The time of day in which it is hardest to distinguish shapes and objects on the road

The answer would be a (foggy) night. When we don't have any available light source, then it's practically impossible or very difficult (also depending on the strength of the moon if you're in the wilderness) for us to see and distinguish things and objects on the road. When we add fog to this equation, the difficulty gets furthered by fog where even a light source does not improve the situation by much.