Someone posted an idea similar to this one under "Real time Commute Monitor", in the 'Public' category.

Having spent a few years with a non-profit called ITS America, I know this has been discussed for over a decade. In fact, the cell-phone carriers are looking to sell aggregate information of triangulated cell-phone signal data as it moves across networks and overlay that on a map of roads.

The REAL holdup is privacy concerns. I'm all for continuing the shame of individual privacy, but at some point we need to let technology win that argument once in a while. Visit www.itsa.org, search for the meeting minutes of sessions they've held over the years, it's all there.