Is there a dashboard widget that displays Google Maps and traffic data overlay?
You can make one yourself using the unsung "Open in Dashboard..." command in Safari.


Open Google Maps in Safari. Adjust the size of the window so the map the size you would like the widget to be.
In Safari, go to the File menu, then "Open in Dashboard." You'll get a little hover in Safari. Place the cursor in the upper left part of the map so that the entire map, and just the entire map, is highlighted. It may be a bit finicky to get the map highlighted properly, so just keep at it and it will be fine. 
Click add in the top of the browser. Done!


You now have a Google Map widget that acts just like Google Maps in the browser. Navigate to where you want to see traffic, enable traffic in the widget, and you're done.