What is the primary function of solid white lines on the road?

The primary function of solid white lines on the road is to show the right edge of the pavement. These lines provide a visual guide for drivers, marking the boundary between the roadway and the shoulder or other areas adjacent to the pavement. This helps to enhance safety by ensuring that drivers stay within the designated travel area, especially in conditions with limited visibility or when driver attention may be distracted.

In contrast, other options serve different purposes and do not accurately describe the role of solid white lines. For example, lines indicating one-way streets typically do not utilize solid white markings but rather specific signs or arrows. Lines that separate lanes of traffic going in opposite directions are actually solid yellow lines, while speed limits are communicated through signs rather than lane markings.