When driving on a one-way street, under what condition may you turn left onto another one-way street?

Answer: B — If traffic on the street moves to the left

Turning left onto a one-way street is permissible when the street you are entering also moves to the left. This is because both streets are designed for similar directional flow, minimizing the risk of collisions and ensuring a smoother traffic pattern. In this scenario, since both the street you are leaving and the street you are entering are one-way and moving in the same leftward direction, it is safe and legal to make the left turn. This allows for a consistent and logical movement of vehicles, facilitating overall traffic ease. The other options do not align with traffic laws or best practices for safe driving. For instance, turning left onto a street where traffic moves in the opposite direction can lead to significant hazards and confusion. Also, the condition related to the number of lanes on the street does not inherently impact the legality of a left turn; it's the direction of vehicular movement that governs the rules in this scenario. Lastly, the presence or absence of cars on the street does not negate the requirement for the turning street to facilitate movement in the same direction for safety and legality.