What condition does a "red light" on a site indicate?

A "red light" on a site typically indicates dangerous seismic conditions. This visual signal alerts engineers and construction personnel that the area is at risk for significant ground movement during an earthquake, prompting a careful consideration of safety measures and design protocols before any construction can proceed. Recognizing the potential for seismic activity is crucial for ensuring the safety of structures and inhabitants, as areas with high seismic risk require specialized engineering solutions to mitigate potential hazards.

The other options do not align with what a "red light" signifies. High soil density and low plasticity may indicate stable soil conditions, while very stable slopes would generally be a positive indicator rather than one that warrants a warning signal.