After a stop to lay a wreath at Russia's Tomb of the Unknown Soldier, Mr.Obama will head to the Kremlin for a private meeting with Russian President Dmitri Medvedev.

VOA: standard.2009.07.06

In other words, they probably bear a distant relation to the semiotics of the stoplight, and that's probably why red was chosen for ambulances and police cars: because they put into your head the notion of "stop."

这很可能是源自于,红灯在一些语义中表示停止的意思，它被应用在了救护车和警车上：,因为他们带有警告的意味“

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