At an airport, a sign with a red background and white inscription (for example, marking a runway designation or an ILS critical area) indicates —

Why: FAA-CT-8080-2H Figure 65 ("U.S. Airport Signs") shows this red-background, white-inscription format on runway-designation and ILS critical-area holding position signs — the standard FAA mandatory instruction sign, marking a location an aircraft or ground vehicle may not proceed beyond without specific ATC clearance.

• Confuses the red mandatory-instruction color convention with a nighttime-visibility color code; red specifically marks mandatory holding locations, not a generic lighting scheme.
• A permanently closed taxiway or runway is marked with an "X" surface marking, not with this red/white sign format.