What action is required if a traffic sign is missing or obstructed?

When a traffic sign is missing or obstructed, the appropriate action to take is to proceed with caution. This choice is correct because a missing or obstructed sign can create uncertainty regarding the rules that apply at that location. Drivers are responsible for understanding and obeying traffic laws, and without clear guidance from signage, they should be particularly vigilant.

Proceeding with caution means reducing speed, being alert for other vehicles, pedestrians, or other potential hazards that may not be clearly indicated due to the sign's absence. This cautious approach helps ensure safety for all road users, as it allows the driver to assess the situation more thoroughly before making decisions.

Other options suggest actions that could lead to unsafe driving conditions. Ignoring speed limits or proceeding at usual speed without the guidance of traffic signs could result in dangerous situations, especially if the missing sign indicated critical information like a speed limit or stop requirement. Yielding to all other vehicles could also lead to confusion or accidents if not all drivers are aware of an uncontrolled situation. Thus, approaching with caution is the most responsible and safe choice.