When is passing not allowed on either side of a highway?

Passing is not allowed on either side of a highway when both sides have solid yellow line markings. Solid yellow lines indicate that passing is prohibited for vehicles traveling in both directions. These markings are used specifically to signal that it is unsafe to overtake another vehicle due to visibility issues, road conditions, or other factors that could make passing dangerous. Therefore, encountering solid yellow lines means drivers must stay in their lanes and cannot proceed to pass other vehicles, ensuring safety and order on the roadway.

The other options may not universally indicate a prohibition on passing. For instance, a speed limit under 30 mph does not inherently restrict passing; it just indicates slower traffic flow. Similarly, a sign prohibiting passing is location-specific but does not apply universally to all areas. Heavy traffic can make passing difficult or unsafe, but it doesn't carry an inherent legal prohibition like the solid yellow line does.