It is an idea that has been around for a while, but now we have the product. From TomTom, the Dutch navigation company, a device that uses mobile phone density to deduce car traffic.
A network operator knows where its users are at each moment. Of course, clever algorithms must be used to manage the vast amount of data and to establish the car flow distinguishing moving cars from pedestrians. Multiple mobile phone operators should also be used, etc.
It is also important to notice the paradign shift in the industry: In the past, the company only sold devices. But now that dozens of competitors have entered the market, prices have plunged. As a result, the company is expanding its business to include services.
From Businessweek, New TomTom Gizmo Tracks Traffic via Cells