I think, at least the way I would approach the problem, would be to look at the speed or flow rate of the phones on a particular road as the primary signal. I believe Google has ways of detecting if the device is in a car/vehicle vs being carried for example so they could filter out "walking" phones. Then looking at the flow of devices alleviates the need to calculate the carrying capacity of a particular road. The speed/flow tells you want you're trying to measure more directly than trying to count phones and decide if that means a road is congested or not, to do that you'd need to develop a heuristic to estimate the capacity of roads which seems like you're unnecessarily ignoring the direct signal in favor of trying to calculate it from a noisier source.