hi! there's always room for more input, but meanwhile, check out this post

How did we come to choose green, yellow and red as the standard colors for traffic lights?

from the Traffic section in the FAQ*

*see the link on the sidebar or the wiki tab

In the mid-19th century, semaphores were used to signal to trains. Red meant danger and, funnily enough, green meant roughly what amber means today.

However, you can look further back at the colour red itself meaning danger. In his book "The Uses of Enchantment", Bruno Bettelheim analyses the colour red in folkloric tales that go back many centuries. One example would be the story of Little Red Riding Hood.

So, red has had a connotation of danger for many, many centuries (the devil is often depicted in red or wearing red) and still does today: the femme fatale in movies often wears red, and dangerous but sexy cars are often painted red (e.g. Ferrari Rosso or Alfa Red 501).