noun
The most common meaning; refers to the red-yellow-green lights controlling road traffic.
次の交差点の信号機が赤になった。
The traffic light at the next intersection turned red.
信号機
Because the traffic signal is broken, a police officer is directing traffic.
noun
signal device; semaphore
Broader meaning: any mechanical or electrical signaling apparatus, such as railway signals or industrial indicators. Less common than sense 1.
鉄道の信号機は、列車の安全を守る重要な装置だ。
Railway signals are important devices that ensure train safety.