You'd be amazed what the human body can survive - there are simply too many variables here. What part of your body is hit, whether you are thrown (and onto what) or are run over, timeliness of assistance, whether an ambitious doc is on duty at the ER that day, and on and on and on...
There are folks who survive all sorts of situations that should have resulted in death, and a few of them are here on SS. Even the best laid plans for suicide can go awry, and this wouldn't even qualify as one of those plans.
Google "miraculous survival stories" and you'll quickly learn that life is completely unpredictable - in good ways and bad.