real time traffic networks

These are starting to come online now. But for the last few years I was thinking about developing one that uses cell phones that already have GPS information. The goal would be to use them to get full coverage of all roads, not just ones with traffic cameras. The cell phones would tell you the driver's speed and location so you would know how fast traffic was going.

I think GM and OnStar are missing the boat on this one, or maybe they are already doing it.