Q: If I missed something on the road and get into an accident, is it my fault?
A: While driving, you are expected to see all the things on the road that are clearly visible. If you look in the direction of a clearly visible thing but don't note it, missing it is your mistake, and you can be held responsible.
While the courts 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 pedestrians. When pedestrians see a car coming, sometimes they make mistakes figuring out how fast a car is going, and they get hurt. The courts have agreed that it's tricky for a person walking to figure out the speed of a car. 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.