Got bluetooth on your phone? Some cities are using bluetooth IDs to measure traffic flow, a phone or car's bluetooth ID as it passed sensors and calculating the speed based on the time taken to cover the distance. This isn't used for speed enforcement, they don't match the IDs to individuals/cars, but it certainly could be.

https://www.econolite.com/products/software/bluetoad/

"Advanced Traffic Management Systems Bluetooth Detection. TrafficCast proven algorithms for filtering and processing data inputs to compute real-time travel times and speeds."