What is the purpose of a runway boundary sign?

• A. To mark the boundary of the runway environment and indicate you should not cross without clearance.
• B. To indicate the location of the control tower.
• C. To show the distance to the terminal.
• D. To indicate you should not cross the boundary without clearance.

Answer: (A)

The main idea being tested is understanding how runway boundary signs control access to the runway environment. A runway boundary sign marks the edge of the runway environment and tells you that you must have explicit clearance before crossing into that area. This is a safety mechanism to prevent inadvertent incursions onto an active runway where aircraft are taxiing or taking off and could collide with vehicles or personnel.

That’s why the best answer is the one that explicitly mentions both marking the boundary of the runway environment and the requirement to obtain clearance before crossing. It captures the purpose of the sign as a boundary indicator tied to controlled access. The other options point to things like the control tower location or terminal distance, which aren’t the function of this sign, and a wording that simply says not to cross without clearance, while correct in spirit, doesn’t specify that it’s the boundary of the runway environment itself.