Traffic lights -The complet story

Traffic lights are such a common part of our daily lives that we rarely give them a second thought. We just do what they tell us to. This wasn’t always the case, however. In 1917, when the first interconnected traffic signal was installed, motorists thought it was some sort of prank and simply ignored it. Police officers had to be dispatched to make sure drivers understood the lights and obeyed them.

The lights most of us are familiar with are the classic three-aspect models.

One question that comes up fairly often is what led to the standard color scheme, with red on top, yellow (or amber) in the center, and green on the bottom. The answer hearkens back to the early days of rail travel, when workers needed a way to warn trains that there was danger ahead. They chose red to mean “stop!” because of its allusions to blood. Green was originally the symbol for caution, and a clear, or white, light meant it was safe to proceed. The problem with this arrangement is that engineers frequently mistook the light of the moon or stars for the “all clear” signal. This led to tragic derailments and train collisions. So the arranegment was changed, with yellow representing caution and green meaning “go.” And that, my friends, is how it all began.

http://autos.yahoo.com/news/why-traffic-lights-red-green-yellow-19004905...