I don't think he would be hit by a side view mirror either. That driver would know they hit something and would have had an opportunity to see Steven in the road. But I'm thinking it is something that the driver didn't know was being dragged behind them, or hanging off the side of a truck/trailer and Steven being on the side of the road and not visible to the driver. Maybe a chain, work equipment, heck one of the warning flags from a logging truck bouncing around. At that speed, a small item could cause a fatal injury to the head and its still technically a "hit and run." Whatever it was was ultimately, but not immediately fatal, and it disoriented him enough that he walked and fell in the middle of the road.
Maybe I'm jaded from driving in Atl traffic and have seen it all. Its like a frogger game dodging crap falling off painting vans! The driver has no idea he's dragging an extension ladder behind his van.