Someone appears to be ‘mashing’ Google maps with traffic info: – this seems to cover hundreds of towns and cities in USA as well as much of the UK!! (zoom right out to see). It’s a fabulous example of what can be done if the information is made available in a form that allows developers to work with it.
No info on icons or meanings of colours, but I imagine it’s based on the same as Highways Agency i.e. green = no delay, yellow = moderate delay, red = severe delay, or possibly Realtime Traffic (which confusingly uses the same colours to mean something different.