Flutterby™! : Every move you make

Missouri to track traffic by looking at movement of individual cell phones. This is with current cell phones:

Instead, it takes the frequent signals that wireless phones send to towers and follows the movement of the phones from one tower to another. Then it overlays that movement with highway maps to determine what road the phones are on and how fast they are moving. Lumping dozens, hundreds or thousands of those signals together can measure traffic flow.

But they are individually tracked.