The concept of "red = stop" and "yellow = slow" existed prior to electricity and artificial light: in rail transportation, a yellow flag meant proceed with caut

The concept of "red = stop" and "yellow = slow" existed prior to electricity and artificial light: in rail transportation, a yellow flag meant proceed with caution. When traffic lights were first created, they simply didn't have the technology to create a great pure yellow, so amber was used instead, and people understood the color they were aiming for.