It does help to know others tried, as I know from experience. Yesterday was the anniversary of my husband's death in a similar fashion, except the was on his bike. The car behind stopped and the driver, who had first aid training, gave him CPR until the ambulance came and they (and the doctors in the hospital) worked on him for some time. I'm thankful to know he wasn't alone, even though unconscious and that everything that could have been done, was.
Most years we plant primroses at the place. The car driver would not let the police tell us who he was.
I was also told he would almost certainly have been dead by the time he hit the road.