WHILE TRAVELLING AT AN APPROXIMATE SPEED OF 45/50, I HAD REASON TO RAPIDLY DECREASE FORWARD AND APPLY MORE THAN NORMAL BRAKE ACTION TO AVERT ANOTHER VEHICLE THAT HAD OVERRUN A STOP SIGN. MY WIFE, WHO WAS IN THE FRONT PASSENGER SEAT AND PROPERLY BELTED, WAS THRUST FORWARD TO A POINT WHERE HER RIGHT SHOULDER MADE CONTACT WITH THE DASHOARD AND HER HEAD MADE LIGHT CONTACT WITH THE WINDSHIELD. SHE WAS SHAKEN BUT HAD NO NOTICEABLE INJURY AT THAT TIME.WE DROVE HOME, AND DURING THAT 7 MILE TRIP WE DECIDED TO RECHECK THE BELT ACTION. ON SEVERAL INSTANCES WE ACCELERATED TO 45/50 AND COMMENCED HARD BRAKING ACTION TO CHECK WHETHER WE COULD DUPLICATE THE PREVIOUS SEAT BELT FAILURE. OF THE TIMES THAT WE PERFORMED THIS ACTION, THE SEAT BELTS FAILED OVER HALF THE TIME. THIS TIME I REALIZED THAT THE DRIVERS BELT WAS ALSO FAILING.DEALER WAS NOTIFIED AND AN APPOINTMENT WAS MADE. NLM