stoplight vs visual signal

stoplight and visual signal both are nouns.

Word	Noun	Adjective	Verb	Adverb
stoplight	Yes	No	No	No
visual signal	Yes	No	No	No

As nouns, visual signal is a hypernym of stoplight; that is, visual signal is a word with a broader meaning than stoplight:

stoplight: a red light on the rear of a motor vehicle that signals when the brakes are applied to slow or stop
visual signal: a signal that involves visual communication

stoplight (noun)	visual signal (noun)
a visual signal to control the flow of traffic at intersections	a signal that involves visual communication
a red light on the rear of a motor vehicle that signals when the brakes are applied to slow or stop