I WAS DRIVING MY 2013 KIA OPTIMA ON A DRY ROAD SURFACE. I HAD JUST GONE THROUGH A TRAFFIC LIGHT THAT WAS GREEN FOR ME TO GO. THERE WAS A SLIGHT DOWN GRADE SO I STARTED TO APPLY MY BRAKES BECAUSE OF TRAFFIC AHEAD. WHEN I PRESSED MY BRAKE PEDAL THE CAR WOULD NOT STOP CAUSING ME TO REAR END ANOTHER VEHICLE. THIS HAPPENED IN A VERY BUSY CITY STREET.