What does a solid white line indicate?

A solid white line indicates that drivers must stay in their lane unless it is necessary to change lanes. This line serves as a boundary to help maintain safety on the road. It indicates that lane changes should be made with caution and only when there is a clear and justifiable reason to do so, such as moving around an obstacle or preparing to exit the roadway. The presence of the solid line minimizes distractions and keeps traffic orderly by discouraging unnecessary lane changes.

In contrast, the other options address concepts that are not aligned with the meaning of a solid white line. For instance, freely changing lanes is not permitted when there is a solid white line, since it signifies the need to stay put unless absolutely required. Slowing down may be relevant in certain scenarios on the road but does not directly relate to the purpose of a solid white line. Lastly, while road edges are marked by different lines or barriers, a solid white line specifically serves the purpose of indicating lane discipline rather than marking the road’s edge.