Final answer:
Riders switch from a staggered formation to single-file to increase safety in specific scenarios such as night riding, heavy traffic, at intersections, and turning corners.
Step-by-step explanation:
When a group of riders moves from a staggered formation into a single-file formation, there are various scenarios where this change would enhance safety and visibility. The most appropriate time for riders to transition into a single-file formation include when riding at night to increase visibility and reduce the space they occupy on the road, during heavy traffic to streamline their profile and reduce the risk of accidents, when stopping at intersections to simplify traffic patterns and prevent blocking, and while turning corners to reduce their footprint on the road and minimize the risk of a collision with other vehicles. Each of these scenarios prioritizes the safety and organization of the riders within the group.