Which are a tiny minority. The largest causes of crashes in the US are attention/cognition problems, not vision problems. Most traffic systems in western countries (probably in others, too, but I don't have personal experience), and in particular the US, are designed to limit visibility problems and do so very effectively.
* someone parking carefully, misjudges depth perception, bumps an object
* person driving at night, their eyes failed to perceive a poorly lit feature of the road/markings/obstacles
* person driving and suddenly blinded by bright object (the sun, bright lights at night)
* person pulling out in traffic who misinterprets their depth perception and therefore misjudges the speed of approaching traffic
* people can only focus their eyes at one distance at a time, and it takes time to focus at a different distance. It is neither unsafe nor unexpected for humans to check their instruments while driving -- but it can take the human eye hundreds of milliseconds to focus under normal circumstances -- If you look down, focus, look back up, and focus, as quick as you can at highway speeds, you will have travelled quite a long distance.
These type of failures can happen not as a result of poor decision making, but of poor perception.