According to Reuters, Google Maps for mobile phones will include starting today live information about the traffic for 30 major metropolitan areas in the US. You just have to select your destination, and Google Maps will show the route with color flags for different conditions of the traffic (red, yellow and green).
Google Maps for Mobile can be accessed at http://google.com/gmm on your Java-enabled mobile phone. After downloading the software, go to the desired location within the application and select "show traffic" in the menu.
You can also check the demo.
Another new feature is that you can store your favorite locations, so you can access faster frequently used routes.
Related:
Edit your mobile personalized homepage
Real-time maps of the world
View geographic data on Google Maps