How do guide dogs know when to stop or go at a traffic light?

It's a common misconception that dogs are color blind. They aren't. Dogs don't see the same range of colors that humans do, but they do see color.

However, guide dogs are trained to automatically stop at all intersections, regardless of traffic lights. The dog is unaware of the lights and is focused only on finding the curb or other obstacle that might be in his master's path. It is then up to the human handler to listen for the sounds of traffic to determine which direction it is going and when it is safe to cross.