Simple, "at fault" takes on a different meaning in driving. Often it's a cluster of mistakes that causes an accident, mistakes by either vehicle or even mistakes by someone not a part of the collision itself. If I'm 51% at fault should I go to jail?
But also everyone makes mistakes occasionally behind the wheel, most often nothing bad happens. And by everyone I mean every single driver without exception. That's why the law mostly operates with concept of Mens Rea, a guilty mind. Negligence only comes into play under a reasonable person standard and since we know those reasonable persons are also making same mistakes... it goes to civil court.
"There is credible subjective evidence that the particular accused neither foresaw nor desired the particular outcome, thus potentially excluding both intention and recklessness. But a reasonable person with the same abilities and skills as the accused would have foreseen and taken precautions to prevent the loss and damage being sustained"
But also everyone makes mistakes occasionally behind the wheel, most often nothing bad happens. And by everyone I mean every single driver without exception. That's why the law mostly operates with concept of Mens Rea, a guilty mind. Negligence only comes into play under a reasonable person standard and since we know those reasonable persons are also making same mistakes... it goes to civil court.
http://en.wikipedia.org/wiki/Mens_rea#Criminal_negligence :
"There is credible subjective evidence that the particular accused neither foresaw nor desired the particular outcome, thus potentially excluding both intention and recklessness. But a reasonable person with the same abilities and skills as the accused would have foreseen and taken precautions to prevent the loss and damage being sustained"