- SignalTrace can correlate wireless device signals with license plate reader data and recurring travel patterns. - SignalTrace itself does not identify people, but investigators can combine its electronic signatures with outside records to develop leads. - Researchers have shown that movement and proximity data can reveal identities and social connections over time. - The Supreme Court has strengthened protections for cellphone location data, but SignalTrace raises new legal questions. Your phone may be giving a roadside sensor clues about where you go and which devices regularly travel alongside you. Say you carpool to work with the same person most mornings. Your car passes a license plate reader, which records the plate along with the time and location. Meanwhile, another sensor detects wireless signals coming from devices traveling nearby. Your phone might be there. So could a smartwatch, wireless headphones or electronics built into the car. After enough trips, software may recognize that some of those signals tend to move together. Now there is a recurring electronic pattern associated with that vehicle. Weeks later, one of those signals appears somewhere else, perhaps near a different vehicle connected to an investigation. Police may not know who owns the device yet. However, its previous connections could give investigators a place to start. That is the idea behind SignalTrace, a technology marketed to law enforcement by security company Leonardo. And it raises a big privacy question: How anonymous is a device signal once it can be tied to a vehicle, repeated locations and other devices traveling alongside it? How SignalTrace device tracking works SignalTrace works alongside automatic license plate reader technology, commonly called ALPR. Traditional license plate readers photograph passing vehicles. They can record the plate number, vehicle information and the time and location of the sighting. We recently looked at how license