If only 5% of users have their CPU and data currently in use, then why wake up other phones? You don't need data from literally everyone who's running it, and it doesn't need to be going all the time. Besides, I bet a lot of phones wake in SOME way when they start moving.
The only useful signal is when a phone goes from "long-term stationary" to "moving": if someone is carrying their phone, actively using it, or riding a vehicle, data from them is worthless. Send one signal when the phone first starts moving after, oh, 2+ minutes of being at rest, and only alert if you see a large spike around a single cell tower within a five-second window. Data use would be very low.