WHILE STOPPED AND TRYING TO TURN LEFT ON A BUSY HIGHWAY I WAS HIT FROM BEHIND BY A DRIVER WHO DID NOT SEE ME UNTIL IT WAS TO LATE. I SAW HIM IN MY REAR VIEW MIRROR, HOWEVER, BARRELING DOWN AT ME AT A HIGH RATE OF SPEED. I HIT THE GAS TO THE FLOOR TRYING TO GET AWAY FROM HIM BUT MY 2015 V6 COLORADO HESITATED FOR A 1/2 SECOND OR 1 SECOND, I DON'T KNOW. IT SEEMED LIKE FOREVER. IT THEN KICKED IN AND WE GOT ABOUT 100' BEFORE HE SLAMMED INTO THE RIGHT REAR BUMPER. THE IMPACT DROVE THE COLORADO FORWARD ABOUT 10' AND KILLED THE ENGINE. MY WIFE AND I BOTH HIT THE BACK OF OUR SEATS AND BOUNCED FORWARD AND THE SEAT BELTS CAUGHT US. JUST PRIOR TO THE HIT, I WAS LEANING FORWARD AND THUS BOUNCED HARD AGAINST THE SEAT AND THEN BOUNCED HARD AGAINST THE SEAT BELTS; SEEMINGLY IN A VERY SMALL FRACTION OF A SECOND. IT BEEN A YEAR NOW AND I STILL HAVE WHIPLASH SYMPTOMS. MY WIFE WAS ALREADY LEANING BACK AGAINST HER SEAT AND APPARENTLY DID NOT SUFFER ANY WHIPLASH. THE CAR THAT HIT ME WAS TOTALED BUT THE 80 YEAR OLD OCCUPANT WAS JUST SHAKEN UP. HE LEFT ABOUT 30' OF SKID MARKS AND WAS TRYING TO GET AROUND ME TO THE RIGHT. IF THE COLORADO HAD NOT HESITATED, I AM CONFIDENT HE WOULD MISSED ME. THE HIT WAS SO HARD BECAUSE HE HIT THE THE COLORADO'S TOW BAR WHICH DID NOT BEND MUCH THUS TRANSMITTING MOST OF THE FORCE TO THE REST OF THE VEHICLE. FOR A FRACTION OF A SECOND, ACCELERATION WAS INTENSE. I COMPLAINED TO THE LOCAL GM DEALER BUT THEY TOLD ME THE HESITATION WAS DESIGNED INTO THE VEHICLE AND THEY