It's not quite true. Morgan invented a traffic
signal, but it's not the red/yellow/green lamps we think of today. It was this:
I say this not to discount the man's work, but because I think that Morgan's signal is really cool and it's a shame that his actual ingenuity and innovation tends to get overlooked in favor of some blinking lights.
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Also, it's worth noting that early traffic signals were not automated; a traffic cop had to manually operate them.