I Just Missed it, People Make Mistakes, Right?
While you are driving,you are expected to see all the things on the road that are clearly visible. If you look in the direction of at a clearly visible thing, but don't take note of it, your missing it is your mistake, and you can be held responsible.
While the courts do look carefully at the facts in every case, the general rule is that if you missed, it, it's your mistake, and will be considered in deciding who is responsible for the accident.
The only exception to this general rule is for people not in cars (pedestrians). When pedestrians see a car coming, sometimes they make mistakes figuring out how fast a car is going and they end up getting hurt. The courts have agreed that it's tricky for a person walking to figure out the speed of a car. (See "Pedestrians"). Otherwise, if something is in plain sight, you've got a duty to see it, even if you did look that way but somehow missed it.