You don't see many firms and apps with names ALL IN CAPS anymore. But, here's a free iPhone app that shouts its name at you (the exclamation point is part of the name like Yahoo!)...

INRIX TRAFFIC!

INRIX says that their predictive ability comes from more than 1 million drivers anonymously sending data about speed, position, and heading to their network. However, I don't think many iPhone users of their app will send much data since the app needs to be active in order to gather that data. And, since the iPhone is essentially single tasking, TRAFFIC! can't gather that data if another task is running on the display.

The app also gathers data about scheduled road closures and large events that could affect traffic. It displays this information marked by traffic cones and stars on its street map. My neck of the woods doesn't have much in the way of real-time traffic information. So, I didn't see much on the map except for scheduled road closures/work. That in itself is quite useful, however.