When is it unsafe or illegal to pass another vehicle?

Passing is unsafe or prohibited in several situations: where a solid yellow line marks your side of the road (no-passing zone), when approaching or within an intersection or railroad crossing, on hills or curves where you cannot see far enough ahead, when a vehicle ahead has stopped for a pedestrian, and when a school bus is stopped with its red lights flashing. Pass only when it is legal and you can clearly see that the lane is clear far enough ahead to complete the pass and return safely. Recognizing when not to pass is critical, because passing errors cause severe head-on collisions.