On a two-way, two-lane road, what does a single broken yellow centerline mean?

Correct answer: B. – Drivers on either side may pass when conditions are safe
A single broken yellow line marks the center of a two-way, two-lane road. Passing is permitted for traffic on either side when safe conditions exist. Because a passing driver must temporarily use a lane assigned to oncoming traffic, the driver must verify that the roadway ahead is clear before crossing the line.
Source: Pennsylvania Driver's Manual — Ch. 2, “Pavement Markings”




