTORY CROSSING THE EASTBOUND LANES. I IMMEDIATELY AND FULLY HIT THE BRAKE PEDAL BUT THE BRAKES DIDN'T RESPOND (PEDAL SEEMED STUCK), AND THE CAR CONTINUED TO MOVE AT SPEED (NO ACCELERATION) TOWARDS THE RESTAURANT ON THE SOUTH SIDE OF SCRR. I TRIED TO TURN WHEEL TO THE LEFT TO AVOID THE CURB, AND PERHAPS PULL A U-TURN, BUT THE WHEEL DIDN'T BUDGE EITHER; ALL THE WHILE I KEPT PRESSURE ON THE BRAKE PEDAL, EXCEPT FOR ONE OR TWO PUMPING ATTEMPTS TO SEE IF THE BRAKES WOULD KICK IN, BUT NO NOTICEABLE CHANGE IN PEDAL RESPONSE OR BRAKING ACTION ENSUED. THE CAR STAYED ON THE COURSE IT GOT ONTO BY THE PRECEDING COLLISION, CROSSED THE EASTBOUND LANES TOWARD THE RESTAURANT, JUMPED THE CURB, WENT THROUGH A GREEN STRIP OF LOW (1-2') JUNIPER BUSHES ONTO THE PAVED RESTAURANT GROUNDS, AND PUSHED A PICNIC TABLE INTO A BUSINESS SIGN BEFORE COMING TO A FULL STOP AGAINST THE BUILDING. THERE WAS NO CLEAR DAMAGE TO THE BUILDING ITSELF. I DO NOT RECALL HAVING TURNED OFF THE IGNITION AT ANY TIME DURING THIS INCIDENT, EITHER WHILE DRIVING OR AFTER I CAME TO A STOP. I RECALL HAVING TO RESTART THE CAR TO LOWER THE DRIVER'S SIDE DOOR WINDOW, BECAUSE THE DOOR ITSELF WAS BLOCKED BY ANOTHER PICNIC TABLE. I