noun
Literal traffic signal meaning: the red light that tells vehicles or pedestrians to stop.
赤信号で止まってください。
Please stop at the red light.
Figurative use: a warning sign that something is wrong, risky, or should be stopped. Often used in business, health, or project contexts.
このプロジェクトはもう赤信号だ。
This project is already a red light (in trouble).
健康診断の結果に赤信号がついた。
My health checkup results showed a red flag.