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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.

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"