[rrrrrrrrrrrryan]

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.

[standardUser]

Makes sense, since Yield signs and similar warning signs are all yellow as well. But being born into a world where everyone calls an orange light yellow is confusing nonetheless!