Under what circumstances is it acceptable to overtake on the left?

Overtaking on the left is permissible under specific circumstances where the road layout supports it for safety and efficiency. On a one-way street with two lanes, vehicles are generally moving in the same direction, and overtaking on the left is allowed since there is no opposing traffic. This makes the maneuver safer and more logical, as it aligns with the movement of vehicles in that environment.

Similarly, on a four-lane road, overtaking on the left can also be acceptable, especially if the right side is congested or if vehicles are stationary. Such roads are designed to accommodate multiple lanes of traffic moving in the same direction, thus providing the opportunity to pass slower vehicles safely on the left without the risks associated with navigating around vehicles that are turning or slowing down.

In contrast, overtaking on a two-lane road is typically discouraged or illegal because it often means crossing over into oncoming traffic, which poses significant danger. In summary, the acceptable scenarios for overtaking on the left align with the carriageway's configuration and traffic flow, ensuring that drivers can perform this maneuver safely.