Under what condition are you allowed to pass on a two-lane highway?

You are allowed to pass on a two-lane highway when there is a dashed yellow line on your side of the center dividing line. A dashed yellow line indicates that the lane is designated for passing and that it is legal to cross the line to overtake another vehicle, provided it is safe to do so. This marking provides a clear visual cue to drivers that passing is permitted, as long as visibility is good, and no oncoming traffic is approaching.

The other situations do not legally permit passing. Heavy traffic often creates unsafe conditions for passing, as there may not be enough space or time to overtake safely. Driving below the speed limit does not inherently justify passing, as it doesn't ensure the road conditions warrant such action. Similarly, although a driver ahead signaling a turn may indicate they are slowing down or preparing to change direction, it does not provide a legal basis for passing; passing in this scenario could be unsafe if done without proper caution. Thus, the presence of a dashed yellow line is crucial for ensuring that passing maneuvers are done safely and legally.