Which element of runway signage is critical for pilots during low visibility conditions?

Runway edge lights are critical for pilots during low visibility conditions as they provide essential visual references that aid in determining the runway's boundaries. These lights define the edges of the runway, illuminating the path even when natural lighting is poor due to fog, rain, or darkness. Pilots rely on the clear visibility of these lights to maintain alignment and ensure safe operations, particularly during takeoff and landing when precision is paramount.

Other elements such as the mandatory instruction signs or taxiway center lines may offer necessary information; however, they do not serve the same purpose in terms of direct visual assistance along the runway edges. The runway edge lights specifically enhance safety by marking the separation from surrounding terrain or obstacles, which is crucial in low-visibility scenarios.