danger-signal

Noun

danger signal (plural danger signals)

1. A signal set at danger and showing a red light or other stop indication, which a train driver must not pass until it changes or until manually cleared past the signal by a dispatcher.
   • On a train travelling at say 40 miles per hour a driver would have to react quite quickly to bring his train to a halt at a danger signal even if he could see that signal half a mile away.2005, "Practical Railway Engineering" by Clifford F. Bonnett, Imperial College Press →ISBN https://books.google.no/books?id=qcMgBJ1exfIC&pg=PA137&lpg=PA137&dq=%22at+a+danger+signal%22&source=bl&ots=c4...
2. (figuratively) An indication that a potentially bad or dangerous situation could develop.