If you're talking about the ones I think you are, those are preemption signal devices [0] for emergency vehicles.
[0]: https://ops.fhwa.dot.gov/publications/fhwahop08024/chapter9....
[0]: https://ops.fhwa.dot.gov/publications/fhwahop08024/chapter9....
Cobb County at least on their website says they use camera sensors as well as buried induction loops.