WHILE DRIVING AT HIGHWAY SPEEDS ON THE US 101 NORTHBOUND INTO SAN FRANCISCO MY 2011 SONATA SUDDENLY BEGAN VEERING OUT OF ITS LANE AND WAS HARD TO STEER OR CONTROL. I WAS ABLE TO BRAKE AND STEER JUST ENOUGH TO GET TO THE RIGHT HIGHWAY SHOULDER WITHOUT HITTING ANYTHING AS THERE WERE NO CARS IMMEDIATELY TO MY RIGHT. I WAS STUCK ON A DANGEROUS PART OF THE HIGHWAY SHOULDER FOR AN HOUR WAITING TO BE TOWED, WITH THE VEHICLE SEEMING DANGEROUS TO DRIVE TO A SAFER SPOT. WHILE WAITING FOR A TOW ON THE HIGHWAY SHOULDER I WAS ABLE TO LOOK AT THE REAR-LEFT WHEEL WHICH WAS INCLINED INTO THE WHEEL-WELL AT ABOUT 30 DEGREES ON THE TOP SIDE. AFTER GETTING TOWED OFF THE HIGHWAY THE TOW-TRUCK DRIVER WAS ABLE TO LOOK UNDER THE CAR AND NOTICED THAT THE UPPER REAR CONTROL ARM OF THE BACK LEFT WHEEL HAD BROKEN. THE BACK LEFT WHEEL WAS LOOSELY MOVEABLE USING JUST OUR HANDS. THIS FAILURE COULD HAVE CAUSED A MUCH MORE SERIOUS ACCIDENT! I AM WAITING TO GET A FULL INSPECTION REPORT FROM A MECHANIC.