Google released today a very useful new overlay for cities in the US showing real-time traffic information for major roads and highways (via GoogleMapsMania). When traffic is available you will see a “Traffic” choice in the upper right next to “Map | Satellite | Hybrid“. When zoomed out you also see a little traffic signal placemark icon telling you to zoom in to see traffic details. For example, see the San Francisco traffic. A fair number of larger cities have the data, but not all cities. Oh, and the data is also available in the Google Maps Mobile version (so you can see this on your phone).
This traffic data is not available yet in Google Earth. Hopefully, Google will implement a new sub-layer under “Roads” or “Transportation” in the form of a network link which would show this traffic data in 3D inside GE. But, in the past, specialized road data appearing in Google Maps has sometimes taken quite a while to migrate its way to GE. The advantage to doing it in GE is that the layer could be dynamic. You could sit there and watch the status of the traffic situations change as soon as new data is available if a network link is used inside of GE.
[UPDATE 1735: Here's Google's announcement of the new traffic feature for Google Maps. They say there are more than 30 cities with the data so far.]