Google Maps keep getting better. They've recently added traffic patterns for major cities (an option that was only previously available on mobile versions of Google). To check it out just click on the "Traffic" button that now appears next to "Map." Gray is for the places where they don't have data, Red means that traffic is at a crawl (less than 25 miles per hour), Yellow means that traffic's going between 25-50 miles per hour, and Green means everything's moving along just fine.
(thanks google operating system)