Final answer:
A solid line down the center of a two-way roadway signifies that no passing is allowed from either direction for safety reasons.
Step-by-step explanation:
A solid line down the center of a two-way roadway indicates that passing is not allowed from either direction. This is a traffic regulation put in place to prevent accidents where visibility or other road conditions make passing unsafe. Such a line signifies that drivers should stay in their lane and not attempt to overtake or cross into oncoming traffic, particularly where there are no breaks in the solid line.