When is it permissible to overtake a vehicle on the right side?

Overtaking a vehicle on the right side is permissible when the driver in front signals a left turn. This is significant because it indicates the driver's intent to move to the left side of the roadway, thereby creating an opening on the right for other vehicles to safely pass. This is a common practice that helps maintain the flow of traffic, especially in situations where the left lane is occupied or there is a delay in the left vehicle's movement.

In contrast, the other options do not justify overtaking on the right. For example, heavy traffic can lead to unpredictable movements from surrounding vehicles, making it less safe to maneuver on the right. Similarly, while clear roads may lend themselves to quicker traffic maneuvers, it is not an absolute condition for safely overtaking on the right. Lastly, when all vehicles are moving slowly, it may not be safe or practical to attempt overtaking, as this can lead to increased risk of accidents. Thus, signaling a left turn is the most definitive and valid scenario for overtaking on the right.