When is it illegal to pass another vehicle?

Passing another vehicle is illegal when there is a solid line marking on the road. Solid lines indicate that passing is prohibited due to safety concerns. These markings are typically found in areas where visibility is limited, such as on curves or hills, where the risk of a collision increases significantly if a driver attempts to overtake another vehicle. The solid line serves as a clear visual cue to drivers that changing lanes or passing is not permitted in that section of the road.

The other options present situations where passing may be legal or dependent on other conditions. Traffic moving below 20 mph does not inherently relate to the legality of passing; rather, it depends on the specific road markings and conditions. Similarly, turning left at an intersection may involve yielding to oncoming traffic but does not directly relate to passing. Lastly, a vehicle signaling does not automatically indicate that it is safe to pass, as the driver may be preparing to turn or change lanes, requiring caution from those behind.