just that really.
www.itv.com/news/story/2016-02-12/eight-children-hit-by-car-outside-school-in-liverpool/
Even if it turns out that the woman involved in this case was generally fit to drive, there have been other accidents when you hear it was an 80+ person at the wheel driving up the wrong side of the motorway or something.
I know other drivers can be dangerous too, but in their case, its more usually that they are choosing not to drive safely at that moment than they can't, so its harder to test for.