It's a great idea: Gathering GPS data from individuals' mobile devices to paint a picture of traffic speeds on local streets. Google announced this week that it's now making such a service available at Google Maps (details available on this blog, with more details here).
Always curious to know about traffic flow, I checked out the city streets around headquarters a little while ago and was surprised to see a red-and-black line signifying congestion on Portland Avenue:
So I walked over to a window for a look:
The map was partly right: Nothing on the road was moving, but that's because the vehicles were parked.
This was completely unscientific, of course, and I fully expect service to improve the longer it's around and as more driver data can be crowdsourced. Give it a whirl and let me know what your results are.