What does a solid double yellow line in the center of the road indicate?

A solid double yellow line in the center of the road serves as a clear indication that no passing is allowed in either direction. This marking is a safety measure, designed to prevent drivers from attempting to overtake vehicles when it could be dangerous, such as on curves or in areas with restricted visibility. The presence of these lines typically signals that traffic on both sides must stay in their respective lanes and not attempt to cross over for the purpose of passing another vehicle.

The other options do not align with the meaning of a solid double yellow line. For instance, passing on both sides would conflict with the line's purpose of promoting safety and preventing dangerous maneuvers. Allowing turns in specific directions isn't implied by a solid double line; turns depend on signs and road markings distinct from lane separation lines. Finally, a bike lane is not indicated by solid double yellow lines, as bike lanes are generally marked with different symbols and colors. Thus, the solid double yellow line is primarily about regulating passing, reinforcing the rule that it is prohibited.