Why does green mean go and red mean stop?

Why is there traffic? How do traffic lights work? How do they know when to turn red and green? Why does green mean go and red mean stop? Why are cars able to go faster than they are allowed to go with the speed limit? Why do we drive on the right side of the road in the United States? Why are some roads bigger than others?  Answers to your traffic questions with Mike Knodler, professor and director of the Transportation Center at the University of Massachusetts. Download our learning guides: PDF | Google Slide | Transcript