noun
advance warning signal; preliminary traffic signal
A traffic signal that gives advance warning of an upcoming signal, often used on high-speed roads or where visibility is limited.
このカーブの先には予告信号が設置されている。
An advance warning signal is installed ahead of this curve.
予告信号が黄色に変わったら、減速して本信号に備えてください。
When the advance warning signal turns yellow, slow down and prepare for the main signal.
Compound of 予告 (advance notice) + 信号 (signal).