What does a solid white line indicate?

A solid white line serves as a clear visual indicator to drivers about lane usage. Specifically, it denotes that lane changes are discouraged. When you see a solid white line separating lanes of traffic moving in the same direction, it is intended to alert drivers to stay in their lane unless absolutely necessary to change for safety reasons. This helps to maintain smooth traffic flow and enhances safety on the road.

In contrast, the other options suggest interpretations that do not align with the purpose of a solid white line. For instance, lane changes being encouraged directly contradicts the meaning of a solid white line, while indicating that one must stop if crossing it misrepresents the line's purpose; it's not a stop line, but rather a guide for maintaining lane discipline. Furthermore, although it’s important to be aware of bike lanes, that aspect pertains more to other markings such as designated symbols or dashed lines, rather than a solid white line.